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diff --git a/doc/RecTutorial/RecTutorial.tex b/doc/RecTutorial/RecTutorial.tex deleted file mode 100644 index df8bc9f1..00000000 --- a/doc/RecTutorial/RecTutorial.tex +++ /dev/null @@ -1,3687 +0,0 @@ -\documentclass[11pt]{article} -\title{A Tutorial on [Co-]Inductive Types in Coq} -\author{Eduardo Gim\'enez\thanks{Eduardo.Gimenez@inria.fr}, -Pierre Cast\'eran\thanks{Pierre.Casteran@labri.fr}} -\date{May 1998 --- \today} - -\usepackage{multirow} -\usepackage{aeguill} -%\externaldocument{RefMan-gal.v} -%\externaldocument{RefMan-ext.v} -%\externaldocument{RefMan-tac.v} -%\externaldocument{RefMan-oth} -%\externaldocument{RefMan-tus.v} -%\externaldocument{RefMan-syn.v} -%\externaldocument{Extraction.v} -\input{recmacros} -\input{coqartmacros} -\newcommand{\refmancite}[1]{{}} -%\newcommand{\refmancite}[1]{\cite{coqrefman}} -%\newcommand{\refmancite}[1]{\cite[#1] {]{coqrefman}} - -\usepackage[latin1]{inputenc} -\usepackage[T1]{fontenc} -\usepackage{makeidx} -%\usepackage{multind} -\usepackage{alltt} -\usepackage{verbatim} -\usepackage{amssymb} -\usepackage{amsmath} -\usepackage{theorem} -\usepackage[dvips]{epsfig} -\usepackage{epic} -\usepackage{eepic} -\usepackage{ecltree} -\usepackage{moreverb} -\usepackage{color} -\usepackage{pifont} -\usepackage{xr} -\usepackage{url} - -\usepackage{alltt} -\renewcommand{\familydefault}{ptm} -\renewcommand{\seriesdefault}{m} -\renewcommand{\shapedefault}{n} -\newtheorem{exercise}{Exercise}[section] -\makeindex -\begin{document} -\maketitle - -\begin{abstract} -This document\footnote{The first versions of this document were entirely written by Eduardo Gimenez. -Pierre Cast\'eran wrote the 2004 and 2006 revisions.} is an introduction to the definition and -use of inductive and co-inductive types in the {\coq} proof environment. It explains how types like natural numbers and infinite streams are defined -in {\coq}, and the kind of proof techniques that can be used to reason -about them (case analysis, induction, inversion of predicates, -co-induction, etc). Each technique is illustrated through an -executable and self-contained {\coq} script. -\end{abstract} -%\RRkeyword{Proof environments, recursive types.} -%\makeRT - -\addtocontents{toc}{\protect \thispagestyle{empty}} -\pagenumbering{arabic} - -\cleardoublepage -\tableofcontents -\clearpage - -\section{About this document} - -This document is an introduction to the definition and use of -inductive and co-inductive types in the {\coq} proof environment. It was born from the -notes written for the course about the version V5.10 of {\coq}, given -by Eduardo Gimenez at -the Ecole Normale Sup\'erieure de Lyon in March 1996. This article is -a revised and improved version of these notes for the version V8.0 of -the system. - - -We assume that the reader has some familiarity with the -proofs-as-programs paradigm of Logic \cite{Coquand:metamathematical} and the generalities -of the {\coq} system \cite{coqrefman}. You would take a greater advantage of -this document if you first read the general tutorial about {\coq} and -{\coq}'s FAQ, both available on \cite{coqsite}. -A text book \cite{coqart}, accompanied with a lot of -examples and exercises \cite{Booksite}, presents a detailed description -of the {\coq} system and its underlying -formalism: the Calculus of Inductive Construction. -Finally, the complete description of {\coq} is given in the reference manual -\cite{coqrefman}. Most of the tactics and commands we describe have -several options, which we do not present exhaustively. -If some script herein uses a non described feature, please refer to -the Reference Manual. - - -If you are familiar with other proof environments -based on type theory and the LCF style ---like PVS, LEGO, Isabelle, -etc--- then you will find not difficulty to guess the unexplained -details. - -The better way to read this document is to start up the {\coq} system, -type by yourself the examples and exercises, and observe the -behavior of the system. All the examples proposed in this tutorial -can be downloaded from the same site as the present document. - - -The tutorial is organised as follows. The next section describes how -inductive types are defined in {\coq}, and introduces some useful ones, -like natural numbers, the empty type, the propositional equality type, -and the logical connectives. Section \ref{CaseAnalysis} explains -definitions by pattern-matching and their connection with the -principle of case analysis. This principle is the most basic -elimination rule associated with inductive or co-inductive types - and follows a -general scheme that we illustrate for some of the types introduced in -Section \ref{Introduction}. Section \ref{CaseTechniques} illustrates -the pragmatics of this principle, showing different proof techniques -based on it. Section \ref{StructuralInduction} introduces definitions -by structural recursion and proofs by induction. -Section~\ref{CaseStudy} presents some elaborate techniques -about dependent case analysis. Finally, Section -\ref{CoInduction} is a brief introduction to co-inductive types ---i.e., types containing infinite objects-- and the principle of -co-induction. - - -Thanks to Bruno Barras, Yves Bertot, Hugo Herbelin, Jean-Fran\c{c}ois Monin -and Michel L\'evy for their help. - -\subsection*{Lexical conventions} -The \texttt{typewriter} font is used to represent text -input by the user, while the \textit{italic} font is used to represent -the text output by the system as answers. - - -Moreover, the mathematical symbols \coqle{}, \coqdiff, \(\exists\), -\(\forall\), \arrow{}, $\rightarrow{}$ \coqor{}, \coqand{}, and \funarrow{} -stand for the character strings \citecoq{<=}, \citecoq{<>}, -\citecoq{exists}, \citecoq{forall}, \citecoq{->}, \citecoq{<-}, -\texttt{\char'134/}, \texttt{/\char'134}, and \citecoq{=>}, -respectively. For instance, the \coq{} statement -%V8 A prendre -% inclusion numero 1 -% traduction numero 1 -\begin{alltt} -\hide{Open Scope nat_scope. Check (}forall A:Type,(exists x : A, forall (y:A), x <> y) -> 2 = 3\hide{).} -\end{alltt} -is written as follows in this tutorial: -%V8 A prendre -% inclusion numero 2 -% traduction numero 2 -\begin{alltt} -\hide{Check (}{\prodsym}A:Type,(\exsym{}x:A, {\prodsym}y:A, x {\coqdiff} y) \arrow{} 2 = 3\hide{).} -\end{alltt} - -When a fragment of \coq{} input text appears in the middle of -regular text, we often place this fragment between double quotes -``\dots.'' These double quotes do not belong to the \coq{} syntax. - -Finally, any -string enclosed between \texttt{(*} and \texttt{*)} is a comment and -is ignored by the \coq{} system. - -\section{Introducing Inductive Types} -\label{Introduction} - -Inductive types are types closed with respect to their introduction -rules. These rules explain the most basic or \textsl{canonical} ways -of constructing an element of the type. In this sense, they -characterize the recursive type. Different rules must be considered as -introducing different objects. In order to fix ideas, let us introduce -in {\coq} the most well-known example of a recursive type: the type of -natural numbers. - -%V8 A prendre -\begin{alltt} -Inductive nat : Set := - | O : nat - | S : nat\arrow{}nat. -\end{alltt} - -The definition of a recursive type has two main parts. First, we -establish what kind of recursive type we will characterize (a set, in -this case). Second, we present the introduction rules that define the -type ({\Z} and {\SUCC}), also called its {\sl constructors}. The constructors -{\Z} and {\SUCC} determine all the elements of this type. In other -words, if $n\mbox{:}\nat$, then $n$ must have been introduced either -by the rule {\Z} or by an application of the rule {\SUCC} to a -previously constructed natural number. In this sense, we can say -that {\nat} is \emph{closed}. On the contrary, the type -$\Set$ is an {\it open} type, since we do not know {\it a priori} all -the possible ways of introducing an object of type \texttt{Set}. - -After entering this command, the constants {\nat}, {\Z} and {\SUCC} are -available in the current context. We can see their types using the -\texttt{Check} command \refmancite{Section \ref{Check}}: - -%V8 A prendre -\begin{alltt} -Check nat. -\it{}nat : Set -\tt{}Check O. -\it{}O : nat -\tt{}Check S. -\it{}S : nat {\arrow} nat -\end{alltt} - -Moreover, {\coq} adds to the context three constants named - $\natind$, $\natrec$ and $\natrect$, which - correspond to different principles of structural induction on -natural numbers that {\coq} infers automatically from the definition. We -will come back to them in Section \ref{StructuralInduction}. - - -In fact, the type of natural numbers as well as several useful -theorems about them are already defined in the basic library of {\coq}, -so there is no need to introduce them. Therefore, let us throw away -our (re)definition of {\nat}, using the command \texttt{Reset}. - -%V8 A prendre -\begin{alltt} -Reset nat. -Print nat. -\it{}Inductive nat : Set := O : nat | S : nat \arrow{} nat -For S: Argument scope is [nat_scope] -\end{alltt} - -Notice that \coq{}'s \emph{interpretation scope} for natural numbers -(called \texttt{nat\_scope}) -allows us to read and write natural numbers in decimal form (see \cite{coqrefman}). For instance, the constructor \texttt{O} can be read or written -as the digit $0$, and the term ``~\texttt{S (S (S O))}~'' as $3$. - -%V8 A prendre -\begin{alltt} -Check O. -\it 0 : nat. -\tt -Check (S (S (S O))). -\it 3 : nat -\end{alltt} - -Let us now take a look to some other -recursive types contained in the standard library of {\coq}. - -\subsection{Lists} -Lists are defined in library \citecoq{List}\footnote{Notice that in versions of -{\coq} -prior to 8.1, the parameter $A$ had sort \citecoq{Set} instead of \citecoq{Type}; -the constant \citecoq{list} was thus of type \citecoq{Set\arrow{} Set}.} - - -\begin{alltt} -Require Import List. -Print list. -\it -Inductive list (A : Type) : Type:= - nil : list A | cons : A {\arrow} list A {\arrow} list A -For nil: Argument A is implicit -For cons: Argument A is implicit -For list: Argument scope is [type_scope] -For nil: Argument scope is [type_scope] -For cons: Argument scopes are [type_scope _ _] -\end{alltt} - -In this definition, \citecoq{A} is a \emph{general parameter}, global -to both constructors. -This kind of definition allows us to build a whole family of -inductive types, indexed over the sort \citecoq{Type}. -This can be observed if we consider the type of identifiers -\citecoq{list}, \citecoq{cons} and \citecoq{nil}. -Notice the notation \citecoq{(A := \dots)} which must be used -when {\coq}'s type inference algorithm cannot infer the implicit -parameter \citecoq{A}. -\begin{alltt} -Check list. -\it list - : Type {\arrow} Type - -\tt Check (nil (A:=nat)). -\it nil - : list nat - -\tt Check (nil (A:= nat {\arrow} nat)). -\it nil - : list (nat {\arrow} nat) - -\tt Check (fun A: Type {\funarrow} (cons (A:=A))). -\it fun A : Type {\funarrow} cons (A:=A) - : {\prodsym} A : Type, A {\arrow} list A {\arrow} list A - -\tt Check (cons 3 (cons 2 nil)). -\it 3 :: 2 :: nil - : list nat - -\tt Check (nat :: bool ::nil). -\it nat :: bool :: nil - : list Set - -\tt Check ((3<=4) :: True ::nil). -\it (3<=4) :: True :: nil - : list Prop - -\tt Check (Prop::Set::nil). -\it Prop::Set::nil - : list Type -\end{alltt} - -\subsection{Vectors.} -\label{vectors} - -Like \texttt{list}, \citecoq{vector} is a polymorphic type: -if $A$ is a type, and $n$ a natural number, ``~\citecoq{vector $A$ $n$}~'' -is the type of vectors of elements of $A$ and size $n$. - - -\begin{alltt} -Require Import Bvector. - -Print vector. -\it -Inductive vector (A : Type) : nat {\arrow} Type := - Vnil : vector A 0 - | Vcons : A {\arrow} {\prodsym} n : nat, vector A n {\arrow} vector A (S n) -For vector: Argument scopes are [type_scope nat_scope] -For Vnil: Argument scope is [type_scope] -For Vcons: Argument scopes are [type_scope _ nat_scope _] -\end{alltt} - - -Remark the difference between the two parameters $A$ and $n$: -The first one is a \textsl{general parameter}, global to all the -introduction rules,while the second one is an \textsl{index}, which is -instantiated differently in the introduction rules. -Such types parameterized by regular -values are called \emph{dependent types}. - -\begin{alltt} -Check (Vnil nat). -\it Vnil nat - : vector nat 0 - -\tt Check (fun (A:Type)(a:A){\funarrow} Vcons _ a _ (Vnil _)). -\it fun (A : Type) (a : A) {\funarrow} Vcons A a 0 (Vnil A) - : {\prodsym} A : Type, A {\arrow} vector A 1 - - -\tt Check (Vcons _ 5 _ (Vcons _ 3 _ (Vnil _))). -\it Vcons nat 5 1 (Vcons nat 3 0 (Vnil nat)) - : vector nat 2 -\end{alltt} - -\subsection{The contradictory proposition.} -Another example of an inductive type is the contradictory proposition. -This type inhabits the universe of propositions, and has no element -at all. -%V8 A prendre -\begin{alltt} -Print False. -\it{} Inductive False : Prop := -\end{alltt} - -\noindent Notice that no constructor is given in this definition. - -\subsection{The tautological proposition.} -Similarly, the -tautological proposition {\True} is defined as an inductive type -with only one element {\I}: - -%V8 A prendre -\begin{alltt} -Print True. -\it{}Inductive True : Prop := I : True -\end{alltt} - -\subsection{Relations as inductive types.} -Some relations can also be introduced in a smart way as an inductive family -of propositions. Let us take as example the order $n \leq m$ on natural -numbers, called \citecoq{le} in {\coq}. - This relation is introduced through -the following definition, quoted from the standard library\footnote{In the interpretation scope -for Peano arithmetic: -\citecoq{nat\_scope}, ``~\citecoq{n <= m}~'' is equivalent to -``~\citecoq{le n m}~'' .}: - - - - -%V8 A prendre -\begin{alltt} -Print le. \it -Inductive le (n:nat) : nat\arrow{}Prop := -| le_n: n {\coqle} n -| le_S: {\prodsym} m, n {\coqle} m \arrow{} n {\coqle} S m. -\end{alltt} - -Notice that in this definition $n$ is a general parameter, -while the second argument of \citecoq{le} is an index (see section -~\ref{vectors}). - This definition -introduces the binary relation $n {\leq} m$ as the family of unary predicates -``\textsl{to be greater or equal than a given $n$}'', parameterized by $n$. - -The introduction rules of this type can be seen as a sort of Prolog -rules for proving that a given integer $n$ is less or equal than another one. -In fact, an object of type $n{\leq} m$ is nothing but a proof -built up using the constructors \textsl{le\_n} and -\textsl{le\_S} of this type. As an example, let us construct -a proof that zero is less or equal than three using {\coq}'s interactive -proof mode. -Such an object can be obtained applying three times the second -introduction rule of \citecoq{le}, to a proof that zero is less or equal -than itself, -which is provided by the first constructor of \citecoq{le}: - -%V8 A prendre -\begin{alltt} -Theorem zero_leq_three: 0 {\coqle} 3. -Proof. -\it{} 1 subgoal - -============================ - 0 {\coqle} 3 - -\tt{}Proof. - constructor 2. - -\it{} 1 subgoal -============================ - 0 {\coqle} 2 - -\tt{} constructor 2. -\it{} 1 subgoal -============================ - 0 {\coqle} 1 - -\tt{} constructor 2 -\it{} 1 subgoal -============================ - 0 {\coqle} 0 - -\tt{} constructor 1. - -\it{}Proof completed -\tt{}Qed. -\end{alltt} - -\noindent When -the current goal is an inductive type, the tactic -``~\citecoq{constructor $i$}~'' \refmancite{Section \ref{constructor}} applies the $i$-th constructor in the -definition of the type. We can take a look at the proof constructed -using the command \texttt{Print}: - -%V8 A prendre -\begin{alltt} -Print Print zero_leq_three. -\it{}zero_leq_three = -zero_leq_three = le_S 0 2 (le_S 0 1 (le_S 0 0 (le_n 0))) - : 0 {\coqle} 3 -\end{alltt} - -When the parameter $i$ is not supplied, the tactic \texttt{constructor} -tries to apply ``~\texttt{constructor $1$}~'', ``~\texttt{constructor $2$}~'',\dots, -``~\texttt{constructor $n$}~'' where $n$ is the number of constructors -of the inductive type (2 in our example) of the conclusion of the goal. -Our little proof can thus be obtained iterating the tactic -\texttt{constructor} until it fails: - -%V8 A prendre -\begin{alltt} -Lemma zero_leq_three': 0 {\coqle} 3. - repeat constructor. -Qed. -\end{alltt} - -Notice that the strict order on \texttt{nat}, called \citecoq{lt} -is not inductively defined: the proposition $n<p$ (notation for \citecoq{lt $n$ $p$}) -is reducible to \citecoq{(S $n$) $\leq$ p}. - -\begin{alltt} -Print lt. -\it -lt = fun n m : nat {\funarrow} S n {\coqle} m - : nat {\arrow} nat {\arrow} Prop -\tt -Lemma zero_lt_three : 0 < 3. -Proof. - repeat constructor. -Qed. - -Print zero_lt_three. -\it zero_lt_three = le_S 1 2 (le_S 1 1 (le_n 1)) - : 0 < 3 -\end{alltt} - - - -\subsection{About general parameters (\coq{} version $\geq$ 8.1)} -\label{parameterstuff} - -Since version $8.1$, it is possible to write more compact inductive definitions -than in earlier versions. - -Consider the following alternative definition of the relation $\leq$ on -type \citecoq{nat}: - -\begin{alltt} -Inductive le'(n:nat):nat -> Prop := - | le'_n : le' n n - | le'_S : forall p, le' (S n) p -> le' n p. - -Hint Constructors le'. -\end{alltt} - -We notice that the type of the second constructor of \citecoq{le'} -has an argument whose type is \citecoq{le' (S n) p}. -This constrasts with earlier versions -of {\coq}, in which a general parameter $a$ of an inductive -type $I$ had to appear only in applications of the form $I\,\dots\,a$. - -Since version $8.1$, if $a$ is a general parameter of an inductive -type $I$, the type of an argument of a constructor of $I$ may be -of the form $I\,\dots\,t_a$ , where $t_a$ is any term. -Notice that the final type of the constructors must be of the form -$I\,\dots\,a$, since these constructors describe how to form -inhabitants of type $I\,\dots\,a$ (this is the role of parameter $a$). - -Another example of this new feature is {\coq}'s definition of accessibility -(see Section~\ref{WellFoundedRecursion}), which has a general parameter -$x$; the constructor for the predicate -``$x$ is accessible'' takes an argument of type ``$y$ is accessible''. - - - -In earlier versions of {\coq}, a relation like \citecoq{le'} would have to be -defined without $n$ being a general parameter. - -\begin{alltt} -Reset le'. - -Inductive le': nat-> nat -> Prop := - | le'_n : forall n, le' n n - | le'_S : forall n p, le' (S n) p -> le' n p. -\end{alltt} - - - - -\subsection{The propositional equality type.} \label{equality} -In {\coq}, the propositional equality between two inhabitants $a$ and -$b$ of -the same type $A$ , -noted $a=b$, is introduced as a family of recursive predicates -``~\textsl{to be equal to $a$}~'', parameterised by both $a$ and its type -$A$. This family of types has only one introduction rule, which -corresponds to reflexivity. -Notice that the syntax ``\citecoq{$a$ = $b$}~'' is an abbreviation -for ``\citecoq{eq $a$ $b$}~'', and that the parameter $A$ is \emph{implicit}, -as it can be infered from $a$. -%V8 A prendre -\begin{alltt} -Print eq. -\it{} Inductive eq (A : Type) (x : A) : A \arrow{} Prop := - refl_equal : x = x -For eq: Argument A is implicit -For refl_equal: Argument A is implicit -For eq: Argument scopes are [type_scope _ _] -For refl_equal: Argument scopes are [type_scope _] -\end{alltt} - -Notice also that the first parameter $A$ of \texttt{eq} has type -\texttt{Type}. The type system of {\coq} allows us to consider equality between -various kinds of terms: elements of a set, proofs, propositions, -types, and so on. -Look at \cite{coqrefman, coqart} to get more details on {\coq}'s type -system, as well as implicit arguments and argument scopes. - - -\begin{alltt} -Lemma eq_3_3 : 2 + 1 = 3. -Proof. - reflexivity. -Qed. - -Lemma eq_proof_proof : refl_equal (2*6) = refl_equal (3*4). -Proof. - reflexivity. -Qed. - -Print eq_proof_proof. -\it eq_proof_proof = -refl_equal (refl_equal (3 * 4)) - : refl_equal (2 * 6) = refl_equal (3 * 4) -\tt - -Lemma eq_lt_le : ( 2 < 4) = (3 {\coqle} 4). -Proof. - reflexivity. -Qed. - -Lemma eq_nat_nat : nat = nat. -Proof. - reflexivity. -Qed. - -Lemma eq_Set_Set : Set = Set. -Proof. - reflexivity. -Qed. -\end{alltt} - -\subsection{Logical connectives.} \label{LogicalConnectives} -The conjunction and disjunction of two propositions are also examples -of recursive types: - -\begin{alltt} -Inductive or (A B : Prop) : Prop := - or_introl : A \arrow{} A {\coqor} B | or_intror : B \arrow{} A {\coqor} B - -Inductive and (A B : Prop) : Prop := - conj : A \arrow{} B \arrow{} A {\coqand} B - -\end{alltt} - -The propositions $A$ and $B$ are general parameters of these -connectives. Choosing different universes for -$A$ and $B$ and for the inductive type itself gives rise to different -type constructors. For example, the type \textsl{sumbool} is a -disjunction but with computational contents. - -\begin{alltt} -Inductive sumbool (A B : Prop) : Set := - left : A \arrow{} \{A\} + \{B\} | right : B \arrow{} \{A\} + \{B\} -\end{alltt} - - - -This type --noted \texttt{\{$A$\}+\{$B$\}} in {\coq}-- can be used in {\coq} -programs as a sort of boolean type, to check whether it is $A$ or $B$ -that is true. The values ``~\citecoq{left $p$}~'' and -``~\citecoq{right $q$}~'' replace the boolean values \textsl{true} and -\textsl{false}, respectively. The advantage of this type over -\textsl{bool} is that it makes available the proofs $p$ of $A$ or $q$ -of $B$, which could be necessary to construct a verification proof -about the program. -For instance, let us consider the certified program \citecoq{le\_lt\_dec} -of the Standard Library. - -\begin{alltt} -Require Import Compare_dec. -Check le_lt_dec. -\it -le_lt_dec - : {\prodsym} n m : nat, \{n {\coqle} m\} + \{m < n\} - -\end{alltt} - -We use \citecoq{le\_lt\_dec} to build a function for computing -the max of two natural numbers: - -\begin{alltt} -Definition max (n p :nat) := match le_lt_dec n p with - | left _ {\funarrow} p - | right _ {\funarrow} n - end. -\end{alltt} - -In the following proof, the case analysis on the term -``~\citecoq{le\_lt\_dec n p}~'' gives us an access to proofs -of $n\leq p$ in the first case, $p<n$ in the other. - -\begin{alltt} -Theorem le_max : {\prodsym} n p, n {\coqle} p {\arrow} max n p = p. -Proof. - intros n p ; unfold max ; case (le_lt_dec n p); simpl. -\it -2 subgoals - - n : nat - p : nat - ============================ - n {\coqle} p {\arrow} n {\coqle} p {\arrow} p = p - -subgoal 2 is: - p < n {\arrow} n {\coqle} p {\arrow} n = p -\tt - trivial. - intros; absurd (p < p); eauto with arith. -Qed. -\end{alltt} - - - Once the program verified, the proofs are -erased by the extraction procedure: - -\begin{alltt} -Extraction max. -\it -(** val max : nat {\arrow} nat {\arrow} nat **) - -let max n p = - match le_lt_dec n p with - | Left {\arrow} p - | Right {\arrow} n -\end{alltt} - -Another example of use of \citecoq{sumbool} is given in Section -\ref{WellFoundedRecursion}: the theorem \citecoq{eq\_nat\_dec} of -library \citecoq{Coq.Arith.Peano\_dec} is used in an euclidean division -algorithm. - -\subsection{The existential quantifier.}\label{ex-def} -The existential quantifier is yet another example of a logical -connective introduced as an inductive type. - -\begin{alltt} -Inductive ex (A : Type) (P : A \arrow{} Prop) : Prop := - ex_intro : {\prodsym} x : A, P x \arrow{} ex P -\end{alltt} - -Notice that {\coq} uses the abreviation ``~\citecoq{\exsym\,$x$:$A$, $B$}~'' -for \linebreak ``~\citecoq{ex (fun $x$:$A$ \funarrow{} $B$)}~''. - - -\noindent The former quantifier inhabits the universe of propositions. -As for the conjunction and disjunction connectives, there is also another -version of existential quantification inhabiting the universes $\Type_i$, -which is noted \texttt{sig $P$}. The syntax -``~\citecoq{\{$x$:$A$ | $B$\}}~'' is an abreviation for ``~\citecoq{sig (fun $x$:$A$ {\funarrow} $B$)}~''. - - - -%\paragraph{The logical connectives.} Conjuction and disjuction are -%also introduced as recursive types: -%\begin{alltt} -%Print or. -%\end{alltt} -%begin{alltt} -%Print and. -%\end{alltt} - - -\subsection{Mutually Dependent Definitions} -\label{MutuallyDependent} - -Mutually dependent definitions of recursive types are also allowed in -{\coq}. A typical example of these kind of declaration is the -introduction of the trees of unbounded (but finite) width: -\label{Forest} -\begin{alltt} -Inductive tree(A:Type) : Type := - node : A {\arrow} forest A \arrow{} tree A -with forest (A: Set) : Type := - nochild : forest A | - addchild : tree A \arrow{} forest A \arrow{} forest A. -\end{alltt} -\noindent Yet another example of mutually dependent types are the -predicates \texttt{even} and \texttt{odd} on natural numbers: -\label{Even} -\begin{alltt} -Inductive - even : nat\arrow{}Prop := - evenO : even O | - evenS : {\prodsym} n, odd n \arrow{} even (S n) -with - odd : nat\arrow{}Prop := - oddS : {\prodsym} n, even n \arrow{} odd (S n). -\end{alltt} - -\begin{alltt} -Lemma odd_49 : odd (7 * 7). - simpl; repeat constructor. -Qed. -\end{alltt} - - - -\section{Case Analysis and Pattern-matching} -\label{CaseAnalysis} -\subsection{Non-dependent Case Analysis} -An \textsl{elimination rule} for the type $A$ is some way to use an -object $a:A$ in order to define an object in some type $B$. -A natural elimination rule for an inductive type is \emph{case analysis}. - - -For instance, any value of type {\nat} is built using either \texttt{O} or \texttt{S}. -Thus, a systematic way of building a value of type $B$ from any -value of type {\nat} is to associate to \texttt{O} a constant $t_O:B$ and -to every term of the form ``~\texttt{S $p$}~'' a term $t_S:B$. The following -construction has type $B$: -\begin{alltt} -match \(n\) return \(B\) with O \funarrow \(t\sb{O}\) | S p \funarrow \(t\sb{S}\) end -\end{alltt} - - -In most of the cases, {\coq} is able to infer the type $B$ of the object -defined, so the ``\texttt{return $B$}'' part can be omitted. - -The computing rules associated with this construct are the expected ones -(the notation $t_S\{q/\texttt{p}\}$ stands for the substitution of $p$ by -$q$ in $t_S$ :) - -\begin{eqnarray*} -\texttt{match $O$ return $b$ with O {\funarrow} $t_O$ | S p {\funarrow} $t_S$ end} &\Longrightarrow& t_O\\ -\texttt{match $S\;q$ return $b$ with O {\funarrow} $t_O$ | S p {\funarrow} $t_S$ end} &\Longrightarrow& t_S\{q/\texttt{p}\} -\end{eqnarray*} - - -\subsubsection{Example: the predecessor function.}\label{firstpred} -An example of a definition by case analysis is the function which -computes the predecessor of any given natural number: -\begin{alltt} -Definition pred (n:nat) := match n with - | O {\funarrow} O - | S m {\funarrow} m - end. - -Eval simpl in pred 56. -\it{} = 55 - : nat -\tt -Eval simpl in pred 0. -\it{} = 0 - : nat - -\tt{}Eval simpl in fun p {\funarrow} pred (S p). -\it{} = fun p : nat {\funarrow} p - : nat {\arrow} nat -\end{alltt} - -As in functional programming, tuples and wild-cards can be used in -patterns \refmancite{Section \ref{ExtensionsOfCases}}. Such -definitions are automatically compiled by {\coq} into an expression which -may contain several nested case expressions. For example, the -exclusive \emph{or} on booleans can be defined as follows: -\begin{alltt} -Definition xorb (b1 b2:bool) := - match b1, b2 with - | false, true {\funarrow} true - | true, false {\funarrow} true - | _ , _ {\funarrow} false - end. -\end{alltt} - -This kind of definition is compiled in {\coq} as follows\footnote{{\coq} uses -the conditional ``~\citecoq{if $b$ then $a$ else $b$}~'' as an abreviation to -``~\citecoq{match $b$ with true \funarrow{} $a$ | false \funarrow{} $b$ end}~''.}: - -\begin{alltt} -Print xorb. -xorb = -fun b1 b2 : bool {\funarrow} -if b1 then if b2 then false else true - else if b2 then true else false - : bool {\arrow} bool {\arrow} bool -\end{alltt} - -\subsection{Dependent Case Analysis} -\label{DependentCase} - -For a pattern matching construct of the form -``~\citecoq{match n with \dots end}~'' a more general typing rule -is obtained considering that the type of the whole expression -may also depend on \texttt{n}. - For instance, let us consider some function -$Q:\texttt{nat}\arrow{}\texttt{Type}$, and $n:\citecoq{nat}$. -In order to build a term of type $Q\;n$, we can associate -to the constructor \texttt{O} some term $t_O: Q\;\texttt{O}$ and to -the pattern ``~\texttt{S p}~'' some term $t_S : Q\;(S\;p)$. -Notice that the terms $t_O$ and $t_S$ do not have the same type. - -The syntax of the \emph{dependent case analysis} and its -associated typing rule make precise how the resulting -type depends on the argument of the pattern matching, and -which constraint holds on the branches of the pattern matching: - -\label{Prod-sup-rule} -\[ -\begin{array}[t]{l} -Q: \texttt{nat}{\arrow}\texttt{Type}\quad{t_O}:{{Q\;\texttt{O}}} \quad -\smalljuge{p:\texttt{nat}}{t_p}{{Q\;(\texttt{S}\;p)}} \quad n:\texttt{nat} \\ -\hline -{\texttt{match \(n\) as \(n\sb{0}\) return \(Q\;n\sb{0}\) with | O \funarrow \(t\sb{O}\) | S p \funarrow \(t\sb{S}\) end}}:{{Q\;n}} -\end{array} -\] - - -The interest of this rule of \textsl{dependent} pattern-matching is -that it can also be read as the following logical principle (when $Q$ has type \citecoq{nat\arrow{}Prop} -by \texttt{Prop} in the type of $Q$): in order to prove -that a property $Q$ holds for all $n$, it is sufficient to prove that -$Q$ holds for {\Z} and that for all $p:\nat$, $Q$ holds for -$(\SUCC\;p)$. The former, non-dependent version of case analysis can -be obtained from this latter rule just taking $Q$ as a constant -function on $n$. - -Notice that destructuring $n$ into \citecoq{O} or ``~\citecoq{S p}~'' - doesn't -make appear in the goal the equalities ``~$n=\citecoq{O}$~'' - and ``~$n=\citecoq{S p}$~''. -They are ``internalized'' in the rules above (see section~\ref{inversion}.) - -\subsubsection{Example: strong specification of the predecessor function.} - -In Section~\ref{firstpred}, the predecessor function was defined directly -as a function from \texttt{nat} to \texttt{nat}. It remains to prove -that this function has some desired properties. Another way to proceed -is to, first introduce a specification of what is the predecessor of a -natural number, under the form of a {\coq} type, then build an inhabitant -of this type: in other words, a realization of this specification. This way, the correctness -of this realization is ensured by {\coq}'s type system. - -A reasonable specification for $\pred$ is to say that for all $n$ -there exists another $m$ such that either $m=n=0$, or $(\SUCC\;m)$ -is equal to $n$. The function $\pred$ should be just the way to -compute such an $m$. - -\begin{alltt} -Definition pred_spec (n:nat) := - \{m:nat | n=0{\coqand} m=0 {\coqor} n = S m\}. - -Definition predecessor : {\prodsym} n:nat, pred_spec n. - intro n; case n. -\it{} - n : nat - ============================ - pred_spec 0 - -\tt{} unfold pred_spec;exists 0;auto. -\it{} - ========================================= - {\prodsym} n0 : nat, pred_spec (S n0) -\tt{} - unfold pred_spec; intro n0; exists n0; auto. -Defined. -\end{alltt} - -If we print the term built by {\coq}, we can observe its dependent pattern-matching structure: - -\begin{alltt} -predecessor = fun n : nat {\funarrow} -\textbf{match n as n0 return (pred_spec n0) with} -\textbf{| O {\funarrow}} - exist (fun m : nat {\funarrow} 0 = 0 {\coqand} m = 0 {\coqor} 0 = S m) 0 - (or_introl (0 = 1) - (conj (refl_equal 0) (refl_equal 0))) -\textbf{| S n0 {\funarrow}} - exist (fun m : nat {\funarrow} S n0 = 0 {\coqand} m = 0 {\coqor} S n0 = S m) n0 - (or_intror (S n0 = 0 {\coqand} n0 = 0) (refl_equal (S n0))) -\textbf{end} : {\prodsym} n : nat, \textbf{pred_spec n} -\end{alltt} - - -Notice that there are many variants to the pattern ``~\texttt{intros \dots; case \dots}~''. Look at the reference manual and/or the book: tactics -\texttt{destruct}, ``~\texttt{intro \emph{pattern}}~'', etc. - -\noindent The command \texttt{Extraction} \refmancite{Section -\ref{ExtractionIdent}} can be used to see the computational -contents associated to the \emph{certified} function \texttt{predecessor}: -\begin{alltt} -Extraction predecessor. -\it -(** val predecessor : nat {\arrow} pred_spec **) - -let predecessor = function - | O {\arrow} O - | S n0 {\arrow} n0 -\end{alltt} - - -\begin{exercise} \label{expand} -Prove the following theorem: -\begin{alltt} -Theorem nat_expand : {\prodsym} n:nat, - n = match n with - | 0 {\funarrow} 0 - | S p {\funarrow} S p - end. -\end{alltt} -\end{exercise} - -\subsection{Some Examples of Case Analysis} -\label{CaseScheme} -The reader will find in the Reference manual all details about -typing case analysis (chapter 4: Calculus of Inductive Constructions, -and chapter 15: Extended Pattern-Matching). - -The following commented examples will show the different situations to consider. - - -%\subsubsection{General Scheme} - -%Case analysis is then the most basic elimination rule that {\coq} -%provides for inductive types. This rule follows a general schema, -%valid for any inductive type $I$. First, if $I$ has type -%``~$\forall\,(z_1:A_1)\ldots(z_r:A_r),S$~'', with $S$ either $\Set$, $\Prop$ or -%$\Type$, then a case expression on $p$ of type ``~$R\;a_1\ldots a_r$~'' -% inhabits ``~$Q\;a_1\ldots a_r\;p$~''. The types of the branches of the case expression -%are obtained from the definition of the type in this way: if the type -%of the $i$-th constructor $c_i$ of $R$ is -%``~$\forall\, (x_1:T_1)\ldots -%(x_n:T_n),(R\;q_1\ldots q_r)$~'', then the $i-th$ branch must have the -%form ``~$c_i\; x_1\; \ldots \;x_n\; \funarrow{}\; t_i$~'' where -%$$(x_1:T_1),\ldots, (x_n:T_n) \vdash t_i : Q\;q_1\ldots q_r)$$ -% for non-dependent case -%analysis, and $$(x_1:T_1)\ldots (x_n:T_n)\vdash t_i :Q\;q_1\ldots -%q_r\;({c}_i\;x_1\;\ldots x_n)$$ for dependent one. In the -%following section, we illustrate this general scheme for different -%recursive types. -%%\textbf{A vérifier} - -\subsubsection{The Empty Type} - -In a definition by case analysis, there is one branch for each -introduction rule of the type. Hence, in a definition by case analysis -on $p:\False$ there are no cases to be considered. In other words, the -rule of (non-dependent) case analysis for the type $\False$ is -(for $s$ in \texttt{Prop}, \texttt{Set} or \texttt{Type}): - -\begin{center} -\snregla {\JM{Q}{s}\;\;\;\;\; - \JM{p}{\False}} - {\JM{\texttt{match $p$ return $Q$ with end}}{Q}} -\end{center} - -As a corollary, if we could construct an object in $\False$, then it -could be possible to define an object in any type. The tactic -\texttt{contradiction} \refmancite{Section \ref{Contradiction}} -corresponds to the application of the elimination rule above. It -searches in the context for an absurd hypothesis (this is, a -hypothesis whose type is $\False$) and then proves the goal by a case -analysis of it. - -\begin{alltt} -Theorem fromFalse : False \arrow{} 0=1. -Proof. - intro H. - contradiction. -Qed. -\end{alltt} - - -In {\coq} the negation is defined as follows : - -\begin{alltt} -Definition not (P:Prop) := P {\arrow} False -\end{alltt} - -The proposition ``~\citecoq{not $A$}~'' is also written ``~$\neg A$~''. - -If $A$ and $B$ are propositions, $a$ is a proof of $A$ and -$H$ is a proof of $\neg A$, -the term ``~\citecoq{match $H\;a$ return $B$ with end}~'' is a proof term of -$B$. -Thus, if your goal is $B$ and you have some hypothesis $H:\neg A$, -the tactic ``~\citecoq{case $H$}~'' generates a new subgoal with -statement $A$, as shown by the following example\footnote{Notice that -$a\coqdiff b$ is just an abreviation for ``~\coqnot a= b~''}. - -\begin{alltt} -Fact Nosense : 0 {\coqdiff} 0 {\arrow} 2 = 3. -Proof. - intro H; case H. -\it -=========================== - 0 = 0 -\tt - reflexivity. -Qed. -\end{alltt} - -The tactic ``~\texttt{absurd $A$}~'' (where $A$ is any proposition), -is based on the same principle, but -generates two subgoals: $A$ and $\neg A$, for solving $B$. - -\subsubsection{The Equality Type} - -Let $A:\Type$, $a$, $b$ of type $A$, and $\pi$ a proof of -$a=b$. Non dependent case analysis of $\pi$ allows us to -associate to any proof of ``~$Q\;a$~'' a proof of ``~$Q\;b$~'', -where $Q:A\arrow{} s$ (where $s\in\{\Prop, \Set, \Type\}$). -The following term is a proof of ``~$Q\;a\, \arrow{}\, Q\;b$~''. - -\begin{alltt} -fun H : Q a {\funarrow} - match \(\pi\) in (_ = y) return Q y with - refl_equal {\funarrow} H - end -\end{alltt} -Notice the header of the \texttt{match} construct. -It expresses how the resulting type ``~\citecoq{Q y}~'' depends on -the \emph{type} of \texttt{p}. -Notice also that in the pattern introduced by the keyword \texttt{in}, -the parameter \texttt{a} in the type ``~\texttt{a = y}~'' must be -implicit, and replaced by a wildcard '\texttt{\_}'. - - -Therefore, case analysis on a proof of the equality $a=b$ -amounts to replacing all the occurrences of the term $b$ with the term -$a$ in the goal to be proven. Let us illustrate this through an -example: the transitivity property of this equality. -\begin{alltt} -Theorem trans : {\prodsym} n m p:nat, n=m \arrow{} m=p \arrow{} n=p. -Proof. - intros n m p eqnm. -\it{} - n : nat - m : nat - p : nat - eqnm : n = m - ============================ - m = p {\arrow} n = p -\tt{} case eqnm. -\it{} - n : nat - m : nat - p : nat - eqnm : n = m - ============================ - n = p {\arrow} n = p -\tt{} trivial. -Qed. -\end{alltt} - -%\noindent The case analysis on the hypothesis $H:n=m$ yields the -%tautological subgoal $n=p\rightarrow n=p$, that is directly proven by -%the tactic \texttt{Trivial}. - -\begin{exercise} -Prove the symmetry property of equality. -\end{exercise} - -Instead of using \texttt{case}, we can use the tactic -\texttt{rewrite} \refmancite{Section \ref{Rewrite}}. If $H$ is a proof -of $a=b$, then -``~\citecoq{rewrite $H$}~'' - performs a case analysis on a proof of $b=a$, obtained by applying a -symmetry theorem to $H$. This application of symmetry allows us to rewrite -the equality from left to right, which looks more natural. An optional -parameter (either \texttt{\arrow{}} or \texttt{$\leftarrow$}) can be used to precise -in which sense the equality must be rewritten. By default, -``~\texttt{rewrite} $H$~'' corresponds to ``~\texttt{rewrite \arrow{}} $H$~'' -\begin{alltt} -Lemma Rw : {\prodsym} x y: nat, y = y * x {\arrow} y * x * x = y. - intros x y e; do 2 rewrite <- e. -\it -1 subgoal - - x : nat - y : nat - e : y = y * x - ============================ - y = y -\tt - reflexivity. -Qed. -\end{alltt} - -Notice that, if $H:a=b$, then the tactic ``~\texttt{rewrite $H$}~'' - replaces \textsl{all} the -occurrences of $a$ by $b$. However, in certain situations we could be -interested in rewriting some of the occurrences, but not all of them. -This can be done using the tactic \texttt{pattern} \refmancite{Section -\ref{Pattern}}. Let us consider yet another example to -illustrate this. - -Let us start with some simple theorems of arithmetic; two of them -are already proven in the Standard Library, the last is left as an exercise. - -\begin{alltt} -\it -mult_1_l - : {\prodsym} n : nat, 1 * n = n - -mult_plus_distr_r - : {\prodsym} n m p : nat, (n + m) * p = n * p + m * p - -mult_distr_S : {\prodsym} n p : nat, n * p + p = (S n)* p. -\end{alltt} - -Let us now prove a simple result: - -\begin{alltt} -Lemma four_n : {\prodsym} n:nat, n+n+n+n = 4*n. -Proof. - intro n;rewrite <- (mult_1_l n). -\it - n : nat - ============================ - 1 * n + 1 * n + 1 * n + 1 * n = 4 * (1 * n) -\end{alltt} - -We can see that the \texttt{rewrite} tactic call replaced \emph{all} -the occurrences of \texttt{n} by the term ``~\citecoq{1 * n}~''. -If we want to do the rewriting ony on the leftmost occurrence of -\texttt{n}, we can mark this occurrence using the \texttt{pattern} -tactic: - - -\begin{alltt} - Undo. - intro n; pattern n at 1. - \it - n : nat - ============================ - (fun n0 : nat {\funarrow} n0 + n + n + n = 4 * n) n -\end{alltt} -Applying the tactic ``~\citecoq{pattern n at 1}~'' allowed us -to explicitly abstract the first occurrence of \texttt{n} from the -goal, putting this goal under the form ``~\citecoq{$Q$ n}~'', -thus pointing to \texttt{rewrite} the particular predicate on $n$ -that we search to prove. - - -\begin{alltt} - rewrite <- mult_1_l. -\it -1 subgoal - - n : nat - ============================ - 1 * n + n + n + n = 4 * n -\tt - repeat rewrite mult_distr_S. -\it - n : nat - ============================ - 4 * n = 4 * n -\tt - trivial. -Qed. -\end{alltt} - -\subsubsection{The Predicate $n {\leq} m$} - - -The last but one instance of the elimination schema that we will illustrate is -case analysis for the predicate $n {\leq} m$: - -Let $n$ and $p$ be terms of type \citecoq{nat}, and $Q$ a predicate -of type $\citecoq{nat}\arrow{}\Prop$. -If $H$ is a proof of ``~\texttt{n {\coqle} p}~'', -$H_0$ a proof of ``~\texttt{$Q$ n}~'' and -$H_S$ a proof of ``~\citecoq{{\prodsym}m:nat, n {\coqle} m {\arrow} Q (S m)}~'', -then the term -\begin{alltt} -match H in (_ {\coqle} q) return (Q q) with - | le_n {\funarrow} H0 - | le_S m Hm {\funarrow} HS m Hm -end -\end{alltt} - is a proof term of ``~\citecoq{$Q$ $p$}~''. - - -The two patterns of this \texttt{match} construct describe -all possible forms of proofs of ``~\citecoq{n {\coqle} m}~'' (notice -again that the general parameter \texttt{n} is implicit in - the ``~\texttt{in \dots}~'' -clause and is absent from the match patterns. - - -Notice that the choice of introducing some of the arguments of the -predicate as being general parameters in its definition has -consequences on the rule of case analysis that is derived. In -particular, the type $Q$ of the object defined by the case expression -only depends on the indexes of the predicate, and not on the general -parameters. In the definition of the predicate $\leq$, the first -argument of this relation is a general parameter of the -definition. Hence, the predicate $Q$ to be proven only depends on the -second argument of the relation. In other words, the integer $n$ is -also a general parameter of the rule of case analysis. - -An example of an application of this rule is the following theorem, -showing that any integer greater or equal than $1$ is the successor of another -natural number: - -\begin{alltt} -Lemma predecessor_of_positive : - {\prodsym} n, 1 {\coqle} n {\arrow} {\exsym} p:nat, n = S p. -Proof. - intros n H;case H. -\it - n : nat - H : 1 {\coqle} n - ============================ - {\exsym} p : nat, 1 = S p -\tt - exists 0; trivial. -\it - - n : nat - H : 1 {\coqle} n - ============================ - {\prodsym} m : nat, 0 {\coqle} m {\arrow} {\exsym} p : nat, S m = S p -\tt - intros m _ . - exists m. - trivial. -Qed. -\end{alltt} - - -\subsubsection{Vectors} - -The \texttt{vector} polymorphic and dependent family of types will -give an idea of the most general scheme of pattern-matching. - -For instance, let us define a function for computing the tail of -any vector. Notice that we shall build a \emph{total} function, -by considering that the tail of an empty vector is this vector itself. -In that sense, it will be slightly different from the \texttt{Vtail} -function of the Standard Library, which is defined only for vectors -of type ``~\citecoq{vector $A$ (S $n$)}~''. - -The header of the function we want to build is the following: - -\begin{verbatim} -Definition Vtail_total - (A : Type) (n : nat) (v : vector A n) : vector A (pred n):= -\end{verbatim} - -Since the branches will not have the same type -(depending on the parameter \texttt{n}), -the body of this function is a dependent pattern matching on -\citecoq{v}. -So we will have : -\begin{verbatim} -match v in (vector _ n0) return (vector A (pred n0)) with -\end{verbatim} - -The first branch deals with the constructor \texttt{Vnil} and must -return a value in ``~\citecoq{vector A (pred 0)}~'', convertible -to ``~\citecoq{vector A 0}~''. So, we propose: -\begin{alltt} -| Vnil {\funarrow} Vnil A -\end{alltt} - -The second branch considers a vector in ``~\citecoq{vector A (S n0)}~'' -of the form -``~\citecoq{Vcons A n0 v0}~'', with ``~\citecoq{v0:vector A n0}~'', -and must return a value of type ``~\citecoq{vector A (pred (S n0))}~'', -which is convertible to ``~\citecoq{vector A n0}~''. -This second branch is thus : -\begin{alltt} -| Vcons _ n0 v0 {\funarrow} v0 -\end{alltt} - -Here is the full definition: - -\begin{alltt} -Definition Vtail_total - (A : Type) (n : nat) (v : vector A n) : vector A (pred n):= -match v in (vector _ n0) return (vector A (pred n0)) with -| Vnil {\funarrow} Vnil A -| Vcons _ n0 v0 {\funarrow} v0 -end. -\end{alltt} - - -\subsection{Case Analysis and Logical Paradoxes} - -In the previous section we have illustrated the general scheme for -generating the rule of case analysis associated to some recursive type -from the definition of the type. However, if the logical soundness is -to be preserved, certain restrictions to this schema are -necessary. This section provides a brief explanation of these -restrictions. - - -\subsubsection{The Positivity Condition} -\label{postypes} - -In order to make sense of recursive types as types closed under their -introduction rules, a constraint has to be imposed on the possible -forms of such rules. This constraint, known as the -\textsl{positivity condition}, is necessary to prevent the user from -naively introducing some recursive types which would open the door to -logical paradoxes. An example of such a dangerous type is the -``inductive type'' \citecoq{Lambda}, whose only constructor is -\citecoq{lambda} of type \citecoq{(Lambda\arrow False)\arrow Lambda}. - Following the pattern -given in Section \ref{CaseScheme}, the rule of (non dependent) case -analysis for \citecoq{Lambda} would be the following: - -\begin{center} -\snregla {\JM{Q}{\Prop}\;\;\;\;\; - \JM{p}{\texttt{Lambda}}\;\;\;\;\; - {h : {\texttt{Lambda}}\arrow\False\; \vdash\; t\,:\,Q}} - {\JM{\citecoq{match $p$ return $Q$ with lambda h {\funarrow} $t$ end}}{Q}} -\end{center} - -In order to avoid paradoxes, it is impossible to construct -the type \citecoq{Lambda} in {\coq}: - -\begin{alltt} -Inductive Lambda : Set := - lambda : (Lambda {\arrow} False) {\arrow} Lambda. -\it -Error: Non strictly positive occurrence of "Lambda" in - "(Lambda {\arrow} False) {\arrow} Lambda" -\end{alltt} - -In order to explain this danger, we -will declare some constants for simulating the construction of -\texttt{Lambda} as an inductive type. - -Let us open some section, and declare two variables, the first one for -\texttt{Lambda}, the other for the constructor \texttt{lambda}. - -\begin{alltt} -Section Paradox. -Variable Lambda : Set. -Variable lambda : (Lambda {\arrow} False) {\arrow}Lambda. -\end{alltt} - -Since \texttt{Lambda} is not a truely inductive type, we can't use -the \texttt{match} construct. Nevertheless, we can simulate it by a -variable \texttt{matchL} such that the term -``~\citecoq{matchL $l$ $Q$ (fun $h$ : Lambda {\arrow} False {\funarrow} $t$)}~'' -should be understood as -``~\citecoq{match $l$ return $Q$ with | lambda h {\funarrow} $t$)}~'' - - -\begin{alltt} -Variable matchL : Lambda {\arrow} - {\prodsym} Q:Prop, ((Lambda {\arrow}False) {\arrow} Q) {\arrow} - Q. -\end{alltt} - ->From these constants, it is possible to define application by case -analysis. Then, through auto-application, the well-known looping term -$(\lambda x.(x\;x)\;\lambda x.(x\;x))$ provides a proof of falsehood. - -\begin{alltt} -Definition application (f x: Lambda) :False := - matchL f False (fun h {\funarrow} h x). - -Definition Delta : Lambda := - lambda (fun x : Lambda {\funarrow} application x x). - -Definition loop : False := application Delta Delta. - -Theorem two_is_three : 2 = 3. -Proof. - elim loop. -Qed. - -End Paradox. -\end{alltt} - -\noindent This example can be seen as a formulation of Russell's -paradox in type theory associating $(\textsl{application}\;x\;x)$ to the -formula $x\not\in x$, and \textsl{Delta} to the set $\{ x \mid -x\not\in x\}$. If \texttt{matchL} would satisfy the reduction rule -associated to case analysis, that is, -$$ \citecoq{matchL (lambda $f$) $Q$ $h$} \Longrightarrow h\;f$$ -then the term \texttt{loop} -would compute into itself. This is not actually surprising, since the -proof of the logical soundness of {\coq} strongly lays on the property -that any well-typed term must terminate. Hence, non-termination is -usually a synonymous of inconsistency. - -%\paragraph{} In this case, the construction of a non-terminating -%program comes from the so-called \textsl{negative occurrence} of -%$\Lambda$ in the type of the constructor $\lambda$. In order to be -%admissible for {\coq}, all the occurrences of the recursive type in its -%own introduction rules must be positive, in the sense on the following -%definition: -% -%\begin{enumerate} -%\item $R$ is positive in $(R\;\vec{t})$; -%\item $R$ is positive in $(x: A)C$ if it does not -%occur in $A$ and $R$ is positive in $C$; -%\item if $P\equiv (\vec{x}:\vec{T})Q$, then $R$ is positive in $(P -%\rightarrow C)$ if $R$ does not occur in $\vec{T}$, $R$ is positive -%in $C$, and either -%\begin{enumerate} -%\item $Q\equiv (R\;\vec{q})$ or -%\item $Q\equiv (J\;\vec{t})$, \label{relax} -% where $J$ is a recursive type, and for any term $t_i$ either : -% \begin{enumerate} -% \item $R$ does not occur in $t_i$, or -% \item $t_i\equiv (z:\vec{Z})(R\;\vec{q})$, $R$ does not occur -% in $\vec{Z}$, $t_i$ instantiates a general -% parameter of $J$, and this parameter is positive in the -% arguments of the constructors of $J$. -% \end{enumerate} -%\end{enumerate} -%\end{enumerate} -%\noindent Those types obtained by erasing option (\ref{relax}) in the -%definition above are called \textsl{strictly positive} types. - - -\subsubsection*{Remark} In this case, the construction of a non-terminating -program comes from the so-called \textsl{negative occurrence} of -\texttt{Lambda} in the argument of the constructor \texttt{lambda}. - -The reader will find in the Reference Manual a complete formal -definition of the notions of \emph{positivity condition} and -\emph{strict positivity} that an inductive definition must satisfy. - - -%In order to be -%admissible for {\coq}, the type $R$ must be positive in the types of the -%arguments of its own introduction rules, in the sense on the following -%definition: - -%\textbf{La définition du manuel de référence est plus complexe: -%la recopier ou donner seulement des exemples? -%} -%\begin{enumerate} -%\item $R$ is positive in $T$ if $R$ does not occur in $T$; -%\item $R$ is positive in $(R\;\vec{t})$ if $R$ does not occur in $\vec{t}$; -%\item $R$ is positive in $(x:A)C$ if it does not -% occur in $A$ and $R$ is positive in $C$; -%\item $R$ is positive in $(J\;\vec{t})$, \label{relax} -% if $J$ is a recursive type, and for any term $t_i$ either : -% \begin{enumerate} -% \item $R$ does not occur in $t_i$, or -% \item $R$ is positive in $t_i$, $t_i$ instantiates a general -% parameter of $J$, and this parameter is positive in the -% arguments of the constructors of $J$. -% \end{enumerate} -%\end{enumerate} - -%\noindent When we can show that $R$ is positive without using the item -%(\ref{relax}) of the definition above, then we say that $R$ is -%\textsl{strictly positive}. - -%\textbf{Changer le discours sur les ordinaux} - -Notice that the positivity condition does not forbid us to -put functional recursive -arguments in the constructors. - -For instance, let us consider the type of infinitely branching trees, -with labels in \texttt{Z}. -\begin{alltt} -Require Import ZArith. - -Inductive itree : Set := -| ileaf : itree -| inode : Z {\arrow} (nat {\arrow} itree) {\arrow} itree. -\end{alltt} - -In this representation, the $i$-th child of a tree -represented by ``~\texttt{inode $z$ $s$}~'' is obtained by applying -the function $s$ to $i$. -The following definitions show how to construct a tree with a single -node, a tree of height 1 and a tree of height 2: - -\begin{alltt} -Definition isingle l := inode l (fun i {\funarrow} ileaf). - -Definition t1 := inode 0 (fun n {\funarrow} isingle (Z_of_nat n)). - -Definition t2 := - inode 0 - (fun n : nat {\funarrow} - inode (Z_of_nat n) - (fun p {\funarrow} isingle (Z_of_nat (n*p)))). -\end{alltt} - - -Let us define a preorder on infinitely branching trees. - In order to compare two non-leaf trees, -it is necessary to compare each of their children - without taking care of the order in which they -appear: - -\begin{alltt} -Inductive itree_le : itree{\arrow} itree {\arrow} Prop := - | le_leaf : {\prodsym} t, itree_le ileaf t - | le_node : {\prodsym} l l' s s', - Zle l l' {\arrow} - ({\prodsym} i, {\exsym} j:nat, itree_le (s i) (s' j)){\arrow} - itree_le (inode l s) (inode l' s'). - -\end{alltt} - -Notice that a call to the predicate \texttt{itree\_le} appears as -a general parameter of the inductive type \texttt{ex} (see Sect.\ref{ex-def}). -This kind of definition is accepted by {\coq}, but may lead to some -difficulties, since the induction principle automatically -generated by the system -is not the most appropriate (see chapter 14 of~\cite{coqart} for a detailed -explanation). - - -The following definition, obtained by -skolemising the -proposition \linebreak $\forall\, i,\exists\, j,(\texttt{itree\_le}\;(s\;i)\;(s'\;j))$ in -the type of \texttt{itree\_le}, does not present this problem: - - -\begin{alltt} -Inductive itree_le' : itree{\arrow} itree {\arrow} Prop := - | le_leaf' : {\prodsym} t, itree_le' ileaf t - | le_node' : {\prodsym} l l' s s' g, - Zle l l' {\arrow} - ({\prodsym} i, itree_le' (s i) (s' (g i))) {\arrow} - itree_le' (inode l s) (inode l' s'). - -\end{alltt} -\iffalse -\begin{alltt} -Lemma t1_le'_t2 : itree_le' t1 t2. -Proof. - unfold t1, t2. - constructor 2 with (fun i : nat {\funarrow} 2 * i). - auto with zarith. - unfold isingle; - intro i ; constructor 2 with (fun i :nat {\funarrow} i). - auto with zarith. - constructor . -Qed. -\end{alltt} -\fi - -%In general, strictly positive definitions are preferable to only -%positive ones. The reason is that it is sometimes difficult to derive -%structural induction combinators for the latter ones. Such combinators -%are automatically generated for strictly positive types, but not for -%the only positive ones. Nevertheless, sometimes non-strictly positive -%definitions provide a smarter or shorter way of declaring a recursive -%type. - -Another example is the type of trees - of unbounded width, in which a recursive subterm -\texttt{(ltree A)} instantiates the type of polymorphic lists: - -\begin{alltt} -Require Import List. - -Inductive ltree (A:Set) : Set := - lnode : A {\arrow} list (ltree A) {\arrow} ltree A. -\end{alltt} - -This declaration can be transformed -adding an extra type to the definition, as was done in Section -\ref{MutuallyDependent}. - - -\subsubsection{Impredicative Inductive Types} - -An inductive type $I$ inhabiting a universe $U$ is \textsl{predicative} -if the introduction rules of $I$ do not make a universal -quantification on a universe containing $U$. All the recursive types -previously introduced are examples of predicative types. An example of -an impredicative one is the following type: -%\textsl{exT}, the dependent product -%of a certain set (or proposition) $x$, and a proof of a property $P$ -%about $x$. - -%\begin{alltt} -%Print exT. -%\end{alltt} -%\textbf{ttention, EXT c'est ex!} -%\begin{alltt} -%Check (exists P:Prop, P {\arrow} not P). -%\end{alltt} - -%This type is useful for expressing existential quantification over -%types, like ``there exists a proposition $x$ such that $(P\;x)$'' -%---written $(\textsl{EXT}\; x:Prop \mid (P\;x))$ in {\coq}. However, - -\begin{alltt} -Inductive prop : Prop := - prop_intro : Prop {\arrow} prop. -\end{alltt} - -Notice -that the constructor of this type can be used to inject any -proposition --even itself!-- into the type. - -\begin{alltt} -Check (prop_intro prop).\it -prop_intro prop - : prop -\end{alltt} - -A careless use of such a -self-contained objects may lead to a variant of Burali-Forti's -paradox. The construction of Burali-Forti's paradox is more -complicated than Russel's one, so we will not describe it here, and -point the interested reader to \cite{Bar98,Coq86}. - - -Another example is the second order existential quantifier for propositions: - -\begin{alltt} -Inductive ex_Prop (P : Prop {\arrow} Prop) : Prop := - exP_intro : {\prodsym} X : Prop, P X {\arrow} ex_Prop P. -\end{alltt} - -%\begin{alltt} -%(* -%Check (match prop_inject with (prop_intro p _) {\funarrow} p end). - -%Error: Incorrect elimination of "prop_inject" in the inductive type -% ex -%The elimination predicate ""fun _ : prop {\funarrow} Prop" has type -% "prop {\arrow} Type" -%It should be one of : -% "Prop" - -%Elimination of an inductive object of sort : "Prop" -%is not allowed on a predicate in sort : "Type" -%because non-informative objects may not construct informative ones. - -%*) -%Print prop_inject. - -%(* -%prop_inject = -%prop_inject = prop_intro prop (fun H : prop {\funarrow} H) -% : prop -%*) -%\end{alltt} - -% \textbf{Et par ça? -%} - -Notice that predicativity on sort \citecoq{Set} forbids us to build -the following definitions. - - -\begin{alltt} -Inductive aSet : Set := - aSet_intro: Set {\arrow} aSet. - -\it{}User error: Large non-propositional inductive types must be in Type -\tt -Inductive ex_Set (P : Set {\arrow} Prop) : Set := - exS_intro : {\prodsym} X : Set, P X {\arrow} ex_Set P. - -\it{}User error: Large non-propositional inductive types must be in Type -\end{alltt} - -Nevertheless, one can define types like \citecoq{aSet} and \citecoq{ex\_Set}, as inhabitants of \citecoq{Type}. - -\begin{alltt} -Inductive ex_Set (P : Set {\arrow} Prop) : Type := - exS_intro : {\prodsym} X : Set, P X {\arrow} ex_Set P. -\end{alltt} - -In the following example, the inductive type \texttt{typ} can be defined, -but the term associated with the interactive Definition of -\citecoq{typ\_inject} is incompatible with {\coq}'s hierarchy of universes: - - -\begin{alltt} -Inductive typ : Type := - typ_intro : Type {\arrow} typ. - -Definition typ_inject: typ. - split; exact typ. -\it Proof completed - -\tt{}Defined. -\it Error: Universe Inconsistency. -\tt -Abort. -\end{alltt} - -One possible way of avoiding this new source of paradoxes is to -restrict the kind of eliminations by case analysis that can be done on -impredicative types. In particular, projections on those universes -equal or bigger than the one inhabited by the impredicative type must -be forbidden \cite{Coq86}. A consequence of this restriction is that it -is not possible to define the first projection of the type -``~\citecoq{ex\_Prop $P$}~'': -\begin{alltt} -Check (fun (P:Prop{\arrow}Prop)(p: ex_Prop P) {\funarrow} - match p with exP_intro X HX {\funarrow} X end). -\it -Error: -Incorrect elimination of "p" in the inductive type -"ex_Prop", the return type has sort "Type" while it should be -"Prop" - -Elimination of an inductive object of sort "Prop" -is not allowed on a predicate in sort "Type" -because proofs can be eliminated only to build proofs. -\end{alltt} - -%In order to explain why, let us consider for example the following -%impredicative type \texttt{ALambda}. -%\begin{alltt} -%Inductive ALambda : Set := -% alambda : (A:Set)(A\arrow{}False)\arrow{}ALambda. -% -%Definition Lambda : Set := ALambda. -%Definition lambda : (ALambda\arrow{}False)\arrow{}ALambda := (alambda ALambda). -%Lemma CaseAL : (Q:Prop)ALambda\arrow{}((ALambda\arrow{}False)\arrow{}Q)\arrow{}Q. -%\end{alltt} -% -%This type contains all the elements of the dangerous type $\Lambda$ -%described at the beginning of this section. Try to construct the -%non-ending term $(\Delta\;\Delta)$ as an object of -%\texttt{ALambda}. Why is it not possible? - -\subsubsection{Extraction Constraints} - -There is a final constraint on case analysis that is not motivated by -the potential introduction of paradoxes, but for compatibility reasons -with {\coq}'s extraction mechanism \refmancite{Appendix -\ref{CamlHaskellExtraction}}. This mechanism is based on the -classification of basic types into the universe $\Set$ of sets and the -universe $\Prop$ of propositions. The objects of a type in the -universe $\Set$ are considered as relevant for computation -purposes. The objects of a type in $\Prop$ are considered just as -formalised comments, not necessary for execution. The extraction -mechanism consists in erasing such formal comments in order to obtain -an executable program. Hence, in general, it is not possible to define -an object in a set (that should be kept by the extraction mechanism) -by case analysis of a proof (which will be thrown away). - -Nevertheless, this general rule has an exception which is important in -practice: if the definition proceeds by case analysis on a proof of a -\textsl{singleton proposition} or an empty type (\emph{e.g.} \texttt{False}), - then it is allowed. A singleton -proposition is a non-recursive proposition with a single constructor -$c$, all whose arguments are proofs. For example, the propositional -equality and the conjunction of two propositions are examples of -singleton propositions. - -%From the point of view of the extraction -%mechanism, such types are isomorphic to a type containing a single -%object $c$, so a definition $\Case{x}{c \Rightarrow b}$ is -%directly replaced by $b$ as an extra optimisation. - -\subsubsection{Strong Case Analysis on Proofs} - -One could consider allowing - to define a proposition $Q$ by case -analysis on the proofs of another recursive proposition $R$. As we -will see in Section \ref{Discrimination}, this would enable one to prove that -different introduction rules of $R$ construct different -objects. However, this property would be in contradiction with the principle -of excluded middle of classical logic, because this principle entails -that the proofs of a proposition cannot be distinguished. This -principle is not provable in {\coq}, but it is frequently introduced by -the users as an axiom, for reasoning in classical logic. For this -reason, the definition of propositions by case analysis on proofs is - not allowed in {\coq}. - -\begin{alltt} - -Definition comes_from_the_left (P Q:Prop)(H:P{\coqor}Q): Prop := - match H with - | or_introl p {\funarrow} True - | or_intror q {\funarrow} False - end. -\it -Error: -Incorrect elimination of "H" in the inductive type -"or", the return type has sort "Type" while it should be -"Prop" - -Elimination of an inductive object of sort "Prop" -is not allowed on a predicate in sort "Type" -because proofs can be eliminated only to build proofs. - -\end{alltt} - -On the other hand, if we replace the proposition $P {\coqor} Q$ with -the informative type $\{P\}+\{Q\}$, the elimination is accepted: - -\begin{alltt} -Definition comes_from_the_left_sumbool - (P Q:Prop)(x:\{P\} + \{Q\}): Prop := - match x with - | left p {\funarrow} True - | right q {\funarrow} False - end. -\end{alltt} - - -\subsubsection{Summary of Constraints} - -To end with this section, the following table summarizes which -universe $U_1$ may inhabit an object of type $Q$ defined by case -analysis on $x:R$, depending on the universe $U_2$ inhabited by the -inductive types $R$.\footnote{In the box indexed by $U_1=\citecoq{Type}$ -and $U_2=\citecoq{Set}$, the answer ``yes'' takes into account the -predicativity of sort \citecoq{Set}. If you are working with the -option ``impredicative-set'', you must put in this box the -condition ``if $R$ is predicative''.} - - -\begin{center} -\renewcommand{\multirowsetup}{\centering} \newlength{\LL} -\settowidth{\LL}{$x : R : s_1$} -\begin{tabular}{|c|c|c|c|c|} -\hline -\multirow{5}{\LL}{$x : R : U_2$} & -\multicolumn{4}{|c|}{$Q : U_1$}\\ -\hline -& &\textsl{Set} & \textsl{Prop} & \textsl{Type}\\ -\cline{2-5} -&\textsl{Set} & yes & yes & yes\\ -\cline{2-5} -&\textsl{Prop} & if $R$ singleton & yes & no\\ -\cline{2-5} -&\textsl{Type} & yes & yes & yes\\ -\hline -\end{tabular} -\end{center} - -\section{Some Proof Techniques Based on Case Analysis} -\label{CaseTechniques} - -In this section we illustrate the use of case analysis as a proof -principle, explaining the proof techniques behind three very useful -{\coq} tactics, called \texttt{discriminate}, \texttt{injection} and -\texttt{inversion}. - -\subsection{Discrimination of introduction rules} -\label{Discrimination} - -In the informal semantics of recursive types described in Section -\ref{Introduction} it was said that each of the introduction rules of a -recursive type is considered as being different from all the others. -It is possible to capture this fact inside the logical system using -the propositional equality. We take as example the following theorem, -stating that \textsl{O} constructs a natural number different -from any of those constructed with \texttt{S}. - -\begin{alltt} -Theorem S_is_not_O : {\prodsym} n, S n {\coqdiff} 0. -\end{alltt} - -In order to prove this theorem, we first define a proposition by case -analysis on natural numbers, so that the proposition is true for {\Z} -and false for any natural number constructed with {\SUCC}. This uses -the empty and singleton type introduced in Sections \ref{Introduction}. - -\begin{alltt} -Definition Is_zero (x:nat):= match x with - | 0 {\funarrow} True - | _ {\funarrow} False - end. -\end{alltt} - -\noindent Then, we prove the following lemma: - -\begin{alltt} -Lemma O_is_zero : {\prodsym} m, m = 0 {\arrow} Is_zero m. -Proof. - intros m H; subst m. -\it{} -================ - Is_zero 0 -\tt{} -simpl;trivial. -Qed. -\end{alltt} - -\noindent Finally, the proof of \texttt{S\_is\_not\_O} follows by the -application of the previous lemma to $S\;n$. - - -\begin{alltt} - - red; intros n Hn. - \it{} - n : nat - Hn : S n = 0 - ============================ - False \tt - - apply O_is_zero with (m := S n). - assumption. -Qed. -\end{alltt} - - -The tactic \texttt{discriminate} \refmancite{Section \ref{Discriminate}} is -a special-purpose tactic for proving disequalities between two -elements of a recursive type introduced by different constructors. It -generalizes the proof method described here for natural numbers to any -[co]-inductive type. This tactic is also capable of proving disequalities -where the difference is not in the constructors at the head of the -terms, but deeper inside them. For example, it can be used to prove -the following theorem: - -\begin{alltt} -Theorem disc2 : {\prodsym} n, S (S n) {\coqdiff} 1. -Proof. - intros n Hn; discriminate. -Qed. -\end{alltt} - -When there is an assumption $H$ in the context stating a false -equality $t_1=t_2$, \texttt{discriminate} solves the goal by first -proving $(t_1\not =t_2)$ and then reasoning by absurdity with respect -to $H$: - -\begin{alltt} -Theorem disc3 : {\prodsym} n, S (S n) = 0 {\arrow} {\prodsym} Q:Prop, Q. -Proof. - intros n Hn Q. - discriminate. -Qed. -\end{alltt} - -\noindent In this case, the proof proceeds by absurdity with respect -to the false equality assumed, whose negation is proved by -discrimination. - -\subsection{Injectiveness of introduction rules} - -Another useful property about recursive types is the -\textsl{injectiveness} of introduction rules, i.e., that whenever two -objects were built using the same introduction rule, then this rule -should have been applied to the same element. This can be stated -formally using the propositional equality: - -\begin{alltt} -Theorem inj : {\prodsym} n m, S n = S m {\arrow} n = m. -Proof. -\end{alltt} - -\noindent This theorem is just a corollary of a lemma about the -predecessor function: - -\begin{alltt} - Lemma inj_pred : {\prodsym} n m, n = m {\arrow} pred n = pred m. - Proof. - intros n m eq_n_m. - rewrite eq_n_m. - trivial. - Qed. -\end{alltt} -\noindent Once this lemma is proven, the theorem follows directly -from it: -\begin{alltt} - intros n m eq_Sn_Sm. - apply inj_pred with (n:= S n) (m := S m); assumption. -Qed. -\end{alltt} - -This proof method is implemented by the tactic \texttt{injection} -\refmancite{Section \ref{injection}}. This tactic is applied to -a term $t$ of type ``~$c\;{t_1}\;\dots\;t_n = c\;t'_1\;\dots\;t'_n$~'', where $c$ is some constructor of -an inductive type. The tactic \texttt{injection} is applied as deep as -possible to derive the equality of all pairs of subterms of $t_i$ and $t'_i$ -placed in the same position. All these equalities are put as antecedents -of the current goal. - - - -Like \texttt{discriminate}, the tactic \citecoq{injection} -can be also applied if $x$ does not -occur in a direct sub-term, but somewhere deeper inside it. Its -application may leave some trivial goals that can be easily solved -using the tactic \texttt{trivial}. - -\begin{alltt} - - Lemma list_inject : {\prodsym} (A:Type)(a b :A)(l l':list A), - a :: b :: l = b :: a :: l' {\arrow} a = b {\coqand} l = l'. -Proof. - intros A a b l l' e. - - -\it - e : a :: b :: l = b :: a :: l' - ============================ - a = b {\coqand} l = l' -\tt - injection e. -\it - ============================ - l = l' {\arrow} b = a {\arrow} a = b {\arrow} a = b {\coqand} l = l' - -\tt{} auto. -Qed. -\end{alltt} - -\subsection{Inversion Techniques}\label{inversion} - -In section \ref{DependentCase}, we motivated the rule of dependent case -analysis as a way of internalizing the informal equalities $n=O$ and -$n=\SUCC\;p$ associated to each case. This internalisation -consisted in instantiating $n$ with the corresponding term in the type -of each branch. However, sometimes it could be better to internalise -these equalities as extra hypotheses --for example, in order to use -the tactics \texttt{rewrite}, \texttt{discriminate} or -\texttt{injection} presented in the previous sections. This is -frequently the case when the element analysed is denoted by a term -which is not a variable, or when it is an object of a particular -instance of a recursive family of types. Consider for example the -following theorem: - -\begin{alltt} -Theorem not_le_Sn_0 : {\prodsym} n:nat, ~ (S n {\coqle} 0). -\end{alltt} - -\noindent Intuitively, this theorem should follow by case analysis on -the hypothesis $H:(S\;n\;\leq\;\Z)$, because no introduction rule allows -to instantiate the arguments of \citecoq{le} with respectively a successor -and zero. However, there -is no way of capturing this with the typing rule for case analysis -presented in section \ref{Introduction}, because it does not take into -account what particular instance of the family the type of $H$ is. -Let us try it: -\begin{alltt} -Proof. - red; intros n H; case H. -\it 2 subgoals - - n : nat - H : S n {\coqle} 0 - ============================ - False - -subgoal 2 is: - {\prodsym} m : nat, S n {\coqle} m {\arrow} False -\tt -Undo. -\end{alltt} - -\noindent What is necessary here is to make available the equalities -``~$\SUCC\;n = \Z$~'' and ``~$\SUCC\;m = \Z$~'' - as extra hypotheses of the -branches, so that the goal can be solved using the -\texttt{Discriminate} tactic. In order to obtain the desired -equalities as hypotheses, let us prove an auxiliary lemma, that our -theorem is a corollary of: - -\begin{alltt} - Lemma not_le_Sn_0_with_constraints : - {\prodsym} n p , S n {\coqle} p {\arrow} p = 0 {\arrow} False. - Proof. - intros n p H; case H . -\it -2 subgoals - - n : nat - p : nat - H : S n {\coqle} p - ============================ - S n = 0 {\arrow} False - -subgoal 2 is: - {\prodsym} m : nat, S n {\coqle} m {\arrow} S m = 0 {\arrow} False -\tt - intros;discriminate. - intros;discriminate. -Qed. -\end{alltt} -\noindent Our main theorem can now be solved by an application of this lemma: -\begin{alltt} -Show. -\it -2 subgoals - - n : nat - p : nat - H : S n {\coqle} p - ============================ - S n = 0 {\arrow} False - -subgoal 2 is: - {\prodsym} m : nat, S n {\coqle} m {\arrow} S m = 0 {\arrow} False -\tt - eapply not_le_Sn_0_with_constraints; eauto. -Qed. -\end{alltt} - - -The general method to address such situations consists in changing the -goal to be proven into an implication, introducing as preconditions -the equalities needed to eliminate the cases that make no -sense. This proof technique is implemented by the tactic -\texttt{inversion} \refmancite{Section \ref{Inversion}}. In order -to prove a goal $G\;\vec{q}$ from an object of type $R\;\vec{t}$, -this tactic automatically generates a lemma $\forall, \vec{x}. -(R\;\vec{x}) \rightarrow \vec{x}=\vec{t}\rightarrow \vec{B}\rightarrow -(G\;\vec{q})$, where the list of propositions $\vec{B}$ correspond to -the subgoals that cannot be directly proven using -\texttt{discriminate}. This lemma can either be saved for later -use, or generated interactively. In this latter case, the subgoals -yielded by the tactic are the hypotheses $\vec{B}$ of the lemma. If the -lemma has been stored, then the tactic \linebreak - ``~\citecoq{inversion \dots using \dots}~'' can be -used to apply it. - -Let us show both techniques on our previous example: - -\subsubsection{Interactive mode} - -\begin{alltt} -Theorem not_le_Sn_0' : {\prodsym} n:nat, ~ (S n {\coqle} 0). -Proof. - red; intros n H ; inversion H. -Qed. -\end{alltt} - - -\subsubsection{Static mode} - -\begin{alltt} - -Derive Inversion le_Sn_0_inv with ({\prodsym} n :nat, S n {\coqle} 0). -Theorem le_Sn_0'' : {\prodsym} n p : nat, ~ S n {\coqle} 0 . -Proof. - intros n p H; - inversion H using le_Sn_0_inv. -Qed. -\end{alltt} - - -In the example above, all the cases are solved using discriminate, so -there remains no subgoal to be proven (i.e. the list $\vec{B}$ is -empty). Let us present a second example, where this list is not empty: - - -\begin{alltt} -TTheorem le_reverse_rules : - {\prodsym} n m:nat, n {\coqle} m {\arrow} - n = m {\coqor} - {\exsym} p, n {\coqle} p {\coqand} m = S p. -Proof. - intros n m H; inversion H. -\it -2 subgoals - - - - - n : nat - m : nat - H : n {\coqle} m - H0 : n = m - ============================ - m = m {\coqor} ({\exsym} p : nat, m {\coqle} p {\coqand} m = S p) - -subgoal 2 is: - n = S m0 {\coqor} ({\exsym} p : nat, n {\coqle} p {\coqand} S m0 = S p) -\tt - left;trivial. - right; exists m0; split; trivial. -\it -Proof completed -\end{alltt} - -This example shows how this tactic can be used to ``reverse'' the -introduction rules of a recursive type, deriving the possible premises -that could lead to prove a given instance of the predicate. This is -why these tactics are called \texttt{inversion} tactics: they go back -from conclusions to premises. - -The hypotheses corresponding to the propositional equalities are not -needed in this example, since the tactic does the necessary rewriting -to solve the subgoals. When the equalities are no longer needed after -the inversion, it is better to use the tactic -\texttt{Inversion\_clear}. This variant of the tactic clears from the -context all the equalities introduced. - -\begin{alltt} -Restart. - intros n m H; inversion_clear H. -\it -\it - - n : nat - m : nat - ============================ - m = m {\coqor} ({\exsym} p : nat, m {\coqle} p {\coqand} m = S p) -\tt - left;trivial. -\it - n : nat - m : nat - m0 : nat - H0 : n {\coqle} m0 - ============================ - n = S m0 {\coqor} ({\exsym} p : nat, n {\coqle} p {\coqand} S m0 = S p) -\tt - right; exists m0; split; trivial. -Qed. -\end{alltt} - - -%This proof technique works in most of the cases, but not always. In -%particular, it could not if the list $\vec{t}$ contains a term $t_j$ -%whose type $T$ depends on a previous term $t_i$, with $i<j$. Remark -%that if this is the case, the propositional equality $x_j=t_j$ is not -%well-typed, since $x_j:T(x_i)$ but $t_j:T(t_i)$, and both types are -%not convertible (otherwise, the problem could be solved using the -%tactic \texttt{Case}). - - - -\begin{exercise} -Consider the following language of arithmetic expression, and -its operational semantics, described by a set of rewriting rules. -%\textbf{J'ai enlevé une règle de commutativité de l'addition qui -%me paraissait bizarre du point de vue de la sémantique opérationnelle} - -\begin{alltt} -Inductive ArithExp : Set := - | Zero : ArithExp - | Succ : ArithExp {\arrow} ArithExp - | Plus : ArithExp {\arrow} ArithExp {\arrow} ArithExp. - -Inductive RewriteRel : ArithExp {\arrow} ArithExp {\arrow} Prop := - | RewSucc : {\prodsym} e1 e2 :ArithExp, - RewriteRel e1 e2 {\arrow} - RewriteRel (Succ e1) (Succ e2) - | RewPlus0 : {\prodsym} e:ArithExp, - RewriteRel (Plus Zero e) e - | RewPlusS : {\prodsym} e1 e2:ArithExp, - RewriteRel e1 e2 {\arrow} - RewriteRel (Plus (Succ e1) e2) - (Succ (Plus e1 e2)). - -\end{alltt} -\begin{enumerate} -\item Prove that \texttt{Zero} cannot be rewritten any further. -\item Prove that an expression of the form ``~$\texttt{Succ}\;e$~'' is always -rewritten -into an expression of the same form. -\end{enumerate} -\end{exercise} - -%Theorem zeroNotCompute : (e:ArithExp)~(RewriteRel Zero e). -%Intro e. -%Red. -%Intro H. -%Inversion_clear H. -%Defined. -%Theorem evalPlus : -% (e1,e2:ArithExp) -% (RewriteRel (Succ e1) e2)\arrow{}(EX e3 : ArithExp | e2=(Succ e3)). -%Intros e1 e2 H. -%Inversion_clear H. -%Exists e3;Reflexivity. -%Qed. - - -\section{Inductive Types and Structural Induction} -\label{StructuralInduction} - -Elements of inductive types are well-founded with -respect to the structural order induced by the constructors of the -type. In addition to case analysis, this extra hypothesis about -well-foundedness justifies a stronger elimination rule for them, called -\textsl{structural induction}. This form of elimination consists in -defining a value ``~$f\;x$~'' from some element $x$ of the inductive type -$I$, assuming that values have been already associated in the same way -to the sub-parts of $x$ of type $I$. - - -Definitions by structural induction are expressed through the -\texttt{Fixpoint} command \refmancite{Section -\ref{Fixpoint}}. This command is quite close to the -\texttt{let-rec} construction of functional programming languages. -For example, the following definition introduces the addition of two -natural numbers (already defined in the Standard Library:) - -\begin{alltt} -Fixpoint plus (n p:nat) \{struct n\} : nat := - match n with - | 0 {\funarrow} p - | S m {\funarrow} S (plus m p) - end. -\end{alltt} - -The definition is by structural induction on the first argument of the -function. This is indicated by the ``~\citecoq{\{struct n\}}~'' -directive in the function's header\footnote{This directive is optional -in the case of a function of a single argument}. - In -order to be accepted, the definition must satisfy a syntactical -condition, called the \textsl{guardedness condition}. Roughly -speaking, this condition constrains the arguments of a recursive call -to be pattern variables, issued from a case analysis of the formal -argument of the function pointed by the \texttt{struct} directive. - In the case of the -function \texttt{plus}, the argument \texttt{m} in the recursive call is a -pattern variable issued from a case analysis of \texttt{n}. Therefore, the -definition is accepted. - -Notice that we could have defined the addition with structural induction -on its second argument: -\begin{alltt} -Fixpoint plus' (n p:nat) \{struct p\} : nat := - match p with - | 0 {\funarrow} n - | S q {\funarrow} S (plus' n q) - end. -\end{alltt} - -%This notation is useful when defining a function whose decreasing -%argument has a dependent type. As an example, consider the following -%recursivly defined proof of the theorem -%$(n,m:\texttt{nat})n<m \rightarrow (S\;n)<(S\;m)$: -%\begin{alltt} -%Fixpoint lt_n_S [n,m:nat;p:(lt n m)] : (lt (S n) (S m)) := -% <[n0:nat](lt (S n) (S n0))> -% Cases p of -% lt_intro1 {\funarrow} (lt_intro1 (S n)) -% | (lt_intro2 m1 p2) {\funarrow} (lt_intro2 (S n) (S m1) (lt_n_S n m1 p2)) -% end. -%\end{alltt} - -%The guardedness condition must be satisfied only by the last argument -%of the enclosed list. For example, the following declaration is an -%alternative way of defining addition: - -%\begin{alltt} -%Reset add. -%Fixpoint add [n:nat] : nat\arrow{}nat := -% Cases n of -% O {\funarrow} [x:nat]x -% | (S m) {\funarrow} [x:nat](add m (S x)) -% end. -%\end{alltt} - -In the following definition of addition, -the second argument of \verb@plus''@ grows at each -recursive call. However, as the first one always decreases, the -definition is sound. -\begin{alltt} -Fixpoint plus'' (n p:nat) \{struct n\} : nat := - match n with - | 0 {\funarrow} p - | S m {\funarrow} plus'' m (S p) - end. -\end{alltt} - - Moreover, the argument in the recursive call -could be a deeper component of $n$. This is the case in the following -definition of a boolean function determining whether a number is even -or odd: - -\begin{alltt} -Fixpoint even_test (n:nat) : bool := - match n - with 0 {\funarrow} true - | 1 {\funarrow} false - | S (S p) {\funarrow} even_test p - end. -\end{alltt} - -Mutually dependent definitions by structural induction are also -allowed. For example, the previous function \textsl{even} could alternatively -be defined using an auxiliary function \textsl{odd}: - -\begin{alltt} -Reset even_test. - - - -Fixpoint even_test (n:nat) : bool := - match n - with - | 0 {\funarrow} true - | S p {\funarrow} odd_test p - end -with odd_test (n:nat) : bool := - match n - with - | 0 {\funarrow} false - | S p {\funarrow} even_test p - end. -\end{alltt} - -%\begin{exercise} -%Define a function by structural induction that computes the number of -%nodes of a tree structure defined in page \pageref{Forest}. -%\end{exercise} - -Definitions by structural induction are computed - only when they are applied, and the decreasing argument -is a term having a constructor at the head. We can check this using -the \texttt{Eval} command, which computes the normal form of a well -typed term. - -\begin{alltt} -Eval simpl in even_test. -\it - = even_test - : nat {\arrow} bool -\tt -Eval simpl in (fun x : nat {\funarrow} even x). -\it - = fun x : nat {\funarrow} even x - : nat {\arrow} Prop -\tt -Eval simpl in (fun x : nat => plus 5 x). -\it - = fun x : nat {\funarrow} S (S (S (S (S x)))) - -\tt -Eval simpl in (fun x : nat {\funarrow} even_test (plus 5 x)). -\it - = fun x : nat {\funarrow} odd_test x - : nat {\arrow} bool -\tt -Eval simpl in (fun x : nat {\funarrow} even_test (plus x 5)). -\it - = fun x : nat {\funarrow} even_test (x + 5) - : nat {\arrow} bool -\end{alltt} - - -%\begin{exercise} -%Prove that the second definition of even satisfies the following -%theorem: -%\begin{verbatim} -%Theorem unfold_even : -% (x:nat) -% (even x)= (Cases x of -% O {\funarrow} true -% | (S O) {\funarrow} false -% | (S (S m)) {\funarrow} (even m) -% end). -%\end{verbatim} -%\end{exercise} - -\subsection{Proofs by Structural Induction} - -The principle of structural induction can be also used in order to -define proofs, that is, to prove theorems. Let us call an -\textsl{elimination combinator} any function that, given a predicate -$P$, defines a proof of ``~$P\;x$~'' by structural induction on $x$. In -{\coq}, the principle of proof by induction on natural numbers is a -particular case of an elimination combinator. The definition of this -combinator depends on three general parameters: the predicate to be -proven, the base case, and the inductive step: - -\begin{alltt} -Section Principle_of_Induction. -Variable P : nat {\arrow} Prop. -Hypothesis base_case : P 0. -Hypothesis inductive_step : {\prodsym} n:nat, P n {\arrow} P (S n). -Fixpoint nat_ind (n:nat) : (P n) := - match n return P n with - | 0 {\funarrow} base_case - | S m {\funarrow} inductive_step m (nat_ind m) - end. - -End Principle_of_Induction. -\end{alltt} - -As this proof principle is used very often, {\coq} automatically generates it -when an inductive type is introduced. Similar principles -\texttt{nat\_rec} and \texttt{nat\_rect} for defining objects in the -universes $\Set$ and $\Type$ are also automatically generated -\footnote{In fact, whenever possible, {\coq} generates the -principle \texttt{$I$\_rect}, then derives from it the -weaker principles \texttt{$I$\_ind} and \texttt{$I$\_rec}. -If some principle has to be defined by hand, the user may try -to build \texttt{$I$\_rect} (if possible). Thanks to {\coq}'s conversion -rule, this principle can be used directly to build proofs and/or -programs.}. The -command \texttt{Scheme} \refmancite{Section \ref{Scheme}} can be -used to generate an elimination combinator from certain parameters, -like the universe that the defined objects must inhabit, whether the -case analysis in the definitions must be dependent or not, etc. For -example, it can be used to generate an elimination combinator for -reasoning on even natural numbers from the mutually dependent -predicates introduced in page \pageref{Even}. We do not display the -combinators here by lack of space, but you can see them using the -\texttt{Print} command. - -\begin{alltt} -Scheme Even_induction := Minimality for even Sort Prop -with Odd_induction := Minimality for odd Sort Prop. -\end{alltt} - -\begin{alltt} -Theorem even_plus_four : {\prodsym} n:nat, even n {\arrow} even (4+n). -Proof. - intros n H. - elim H using Even_induction with (P0 := fun n {\funarrow} odd (4+n)); - simpl;repeat constructor;assumption. -Qed. -\end{alltt} - -Another example of an elimination combinator is the principle -of double induction on natural numbers, introduced by the following -definition: - -\begin{alltt} -Section Principle_of_Double_Induction. -Variable P : nat {\arrow} nat {\arrow}Prop. -Hypothesis base_case1 : {\prodsym} m:nat, P 0 m. -Hypothesis base_case2 : {\prodsym} n:nat, P (S n) 0. -Hypothesis inductive_step : {\prodsym} n m:nat, P n m {\arrow} - \,\, P (S n) (S m). - -Fixpoint nat_double_ind (n m:nat)\{struct n\} : P n m := - match n, m return P n m with - | 0 , x {\funarrow} base_case1 x - | (S x), 0 {\funarrow} base_case2 x - | (S x), (S y) {\funarrow} inductive_step x y (nat_double_ind x y) - end. -End Principle_of_Double_Induction. -\end{alltt} - -Changing the type of $P$ into $\nat\rightarrow\nat\rightarrow\Type$, -another combinator \texttt{nat\_double\_rect} for constructing -(certified) programs can be defined in exactly the same way. -This definition is left as an exercise.\label{natdoublerect} - -\iffalse -\begin{alltt} -Section Principle_of_Double_Recursion. -Variable P : nat {\arrow} nat {\arrow} Type. -Hypothesis base_case1 : {\prodsym} x:nat, P 0 x. -Hypothesis base_case2 : {\prodsym} x:nat, P (S x) 0. -Hypothesis inductive_step : {\prodsym} n m:nat, P n m {\arrow} P (S n) (S m). -Fixpoint nat_double_rect (n m:nat)\{struct n\} : P n m := - match n, m return P n m with - 0 , x {\funarrow} base_case1 x - | (S x), 0 {\funarrow} base_case2 x - | (S x), (S y) {\funarrow} inductive_step x y (nat_double_rect x y) - end. -End Principle_of_Double_Recursion. -\end{alltt} -\fi -For instance the function computing the minimum of two natural -numbers can be defined in the following way: - -\begin{alltt} -Definition min : nat {\arrow} nat {\arrow} nat := - nat_double_rect (fun (x y:nat) {\funarrow} nat) - (fun (x:nat) {\funarrow} 0) - (fun (y:nat) {\funarrow} 0) - (fun (x y r:nat) {\funarrow} S r). -Eval compute in (min 5 8). -\it -= 5 : nat -\end{alltt} - - -%\begin{exercise} -% -%Define the combinator \texttt{nat\_double\_rec}, and apply it -%to give another definition of \citecoq{le\_lt\_dec} (using the theorems -%of the \texttt{Arith} library). -%\end{exercise} - -\subsection{Using Elimination Combinators.} -The tactic \texttt{apply} can be used to apply one of these proof -principles during the development of a proof. - -\begin{alltt} -Lemma not_circular : {\prodsym} n:nat, n {\coqdiff} S n. -Proof. - intro n. - apply nat_ind with (P:= fun n {\funarrow} n {\coqdiff} S n). -\it - - - -2 subgoals - - n : nat - ============================ - 0 {\coqdiff} 1 - - -subgoal 2 is: - {\prodsym} n0 : nat, n0 {\coqdiff} S n0 {\arrow} S n0 {\coqdiff} S (S n0) - -\tt - discriminate. - red; intros n0 Hn0 eqn0Sn0;injection eqn0Sn0;trivial. -Qed. -\end{alltt} - -The tactic \texttt{elim} \refmancite{Section \ref{Elim}} is a -refinement of \texttt{apply}, specially designed for the application -of elimination combinators. If $t$ is an object of an inductive type -$I$, then ``~\citecoq{elim $t$}~'' tries to find an abstraction $P$ of the -current goal $G$ such that $(P\;t)\equiv G$. Then it solves the goal -applying ``~$I\texttt{\_ind}\;P$~'', where $I$\texttt{\_ind} is the -combinator associated to $I$. The different cases of the induction -then appear as subgoals that remain to be solved. -In the previous proof, the tactic call ``~\citecoq{apply nat\_ind with (P:= fun n {\funarrow} n {\coqdiff} S n)}~'' can simply be replaced with ``~\citecoq{elim n}~''. - -The option ``~\citecoq{\texttt{elim} $t$ \texttt{using} $C$}~'' - allows to use a -derived combinator $C$ instead of the default one. Consider the -following theorem, stating that equality is decidable on natural -numbers: - -\label{iseqpage} -\begin{alltt} -Lemma eq_nat_dec : {\prodsym} n p:nat, \{n=p\}+\{n {\coqdiff} p\}. -Proof. - intros n p. -\end{alltt} - -Let us prove this theorem using the combinator \texttt{nat\_double\_rect} -of section~\ref{natdoublerect}. The example also illustrates how -\texttt{elim} may sometimes fail in finding a suitable abstraction $P$ -of the goal. Note that if ``~\texttt{elim n}~'' - is used directly on the -goal, the result is not the expected one. - -\vspace{12pt} - -%\pagebreak -\begin{alltt} - elim n using nat_double_rect. -\it -4 subgoals - - n : nat - p : nat - ============================ - {\prodsym} x : nat, \{x = p\} + \{x {\coqdiff} p\} - -subgoal 2 is: - nat {\arrow} \{0 = p\} + \{0 {\coqdiff} p\} - -subgoal 3 is: - nat {\arrow} {\prodsym} m : nat, \{m = p\} + \{m {\coqdiff} p\} {\arrow} \{S m = p\} + \{S m {\coqdiff} p\} - -subgoal 4 is: - nat -\end{alltt} - -The four sub-goals obtained do not correspond to the premises that -would be expected for the principle \texttt{nat\_double\_rec}. The -problem comes from the fact that -this principle for eliminating $n$ -has a universally quantified formula as conclusion, which confuses -\texttt{elim} about the right way of abstracting the goal. - -%In effect, let us consider the type of the goal before the call to -%\citecoq{elim}: ``~\citecoq{\{n = p\} + \{n {\coqdiff} p\}}~''. - -%Among all the abstractions that can be built by ``~\citecoq{elim n}~'' -%let us consider this one -%$P=$\citecoq{fun n :nat {\funarrow} fun q : nat {\funarrow} {\{q= p\} + \{q {\coqdiff} p\}}}. -%It is easy to verify that -%$P$ has type \citecoq{nat {\arrow} nat {\arrow} Set}, and that, if some -%$q:\citecoq{nat}$ is given, then $P\;q\;$ matches the current goal. -%Then applying \citecoq{nat\_double\_rec} with $P$ generates -%four goals, corresponding to - - - - -Therefore, -in this case the abstraction must be explicited using the tactic -\texttt{pattern}. Once the right abstraction is provided, the rest of -the proof is immediate: - -\begin{alltt} -Undo. - pattern p,n. -\it - n : nat - p : nat - ============================ - (fun n0 n1 : nat {\funarrow} \{n1 = n0\} + \{n1 {\coqdiff} n0\}) p n -\tt - elim n using nat_double_rec. -\it -3 subgoals - - n : nat - p : nat - ============================ - {\prodsym} x : nat, \{x = 0\} + \{x {\coqdiff} 0\} - -subgoal 2 is: - {\prodsym} x : nat, \{0 = S x\} + \{0 {\coqdiff} S x\} -subgoal 3 is: - {\prodsym} n0 m : nat, \{m = n0\} + \{m {\coqdiff} n0\} {\arrow} \{S m = S n0\} + \{S m {\coqdiff} S n0\} - -\tt - destruct x; auto. - destruct x; auto. - intros n0 m H; case H. - intro eq; rewrite eq ; auto. - intro neg; right; red ; injection 1; auto. -Defined. -\end{alltt} - - -Notice that the tactic ``~\texttt{decide equality}~'' -\refmancite{Section\ref{DecideEquality}} generalises the proof -above to a large class of inductive types. It can be used for proving -a proposition of the form -$\forall\,(x,y:R),\{x=y\}+\{x{\coqdiff}y\}$, where $R$ is an inductive datatype -all whose constructors take informative arguments ---like for example -the type {\nat}: - -\begin{alltt} -Definition eq_nat_dec' : {\prodsym} n p:nat, \{n=p\} + \{n{\coqdiff}p\}. - decide equality. -Defined. -\end{alltt} - -\begin{exercise} -\begin{enumerate} -\item Define a recursive function \emph{nat2itree} -mapping any natural number $n$ into an infinitely branching -tree of height $n$. -\item Provide an elimination combinator for these trees. -\item Prove that the relation \citecoq{itree\_le} is a preorder -(i.e. reflexive and transitive). -\end{enumerate} -\end{exercise} - -\begin{exercise} \label{zeroton} -Define the type of lists, and a predicate ``being an ordered list'' -using an inductive family. Then, define the function -$(from\;n)=0::1\;\ldots\; n::\texttt{nil}$ and prove that it always generates an -ordered list. -\end{exercise} - -\begin{exercise} -Prove that \citecoq{le' n p} and \citecoq{n $\leq$ p} are logically equivalent -for all n and p. (\citecoq{le'} is defined in section \ref{parameterstuff}). -\end{exercise} - - -\subsection{Well-founded Recursion} -\label{WellFoundedRecursion} - -Structural induction is a strong elimination rule for inductive types. -This method can be used to define any function whose termination is -based on the well-foundedness of certain order relation $R$ decreasing -at each recursive call. What makes this principle so strong is the -possibility of reasoning by structural induction on the proof that -certain $R$ is well-founded. In order to illustrate this we have -first to introduce the predicate of accessibility. - -\begin{alltt} -Print Acc. -\it -Inductive Acc (A : Type) (R : A {\arrow} A {\arrow} Prop) (x:A) : Prop := - Acc_intro : ({\prodsym} y : A, R y x {\arrow} Acc R y) {\arrow} Acc R x -For Acc: Argument A is implicit -For Acc_intro: Arguments A, R are implicit - -\dots -\end{alltt} - -\noindent This inductive predicate characterizes those elements $x$ of -$A$ such that any descending $R$-chain $\ldots x_2\;R\;x_1\;R\;x$ -starting from $x$ is finite. A well-founded relation is a relation -such that all the elements of $A$ are accessible. -\emph{Notice the use of parameter $x$ (see Section~\ref{parameterstuff}, page -\pageref{parameterstuff}).} - -Consider now the problem of representing in {\coq} the following ML -function $\textsl{div}(x,y)$ on natural numbers, which computes -$\lceil\frac{x}{y}\rceil$ if $y>0$ and yields $x$ otherwise. - -\begin{verbatim} -let rec div x y = - if x = 0 then 0 - else if y = 0 then x - else (div (x-y) y)+1;; -\end{verbatim} - - -The equality test on natural numbers can be represented as the -function \textsl{eq\_nat\_dec} defined page \pageref{iseqpage}. Giving $x$ and -$y$, this function yields either the value $(\textsl{left}\;p)$ if -there exists a proof $p:x=y$, or the value $(\textsl{right}\;q)$ if -there exists $q:a\not = b$. The subtraction function is already -defined in the library \citecoq{Minus}. - -Hence, direct translation of the ML function \textsl{div} would be: - -\begin{alltt} -Require Import Minus. - -Fixpoint div (x y:nat)\{struct x\}: nat := - if eq_nat_dec x 0 - then 0 - else if eq_nat_dec y 0 - then x - else S (div (x-y) y). - -\it Error: -Recursive definition of div is ill-formed. -In environment -div : nat {\arrow} nat {\arrow} nat -x : nat -y : nat -_ : x {\coqdiff} 0 -_ : y {\coqdiff} 0 - -Recursive call to div has principal argument equal to -"x - y" -instead of a subterm of x -\end{alltt} - - -The program \texttt{div} is rejected by {\coq} because it does not verify -the syntactical condition to ensure termination. In particular, the -argument of the recursive call is not a pattern variable issued from a -case analysis on $x$. -We would have the same problem if we had the directive -``~\citecoq{\{struct y\}}~'' instead of ``~\citecoq{\{struct x\}}~''. -However, we know that this program always -stops. One way to justify its termination is to define it by -structural induction on a proof that $x$ is accessible trough the -relation $<$. Notice that any natural number $x$ is accessible -for this relation. In order to do this, it is first necessary to prove -some auxiliary lemmas, justifying that the first argument of -\texttt{div} decreases at each recursive call. - -\begin{alltt} -Lemma minus_smaller_S : {\prodsym} x y:nat, x - y < S x. -Proof. - intros x y; pattern y, x; - elim x using nat_double_ind. - destruct x0; auto with arith. - simpl; auto with arith. - simpl; auto with arith. -Qed. - - -Lemma minus_smaller_positive : - {\prodsym} x y:nat, x {\coqdiff}0 {\arrow} y {\coqdiff} 0 {\arrow} x - y < x. -Proof. - destruct x; destruct y; - ( simpl;intros; apply minus_smaller || - intros; absurd (0=0); auto). -Qed. -\end{alltt} - -\noindent The last two lemmas are necessary to prove that for any pair -of positive natural numbers $x$ and $y$, if $x$ is accessible with -respect to \citecoq{lt}, then so is $x-y$. - -\begin{alltt} -Definition minus_decrease : {\prodsym} x y:nat, Acc lt x {\arrow} - x {\coqdiff} 0 {\arrow} - y {\coqdiff} 0 {\arrow} - Acc lt (x-y). -Proof. - intros x y H; case H. - intros Hz posz posy. - apply Hz; apply minus_smaller_positive; assumption. -Defined. -\end{alltt} - -Let us take a look at the proof of the lemma \textsl{minus\_decrease}, since -the way in which it has been proven is crucial for what follows. -\begin{alltt} -Print minus_decrease. -\it -minus_decrease = -fun (x y : nat) (H : Acc lt x) {\funarrow} -match H in (Acc _ y0) return (y0 {\coqdiff} 0 {\arrow} y {\coqdiff} 0 {\arrow} Acc lt (y0 - y)) with -| Acc_intro z Hz {\funarrow} - fun (posz : z {\coqdiff} 0) (posy : y {\coqdiff} 0) {\funarrow} - Hz (z - y) (minus_smaller_positive z y posz posy) -end - : {\prodsym} x y : nat, Acc lt x {\arrow} x {\coqdiff} 0 {\arrow} y {\coqdiff} 0 {\arrow} Acc lt (x - y) - -\end{alltt} -\noindent Notice that the function call -$(\texttt{minus\_decrease}\;n\;m\;H)$ -indeed yields an accessibility proof that is \textsl{structurally -smaller} than its argument $H$, because it is (an application of) its -recursive component $Hz$. This enables to justify the following -definition of \textsl{div\_aux}: - -\begin{alltt} -Definition div_aux (x y:nat)(H: Acc lt x):nat. - fix 3. - intros. - refine (if eq_nat_dec x 0 - then 0 - else if eq_nat_dec y 0 - then y - else div_aux (x-y) y _). -\it - div_aux : {\prodsym} x : nat, nat {\arrow} Acc lt x {\arrow} nat - x : nat - y : nat - H : Acc lt x - _ : x {\coqdiff} 0 - _0 : y {\coqdiff} 0 - ============================ - Acc lt (x - y) - -\tt - apply (minus_decrease x y H);auto. -Defined. -\end{alltt} - -The main division function is easily defined, using the theorem -\citecoq{lt\_wf} of the library \citecoq{Wf\_nat}. This theorem asserts that -\citecoq{nat} is well founded w.r.t. \citecoq{lt}, thus any natural number -is accessible. -\begin{alltt} -Definition div x y := div_aux x y (lt_wf x). -\end{alltt} - -Let us explain the proof above. In the definition of \citecoq{div\_aux}, -what decreases is not $x$ but the \textsl{proof} of the accessibility -of $x$. The tactic ``~\texttt{fix 3}~'' is used to indicate that the proof -proceeds by structural induction on the third argument of the theorem ---that is, on the accessibility proof. It also introduces a new -hypothesis in the context, named as the current theorem, and with the -same type as the goal. Then, the proof is refined with an incomplete -proof term, containing a hole \texttt{\_}. This hole corresponds to the proof -of accessibility for $x-y$, and is filled up with the (smaller!) -accessibility proof provided by the function \texttt{minus\_decrease}. - - -\noindent Let us take a look to the term \textsl{div\_aux} defined: - -\pagebreak -\begin{alltt} -Print div_aux. -\it -div_aux = -(fix div_aux (x y : nat) (H : Acc lt x) \{struct H\} : nat := - match eq_nat_dec x 0 with - | left _ {\funarrow} 0 - | right _ {\funarrow} - match eq_nat_dec y 0 with - | left _ {\funarrow} y - | right _0 {\funarrow} div_aux (x - y) y (minus_decrease x y H _ _0) - end - end) - : {\prodsym} x : nat, nat {\arrow} Acc lt x {\arrow} nat - -\end{alltt} - -If the non-informative parts from this proof --that is, the -accessibility proof-- are erased, then we obtain exactly the program -that we were looking for. -\begin{alltt} - -Extraction div. - -\it -let div x y = - div_aux x y -\tt - -Extraction div_aux. - -\it -let rec div_aux x y = - match eq_nat_dec x O with - | Left {\arrow} O - | Right {\arrow} - (match eq_nat_dec y O with - | Left {\arrow} y - | Right {\arrow} div_aux (minus x y) y) -\end{alltt} - -This methodology enables the representation -of any program whose termination can be proved in {\coq}. Once the -expected properties from this program have been verified, the -justification of its termination can be thrown away, keeping just the -desired computational behavior for it. - -\section{A case study in dependent elimination}\label{CaseStudy} - -Dependent types are very expressive, but ignoring some useful -techniques can cause some problems to the beginner. -Let us consider again the type of vectors (see section~\ref{vectors}). -We want to prove a quite trivial property: the only value of type -``~\citecoq{vector A 0}~'' is ``~\citecoq{Vnil $A$}~''. - -Our first naive attempt leads to a \emph{cul-de-sac}. -\begin{alltt} -Lemma vector0_is_vnil : - {\prodsym} (A:Type)(v:vector A 0), v = Vnil A. -Proof. - intros A v;inversion v. -\it -1 subgoal - - A : Set - v : vector A 0 - ============================ - v = Vnil A -\tt -Abort. -\end{alltt} - -Another attempt is to do a case analysis on a vector of any length -$n$, under an explicit hypothesis $n=0$. The tactic -\texttt{discriminate} will help us to get rid of the case -$n=\texttt{S $p$}$. -Unfortunately, even the statement of our lemma is refused! - -\begin{alltt} - Lemma vector0_is_vnil_aux : - {\prodsym} (A:Type)(n:nat)(v:vector A n), n = 0 {\arrow} v = Vnil A. - -\it -Error: In environment -A : Type -n : nat -v : vector A n -e : n = 0 -The term "Vnil A" has type "vector A 0" while it is expected to have type - "vector A n" -\end{alltt} - -In effect, the equality ``~\citecoq{v = Vnil A}~'' is ill typed, -because the type ``~\citecoq{vector A n}~'' is not \emph{convertible} -with ``~\citecoq{vector A 0}~''. - -This problem can be solved if we consider the heterogeneous -equality \citecoq{JMeq} \cite{conor:motive} -which allows us to consider terms of different types, even if this -equality can only be proven for terms in the same type. -The axiom \citecoq{JMeq\_eq}, from the library \citecoq{JMeq} allows us to convert a -heterogeneous equality to a standard one. - -\begin{alltt} -Lemma vector0_is_vnil_aux : - {\prodsym} (A:Type)(n:nat)(v:vector A n), - n= 0 {\arrow} JMeq v (Vnil A). -Proof. - destruct v. - auto. - intro; discriminate. -Qed. -\end{alltt} - -Our property of vectors of null length can be easily proven: - -\begin{alltt} -Lemma vector0_is_vnil : {\prodsym} (A:Type)(v:vector A 0), v = Vnil A. - intros a v;apply JMeq_eq. - apply vector0_is_vnil_aux. - trivial. -Qed. -\end{alltt} - -It is interesting to look at another proof of -\citecoq{vector0\_is\_vnil}, which illustrates a technique developed -and used by various people (consult in the \emph{Coq-club} mailing -list archive the contributions by Yves Bertot, Pierre Letouzey, Laurent Théry, -Jean Duprat, and Nicolas Magaud, Venanzio Capretta and Conor McBride). -This technique is also used for unfolding infinite list definitions -(see chapter13 of~\cite{coqart}). -Notice that this definition does not rely on any axiom (\emph{e.g.} \texttt{JMeq\_eq}). - -We first give a new definition of the identity on vectors. Before that, -we make the use of constructors and selectors lighter thanks to -the implicit arguments feature: - -\begin{alltt} -Implicit Arguments Vcons [A n]. -Implicit Arguments Vnil [A]. -Implicit Arguments Vhead [A n]. -Implicit Arguments Vtail [A n]. - -Definition Vid : {\prodsym} (A : Type)(n:nat), vector A n {\arrow} vector A n. -Proof. - destruct n; intro v. - exact Vnil. - exact (Vcons (Vhead v) (Vtail v)). -Defined. -\end{alltt} - - -Then we prove that \citecoq{Vid} is the identity on vectors: - -\begin{alltt} -Lemma Vid_eq : {\prodsym} (n:nat) (A:Type)(v:vector A n), v=(Vid _ n v). -Proof. - destruct v. - -\it - A : Type - ============================ - Vnil = Vid A 0 Vnil - -subgoal 2 is: - Vcons a v = Vid A (S n) (Vcons a v) -\tt - reflexivity. - reflexivity. -Defined. -\end{alltt} - -Why defining a new identity function on vectors? The following -dialogue shows that \citecoq{Vid} has some interesting computational -properties: - -\begin{alltt} -Eval simpl in (fun (A:Type)(v:vector A 0) {\funarrow} (Vid _ _ v)). -\it = fun (A : Type) (_ : vector A 0) {\funarrow} Vnil - : {\prodsym} A : Type, vector A 0 {\arrow} vector A 0 - -\end{alltt} - -Notice that the plain identity on vectors doesn't convert \citecoq{v} -into \citecoq{Vnil}. -\begin{alltt} -Eval simpl in (fun (A:Type)(v:vector A 0) {\funarrow} v). -\it = fun (A : Type) (v : vector A 0) {\funarrow} v - : {\prodsym} A : Type, vector A 0 {\arrow} vector A 0 -\end{alltt} - -Then we prove easily that any vector of length 0 is \citecoq{Vnil}: - -\begin{alltt} -Theorem zero_nil : {\prodsym} A (v:vector A 0), v = Vnil. -Proof. - intros. - change (Vnil (A:=A)) with (Vid _ 0 v). -\it -1 subgoal - - A : Type - v : vector A 0 - ============================ - v = Vid A 0 v -\tt - apply Vid_eq. -Defined. -\end{alltt} - -A similar result can be proven about vectors of strictly positive -length\footnote{As for \citecoq{Vid} and \citecoq{Vid\_eq}, this definition -is from Jean Duprat.}. - -\begin{alltt} - - -Theorem decomp : - {\prodsym} (A : Type) (n : nat) (v : vector A (S n)), - v = Vcons (Vhead v) (Vtail v). -Proof. - intros. - change (Vcons (Vhead v) (Vtail v)) with (Vid _ (S n) v). -\it - 1 subgoal - - A : Type - n : nat - v : vector A (S n) - ============================ - v = Vid A (S n) v - -\tt{} apply Vid_eq. -Defined. -\end{alltt} - - -Both lemmas: \citecoq{zero\_nil} and \citecoq{decomp}, -can be used to easily derive a double recursion principle -on vectors of same length: - - -\begin{alltt} -Definition vector_double_rect : - {\prodsym} (A:Type) (P: {\prodsym} (n:nat),(vector A n){\arrow}(vector A n) {\arrow} Type), - P 0 Vnil Vnil {\arrow} - ({\prodsym} n (v1 v2 : vector A n) a b, P n v1 v2 {\arrow} - P (S n) (Vcons a v1) (Vcons b v2)) {\arrow} - {\prodsym} n (v1 v2 : vector A n), P n v1 v2. - induction n. - intros; rewrite (zero_nil _ v1); rewrite (zero_nil _ v2). - auto. - intros v1 v2; rewrite (decomp _ _ v1);rewrite (decomp _ _ v2). - apply X0; auto. -Defined. -\end{alltt} - -Notice that, due to the conversion rule of {\coq}'s type system, -this function can be used directly with \citecoq{Prop} or \citecoq{Type} -instead of type (thus it is useless to build -\citecoq{vector\_double\_ind} and \citecoq{vector\_double\_rec}) from scratch. - -We finish this example with showing how to define the bitwise -\emph{or} on boolean vectors of the same length, -and proving a little property about this -operation. - -\begin{alltt} -Definition bitwise_or n v1 v2 : vector bool n := - vector_double_rect - bool - (fun n v1 v2 {\funarrow} vector bool n) - Vnil - (fun n v1 v2 a b r {\funarrow} Vcons (orb a b) r) n v1 v2. -\end{alltt} - -Let us define recursively the $n$-th element of a vector. Notice -that it must be a partial function, in case $n$ is greater or equal -than the length of the vector. Since {\coq} only considers total -functions, the function returns a value in an \emph{option} type. - -\begin{alltt} -Fixpoint vector_nth (A:Type)(n:nat)(p:nat)(v:vector A p) - \{struct v\} - : option A := - match n,v with - _ , Vnil {\funarrow} None - | 0 , Vcons b _ _ {\funarrow} Some b - | S n', Vcons _ p' v' {\funarrow} vector_nth A n' p' v' - end. -Implicit Arguments vector_nth [A p]. -\end{alltt} - -We can now prove --- using the double induction combinator --- -a simple property relying \citecoq{vector\_nth} and \citecoq{bitwise\_or}: - -\begin{alltt} -Lemma nth_bitwise : - {\prodsym} (n:nat) (v1 v2: vector bool n) i a b, - vector_nth i v1 = Some a {\arrow} - vector_nth i v2 = Some b {\arrow} - vector_nth i (bitwise_or _ v1 v2) = Some (orb a b). -Proof. - intros n v1 v2; pattern n,v1,v2. - apply vector_double_rect. - simpl. - destruct i; discriminate 1. - destruct i; simpl;auto. - injection 1; injection 2;intros; subst a; subst b; auto. -Qed. -\end{alltt} - - -\section{Co-inductive Types and Non-ending Constructions} -\label{CoInduction} - -The objects of an inductive type are well-founded with respect to -the constructors of the type. In other words, these objects are built -by applying \emph{a finite number of times} the constructors of the type. -Co-inductive types are obtained by relaxing this condition, -and may contain non-well-founded objects \cite{EG96,EG95a}. An -example of a co-inductive type is the type of infinite -sequences formed with elements of type $A$, also called streams. This -type can be introduced through the following definition: - -\begin{alltt} - CoInductive Stream (A: Type) :Type := - | Cons : A\arrow{}Stream A\arrow{}Stream A. -\end{alltt} - -If we are interested in finite or infinite sequences, we consider the type -of \emph{lazy lists}: - -\begin{alltt} -CoInductive LList (A: Type) : Type := - | LNil : LList A - | LCons : A {\arrow} LList A {\arrow} LList A. -\end{alltt} - - -It is also possible to define co-inductive types for the -trees with infinite branches (see Chapter 13 of~\cite{coqart}). - -Structural induction is the way of expressing that inductive types -only contain well-founded objects. Hence, this elimination principle -is not valid for co-inductive types, and the only elimination rule for -streams is case analysis. This principle can be used, for example, to -define the destructors \textsl{head} and \textsl{tail}. - -\begin{alltt} - Definition head (A:Type)(s : Stream A) := - match s with Cons a s' {\funarrow} a end. - - Definition tail (A : Type)(s : Stream A) := - match s with Cons a s' {\funarrow} s' end. -\end{alltt} - -Infinite objects are defined by means of (non-ending) methods of -construction, like in lazy functional programming languages. Such -methods can be defined using the \texttt{CoFixpoint} command -\refmancite{Section \ref{CoFixpoint}}. For example, the following -definition introduces the infinite list $[a,a,a,\ldots]$: - -\begin{alltt} - CoFixpoint repeat (A:Type)(a:A) : Stream A := - Cons a (repeat a). -\end{alltt} - - -However, not every co-recursive definition is an admissible method of -construction. Similarly to the case of structural induction, the -definition must verify a \textsl{guardedness} condition to be -accepted. This condition states that any recursive call in the -definition must be protected --i.e, be an argument of-- some -constructor, and only an argument of constructors \cite{EG94a}. The -following definitions are examples of valid methods of construction: - -\begin{alltt} -CoFixpoint iterate (A: Type)(f: A {\arrow} A)(a : A) : Stream A:= - Cons a (iterate f (f a)). - -CoFixpoint map - (A B:Type)(f: A {\arrow} B)(s : Stream A) : Stream B:= - match s with Cons a tl {\funarrow} Cons (f a) (map f tl) end. -\end{alltt} - -\begin{exercise} -Define two different methods for constructing the stream which -infinitely alternates the values \citecoq{true} and \citecoq{false}. -\end{exercise} -\begin{exercise} -Using the destructors \texttt{head} and \texttt{tail}, define a function -which takes the n-th element of an infinite stream. -\end{exercise} - -A non-ending method of construction is computed lazily. This means -that its definition is unfolded only when the object that it -introduces is eliminated, that is, when it appears as the argument of -a case expression. We can check this using the command -\texttt{Eval}. - -\begin{alltt} -Eval simpl in (fun (A:Type)(a:A) {\funarrow} repeat a). -\it = fun (A : Type) (a : A) {\funarrow} repeat a - : {\prodsym} A : Type, A {\arrow} Stream A -\tt -Eval simpl in (fun (A:Type)(a:A) {\funarrow} head (repeat a)). -\it = fun (A : Type) (a : A) {\funarrow} a - : {\prodsym} A : Type, A {\arrow} A -\end{alltt} - -%\begin{exercise} -%Prove the following theorem: -%\begin{verbatim} -%Theorem expand_repeat : (a:A)(repeat a)=(Cons a (repeat a)). -%\end{verbatim} -%Hint: Prove first the streams version of the lemma in exercise -%\ref{expand}. -%\end{exercise} - -\subsection{Extensional Properties} - -Case analysis is also a valid proof principle for infinite -objects. However, this principle is not sufficient to prove -\textsl{extensional} properties, that is, properties concerning the -whole infinite object \cite{EG95a}. A typical example of an -extensional property is the predicate expressing that two streams have -the same elements. In many cases, the minimal reflexive relation $a=b$ -that is used as equality for inductive types is too small to capture -equality between streams. Consider for example the streams -$\texttt{iterate}\;f\;(f\;x)$ and -$(\texttt{map}\;f\;(\texttt{iterate}\;f\;x))$. Even though these two streams have -the same elements, no finite expansion of their definitions lead to -equal terms. In other words, in order to deal with extensional -properties, it is necessary to construct infinite proofs. The type of -infinite proofs of equality can be introduced as a co-inductive -predicate, as follows: -\begin{alltt} -CoInductive EqSt (A: Type) : Stream A {\arrow} Stream A {\arrow} Prop := - eqst : {\prodsym} s1 s2: Stream A, - head s1 = head s2 {\arrow} - EqSt (tail s1) (tail s2) {\arrow} - EqSt s1 s2. -\end{alltt} - -It is possible to introduce proof principles for reasoning about -infinite objects as combinators defined through -\texttt{CoFixpoint}. However, oppositely to the case of inductive -types, proof principles associated to co-inductive types are not -elimination but \textsl{introduction} combinators. An example of such -a combinator is Park's principle for proving the equality of two -streams, usually called the \textsl{principle of co-induction}. It -states that two streams are equal if they satisfy a -\textit{bisimulation}. A bisimulation is a binary relation $R$ such -that any pair of streams $s_1$ ad $s_2$ satisfying $R$ have equal -heads, and tails also satisfying $R$. This principle is in fact a -method for constructing an infinite proof: - -\begin{alltt} -Section Parks_Principle. -Variable A : Type. -Variable R : Stream A {\arrow} Stream A {\arrow} Prop. -Hypothesis bisim1 : {\prodsym} s1 s2:Stream A, - R s1 s2 {\arrow} head s1 = head s2. - -Hypothesis bisim2 : {\prodsym} s1 s2:Stream A, - R s1 s2 {\arrow} R (tail s1) (tail s2). - -CoFixpoint park_ppl : - {\prodsym} s1 s2:Stream A, R s1 s2 {\arrow} EqSt s1 s2 := - fun s1 s2 (p : R s1 s2) {\funarrow} - eqst s1 s2 (bisim1 s1 s2 p) - (park_ppl (tail s1) - (tail s2) - (bisim2 s1 s2 p)). -End Parks_Principle. -\end{alltt} - -Let us use the principle of co-induction to prove the extensional -equality mentioned above. -\begin{alltt} -Theorem map_iterate : {\prodsym} (A:Type)(f:A{\arrow}A)(x:A), - EqSt (iterate f (f x)) - (map f (iterate f x)). -Proof. - intros A f x. - apply park_ppl with - (R:= fun s1 s2 {\funarrow} - {\exsym} x: A, s1 = iterate f (f x) {\coqand} - s2 = map f (iterate f x)). - - intros s1 s2 (x0,(eqs1,eqs2)); - rewrite eqs1; rewrite eqs2; reflexivity. - intros s1 s2 (x0,(eqs1,eqs2)). - exists (f x0);split; - [rewrite eqs1|rewrite eqs2]; reflexivity. - exists x;split; reflexivity. -Qed. -\end{alltt} - -The use of Park's principle is sometimes annoying, because it requires -to find an invariant relation and prove that it is indeed a -bisimulation. In many cases, a shorter proof can be obtained trying -to construct an ad-hoc infinite proof, defined by a guarded -declaration. The tactic ``~``\texttt{Cofix $f$}~'' can be used to do -that. Similarly to the tactic \texttt{fix} indicated in Section -\ref{WellFoundedRecursion}, this tactic introduces an extra hypothesis -$f$ into the context, whose type is the same as the current goal. Note -that the applications of $f$ in the proof \textsl{must be guarded}. In -order to prevent us from doing unguarded calls, we can define a tactic -that always apply a constructor before using $f$ \refmancite{Chapter -\ref{WritingTactics}} : - -\begin{alltt} -Ltac infiniteproof f := - cofix f; - constructor; - [clear f| simpl; try (apply f; clear f)]. -\end{alltt} - - -In the example above, this tactic produces a much simpler proof -that the former one: - -\begin{alltt} -Theorem map_iterate' : {\prodsym} ((A:Type)f:A{\arrow}A)(x:A), - EqSt (iterate f (f x)) - (map f (iterate f x)). -Proof. - infiniteproof map_iterate'. - reflexivity. -Qed. -\end{alltt} - -\begin{exercise} -Define a co-inductive type $Nat$ containing non-standard -natural numbers --this is, verifying - -$$\exists m \in \mbox{\texttt{Nat}}, \forall\, n \in \mbox{\texttt{Nat}}, n<m$$. -\end{exercise} - -\begin{exercise} -Prove that the extensional equality of streams is an equivalence relation -using Park's co-induction principle. -\end{exercise} - - -\begin{exercise} -Provide a suitable definition of ``being an ordered list'' for infinite lists -and define a principle for proving that an infinite list is ordered. Apply -this method to the list $[0,1,\ldots ]$. Compare the result with -exercise \ref{zeroton}. -\end{exercise} - -\subsection{About injection, discriminate, and inversion} -Since co-inductive types are closed w.r.t. their constructors, -the techniques shown in Section~\ref{CaseTechniques} work also -with these types. - -Let us consider the type of lazy lists, introduced on page~\pageref{CoInduction}. -The following lemmas are straightforward applications - of \texttt{discriminate} and \citecoq{injection}: - -\begin{alltt} -Lemma Lnil_not_Lcons : {\prodsym} (A:Type)(a:A)(l:LList A), - LNil {\coqdiff} (LCons a l). -Proof. - intros;discriminate. -Qed. - -Lemma injection_demo : {\prodsym} (A:Type)(a b : A)(l l': LList A), - LCons a (LCons b l) = LCons b (LCons a l') {\arrow} - a = b {\coqand} l = l'. -Proof. - intros A a b l l' e; injection e; auto. -Qed. - -\end{alltt} - -In order to show \citecoq{inversion} at work, let us define -two predicates on lazy lists: - -\begin{alltt} -Inductive Finite (A:Type) : LList A {\arrow} Prop := -| Lnil_fin : Finite (LNil (A:=A)) -| Lcons_fin : {\prodsym} a l, Finite l {\arrow} Finite (LCons a l). - -CoInductive Infinite (A:Type) : LList A {\arrow} Prop := -| LCons_inf : {\prodsym} a l, Infinite l {\arrow} Infinite (LCons a l). -\end{alltt} - -\noindent -First, two easy theorems: -\begin{alltt} -Lemma LNil_not_Infinite : {\prodsym} (A:Type), ~ Infinite (LNil (A:=A)). -Proof. - intros A H;inversion H. -Qed. - -Lemma Finite_not_Infinite : {\prodsym} (A:Type)(l:LList A), - Finite l {\arrow} ~ Infinite l. -Proof. - intros A l H; elim H. - apply LNil_not_Infinite. - intros a l0 F0 I0' I1. - case I0'; inversion_clear I1. - trivial. -Qed. -\end{alltt} - - -On the other hand, the next proof uses the \citecoq{cofix} tactic. -Notice the destructuration of \citecoq{l}, which allows us to -apply the constructor \texttt{LCons\_inf}, thus satisfying - the guard condition: -\begin{alltt} -Lemma Not_Finite_Infinite : {\prodsym} (A:Type)(l:LList A), - ~ Finite l {\arrow} Infinite l. -Proof. - cofix H. - destruct l. - intro; - absurd (Finite (LNil (A:=A))); - [auto|constructor]. -\it - - - - -1 subgoal - - H : forall (A : Type) (l : LList A), ~ Finite l -> Infinite l - A : Type - a : A - l : LList A - H0 : ~ Finite (LCons a l) - ============================ - Infinite l -\end{alltt} -At this point, one must not apply \citecoq{H}! . It would be possible -to solve the current goal by an inversion of ``~\citecoq{Finite (LCons a l)}~'', but, since the guard condition would be violated, the user -would get an error message after typing \citecoq{Qed}. -In order to satisfy the guard condition, we apply the constructor of -\citecoq{Infinite}, \emph{then} apply \citecoq{H}. - -\begin{alltt} - constructor. - apply H. - red; intro H1;case H0. - constructor. - trivial. -Qed. -\end{alltt} - - - - -The reader is invited to replay this proof and understand each of its steps. - - -\bibliographystyle{abbrv} -\bibliography{manbiblio,morebib} - -\end{document} - |