path: root/doc/sphinx/proof-engine/detailed-tactic-examples.rst

.. _detailedexamplesoftactics:

Detailed examples of tactics
============================

This chapter presents detailed examples of certain tactics, to
illustrate their behavior.

.. _dependent-induction:

dependent induction
-------------------

The tactics ``dependent induction`` and ``dependent destruction`` are another
solution for inverting inductive predicate instances and potentially
doing induction at the same time. It is based on the ``BasicElim`` tactic
of Conor McBride which works by abstracting each argument of an
inductive instance by a variable and constraining it by equalities
afterwards. This way, the usual induction and destruct tactics can be
applied to the abstracted instance and after simplification of the
equalities we get the expected goals.

The abstracting tactic is called generalize_eqs and it takes as
argument an hypothesis to generalize. It uses the JMeq datatype
defined in Coq.Logic.JMeq, hence we need to require it before. For
example, revisiting the first example of the inversion documentation:

.. coqtop:: in

   Require Import Coq.Logic.JMeq.

   Inductive Le : nat -> nat -> Set :=
        | LeO : forall n:nat, Le 0 n
        | LeS : forall n m:nat, Le n m -> Le (S n) (S m).

   Variable P : nat -> nat -> Prop.

   Goal forall n m:nat, Le (S n) m -> P n m.

   intros n m H.

.. coqtop:: all

   generalize_eqs H.

The index ``S n`` gets abstracted by a variable here, but a corresponding
equality is added under the abstract instance so that no information
is actually lost. The goal is now almost amenable to do induction or
case analysis. One should indeed first move ``n`` into the goal to
strengthen it before doing induction, or ``n`` will be fixed in the
inductive hypotheses (this does not matter for case analysis). As a
rule of thumb, all the variables that appear inside constructors in
the indices of the hypothesis should be generalized. This is exactly
what the ``generalize_eqs_vars`` variant does:

.. coqtop:: all

   generalize_eqs_vars H.
   induction H.

As the hypothesis itself did not appear in the goal, we did not need
to use an heterogeneous equality to relate the new hypothesis to the
old one (which just disappeared here). However, the tactic works just
as well in this case, e.g.:

.. coqtop:: in

   Variable Q : forall (n m : nat), Le n m -> Prop.
   Goal forall n m (p : Le (S n) m), Q (S n) m p.

.. coqtop:: all

   intros n m p.
   generalize_eqs_vars p.

One drawback of this approach is that in the branches one will have to
substitute the equalities back into the instance to get the right
assumptions. Sometimes injection of constructors will also be needed
to recover the needed equalities. Also, some subgoals should be
directly solved because of inconsistent contexts arising from the
constraints on indexes. The nice thing is that we can make a tactic
based on discriminate, injection and variants of substitution to
automatically do such simplifications (which may involve the K axiom).
This is what the ``simplify_dep_elim`` tactic from ``Coq.Program.Equality``
does. For example, we might simplify the previous goals considerably:

.. coqtop:: all

   Require Import Coq.Program.Equality.

.. coqtop:: all

   induction p ; simplify_dep_elim.

The higher-order tactic ``do_depind`` defined in ``Coq.Program.Equality``
takes a tactic and combines the building blocks we have seen with it:
generalizing by equalities calling the given tactic with the
generalized induction hypothesis as argument and cleaning the subgoals
with respect to equalities. Its most important instantiations
are ``dependent induction`` and ``dependent destruction`` that do induction or
simply case analysis on the generalized hypothesis. For example we can
redo what we’ve done manually with dependent destruction:

.. coqtop:: in

   Require Import Coq.Program.Equality.

.. coqtop:: in

   Lemma ex : forall n m:nat, Le (S n) m -> P n m.

.. coqtop:: in

   intros n m H.

.. coqtop:: all

   dependent destruction H.

This gives essentially the same result as inversion. Now if the
destructed hypothesis actually appeared in the goal, the tactic would
still be able to invert it, contrary to dependent inversion. Consider
the following example on vectors:

.. coqtop:: in

   Require Import Coq.Program.Equality.

.. coqtop:: in

   Set Implicit Arguments.

.. coqtop:: in

   Variable A : Set.

.. coqtop:: in

   Inductive vector : nat -> Type :=
            | vnil : vector 0
            | vcons : A -> forall n, vector n -> vector (S n).

.. coqtop:: in

   Goal forall n, forall v : vector (S n),
            exists v' : vector n, exists a : A, v = vcons a v'.

.. coqtop:: in

   intros n v.

.. coqtop:: all

   dependent destruction v.

In this case, the ``v`` variable can be replaced in the goal by the
generalized hypothesis only when it has a type of the form ``vector (S n)``,
that is only in the second case of the destruct. The first one is
dismissed because ``S n <> 0``.


A larger example
~~~~~~~~~~~~~~~~

Let’s see how the technique works with induction on inductive
predicates on a real example. We will develop an example application
to the theory of simply-typed lambda-calculus formalized in a
dependently-typed style:

.. coqtop:: in

   Inductive type : Type :=
            | base : type
            | arrow : type -> type -> type.

.. coqtop:: in

   Notation " t --> t' " := (arrow t t') (at level 20, t' at next level).

.. coqtop:: in

   Inductive ctx : Type :=
            | empty : ctx
            | snoc : ctx -> type -> ctx.

.. coqtop:: in

   Notation " G , tau " := (snoc G tau) (at level 20, tau at next level).

.. coqtop:: in

   Fixpoint conc (G D : ctx) : ctx :=
            match D with
            | empty => G
            | snoc D' x => snoc (conc G D') x
            end.

.. coqtop:: in

   Notation " G ; D " := (conc G D) (at level 20).

.. coqtop:: in

   Inductive term : ctx -> type -> Type :=
            | ax : forall G tau, term (G, tau) tau
            | weak : forall G tau,
                       term G tau -> forall tau', term (G, tau') tau
            | abs : forall G tau tau',
                      term (G , tau) tau' -> term G (tau --> tau')
            | app : forall G tau tau',
                      term G (tau --> tau') -> term G tau -> term G tau'.

We have defined types and contexts which are snoc-lists of types. We
also have a ``conc`` operation that concatenates two contexts. The ``term``
datatype represents in fact the possible typing derivations of the
calculus, which are isomorphic to the well-typed terms, hence the
name. A term is either an application of:


+ the axiom rule to type a reference to the first variable in a
  context
+ the weakening rule to type an object in a larger context
+ the abstraction or lambda rule to type a function
+ the application to type an application of a function to an argument


Once we have this datatype we want to do proofs on it, like weakening:

.. coqtop:: in undo

   Lemma weakening : forall G D tau, term (G ; D) tau -> 
                     forall tau', term (G , tau' ; D) tau.

The problem here is that we can’t just use induction on the typing
derivation because it will forget about the ``G ; D`` constraint appearing
in the instance. A solution would be to rewrite the goal as:

.. coqtop:: in

   Lemma weakening' : forall G' tau, term G' tau ->
                      forall G D, (G ; D) = G' ->
                      forall tau', term (G, tau' ; D) tau.

With this proper separation of the index from the instance and the
right induction loading (putting ``G`` and ``D`` after the inducted-on
hypothesis), the proof will go through, but it is a very tedious
process. One is also forced to make a wrapper lemma to get back the
more natural statement. The ``dependent induction`` tactic alleviates this
trouble by doing all of this plumbing of generalizing and substituting
back automatically. Indeed we can simply write:

.. coqtop:: in

   Require Import Coq.Program.Tactics.

.. coqtop:: in

   Lemma weakening : forall G D tau, term (G ; D) tau ->
                     forall tau', term (G , tau' ; D) tau.

.. coqtop:: in

   Proof with simpl in * ; simpl_depind ; auto.

.. coqtop:: in

   intros G D tau H. dependent induction H generalizing G D ; intros.

This call to dependent induction has an additional arguments which is
a list of variables appearing in the instance that should be
generalized in the goal, so that they can vary in the induction
hypotheses. By default, all variables appearing inside constructors
(except in a parameter position) of the instantiated hypothesis will
be generalized automatically but one can always give the list
explicitly.

.. coqtop:: all

   Show.

The ``simpl_depind`` tactic includes an automatic tactic that tries to
simplify equalities appearing at the beginning of induction
hypotheses, generally using trivial applications of ``reflexivity``. In
cases where the equality is not between constructor forms though, one
must help the automation by giving some arguments, using the
``specialize`` tactic for example.

.. coqtop:: in

   destruct D... apply weak; apply ax. apply ax.

.. coqtop:: in

   destruct D...

.. coqtop:: all

   Show.

.. coqtop:: all

   specialize (IHterm G0 empty eq_refl).

Once the induction hypothesis has been narrowed to the right equality,
it can be used directly.

.. coqtop:: all

   apply weak, IHterm.

If there is an easy first-order solution to these equations as in this
subgoal, the ``specialize_eqs`` tactic can be used instead of giving
explicit proof terms:

.. coqtop:: all

   specialize_eqs IHterm.

This concludes our example.

See also: The :tacn:`induction`, :tacn:`case`, and :tacn:`inversion` tactics.


autorewrite
-----------

Here are two examples of ``autorewrite`` use. The first one ( *Ackermann
function*) shows actually a quite basic use where there is no
conditional rewriting. The second one ( *Mac Carthy function*)
involves conditional rewritings and shows how to deal with them using
the optional tactic of the ``Hint Rewrite`` command.


Example 1: Ackermann function

.. coqtop:: in

   Reset Initial.

.. coqtop:: in

   Require Import Arith.

.. coqtop:: in

   Variable Ack : nat -> nat -> nat.

.. coqtop:: in

   Axiom Ack0 : forall m:nat, Ack 0 m = S m.
   Axiom Ack1 : forall n:nat, Ack (S n) 0 = Ack n 1.
   Axiom Ack2 : forall n m:nat, Ack (S n) (S m) = Ack n (Ack (S n) m).

.. coqtop:: in

   Hint Rewrite Ack0 Ack1 Ack2 : base0.

.. coqtop:: all

   Lemma ResAck0 : Ack 3 2 = 29.

.. coqtop:: all

   autorewrite with base0 using try reflexivity.

Example 2: Mac Carthy function

.. coqtop:: in

   Require Import Omega.

.. coqtop:: in

   Variable g : nat -> nat -> nat.

.. coqtop:: in

   Axiom g0 : forall m:nat, g 0 m = m.
   Axiom g1 : forall n m:nat, (n > 0) -> (m > 100) -> g n m = g (pred n) (m - 10).
   Axiom g2 : forall n m:nat, (n > 0) -> (m <= 100) -> g n m = g (S n) (m + 11).


.. coqtop:: in

   Hint Rewrite g0 g1 g2 using omega : base1.

.. coqtop:: in

   Lemma Resg0 : g 1 110 = 100.

.. coqtop:: out

   Show.

.. coqtop:: all

   autorewrite with base1 using reflexivity || simpl.

.. coqtop:: all

   Lemma Resg1 : g 1 95 = 91.

.. coqtop:: all

   autorewrite with base1 using reflexivity || simpl.


.. _quote:

quote
-----

The tactic ``quote`` allows using Barendregt’s so-called 2-level approach
without writing any ML code. Suppose you have a language ``L`` of
'abstract terms' and a type ``A`` of 'concrete terms' and a function ``f : L -> A``.
If ``L`` is a simple inductive datatype and ``f`` a simple fixpoint,
``quote f`` will replace the head of current goal by a convertible term of
the form ``(f t)``. ``L`` must have a constructor of type: ``A -> L``.

Here is an example:

.. coqtop:: in

   Require Import Quote.

.. coqtop:: all

   Parameters A B C : Prop.

.. coqtop:: all

   Inductive formula : Type :=
            | f_and : formula -> formula -> formula (* binary constructor *)
            | f_or : formula -> formula -> formula
            | f_not : formula -> formula (* unary constructor *)
            | f_true : formula (* 0-ary constructor *)
            | f_const : Prop -> formula (* constructor for constants *).

.. coqtop:: all

   Fixpoint interp_f (f:formula) : Prop :=
            match f with
            | f_and f1 f2 => interp_f f1 /\ interp_f f2
            | f_or f1 f2 => interp_f f1 \/ interp_f f2
            | f_not f1 => ~ interp_f f1
            | f_true => True
            | f_const c => c
            end.

.. coqtop:: all

   Goal A /\ (A \/ True) /\ ~ B /\ (A <-> A).

.. coqtop:: all

   quote interp_f.

The algorithm to perform this inversion is: try to match the term with
right-hand sides expression of ``f``. If there is a match, apply the
corresponding left-hand side and call yourself recursively on sub-
terms. If there is no match, we are at a leaf: return the
corresponding constructor (here ``f_const``) applied to the term.


Error messages:


#. quote: not a simple fixpoint

   Happens when ``quote`` is not able to perform inversion properly.



Introducing variables map
~~~~~~~~~~~~~~~~~~~~~~~~~

The normal use of quote is to make proofs by reflection: one defines a
function ``simplify : formula -> formula`` and proves a theorem
``simplify_ok: (f:formula)(interp_f (simplify f)) -> (interp_f f)``. Then,
one can simplify formulas by doing:

.. coqtop:: in

       quote interp_f.
       apply simplify_ok.
       compute.

But there is a problem with leafs: in the example above one cannot
write a function that implements, for example, the logical
simplifications :math:`A \wedge A \rightarrow A` or :math:`A \wedge
\lnot A \rightarrow \mathrm{False}`. This is because ``Prop`` is
impredicative.

It is better to use that type of formulas:

.. coqtop:: in reset

   Require Import Quote.

.. coqtop:: in

   Parameters A B C : Prop.

.. coqtop:: all

   Inductive formula : Set :=
            | f_and : formula -> formula -> formula
            | f_or : formula -> formula -> formula
            | f_not : formula -> formula
            | f_true : formula
            | f_atom : index -> formula.

``index`` is defined in module ``Quote``. Equality on that type is
decidable so we are able to simplify :math:`A \wedge A` into :math:`A`
at the abstract level.

When there are variables, there are bindings, and ``quote`` also
provides a type ``(varmap A)`` of bindings from index to any set
``A``, and a function ``varmap_find`` to search in such maps. The
interpretation function also has another argument, a variables map:

.. coqtop:: all

   Fixpoint interp_f (vm:varmap Prop) (f:formula) {struct f} : Prop :=
            match f with
            | f_and f1 f2 => interp_f vm f1 /\ interp_f vm f2
            | f_or f1 f2 => interp_f vm f1 \/ interp_f vm f2
            | f_not f1 => ~ interp_f vm f1
            | f_true => True
            | f_atom i => varmap_find True i vm
            end.

``quote`` handles this second case properly:

.. coqtop:: all

   Goal A /\ (B \/ A) /\ (A \/ ~ B).

.. coqtop:: all

   quote interp_f.

It builds ``vm`` and ``t`` such that ``(f vm t)`` is convertible with the
conclusion of current goal.


Combining variables and constants
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

One can have both variables and constants in abstracts terms; for
example, this is the case for the :tacn:`ring` tactic. Then one must provide to
``quote`` a list of *constructors of constants*. For example, if the list
is ``[O S]`` then closed natural numbers will be considered as constants
and other terms as variables.

Example:

.. coqtop:: in

   Inductive formula : Type :=
            | f_and : formula -> formula -> formula
            | f_or : formula -> formula -> formula
            | f_not : formula -> formula
            | f_true : formula
            | f_const : Prop -> formula (* constructor for constants *)
            | f_atom : index -> formula.

.. coqtop:: in

   Fixpoint interp_f (vm:varmap Prop) (f:formula) {struct f} : Prop :=
            match f with
            | f_and f1 f2 => interp_f vm f1 /\ interp_f vm f2
            | f_or f1 f2 => interp_f vm f1 \/ interp_f vm f2
            | f_not f1 => ~ interp_f vm f1
            | f_true => True
            | f_const c => c
            | f_atom i => varmap_find True i vm
            end.

.. coqtop:: in

   Goal A /\ (A \/ True) /\ ~ B /\ (C <-> C).

.. coqtop:: all

   quote interp_f [ A B ].


.. coqtop:: all

   Undo.

.. coqtop:: all

   quote interp_f [ B C iff ].

Warning: Since function inversion is undecidable in general case,
don’t expect miracles from it!

.. tacv:: quote @ident in @term using @tactic

   ``tactic`` must be a functional tactic (starting with ``fun x =>``) and
   will be called with the quoted version of term according to ``ident``.

.. tacv:: quote @ident [{+ @ident}] in @term using @tactic          

   Same as above, but will use the additional ``ident`` list to chose
   which subterms are constants (see above).

See also: comments of source file ``plugins/quote/quote.ml``

See also: the :tacn:`ring` tactic.


Using the tactical language
---------------------------


About the cardinality of the set of natural numbers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A first example which shows how to use pattern matching over the
proof contexts is the proof that natural numbers have more than two
elements. The proof of such a lemma can be done as follows:

.. coqtop:: in

   Lemma card_nat : ~ (exists x : nat, exists y : nat, forall z:nat, x = z \/ y = z).
   Proof.

.. coqtop:: in

   red; intros (x, (y, Hy)).

.. coqtop:: in

   elim (Hy 0); elim (Hy 1); elim (Hy 2); intros;

   match goal with
   | [_:(?a = ?b),_:(?a = ?c) |- _ ] =>
            cut (b = c); [ discriminate | transitivity a; auto ]
   end.

.. coqtop:: in

   Qed.

We can notice that all the (very similar) cases coming from the three
eliminations (with three distinct natural numbers) are successfully
solved by a match goal structure and, in particular, with only one
pattern (use of non-linear matching).


Permutation on closed lists
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Another more complex example is the problem of permutation on closed
lists. The aim is to show that a closed list is a permutation of
another one.

First, we define the permutation predicate as shown here:

.. coqtop:: in

   Section Sort.

.. coqtop:: in

   Variable A : Set.

.. coqtop:: in

   Inductive permut : list A -> list A -> Prop :=
            | permut_refl : forall l, permut l l
            | permut_cons : forall a l0 l1, permut l0 l1 -> permut (a :: l0) (a :: l1)
            | permut_append : forall a l, permut (a :: l) (l ++ a :: nil)
            | permut_trans : forall l0 l1 l2, permut l0 l1 -> permut l1 l2 -> permut l0 l2.

.. coqtop:: in

   End Sort.

A more complex example is the problem of permutation on closed lists.
The aim is to show that a closed list is a permutation of another one.
First, we define the permutation predicate as shown above.


.. coqtop:: none

   Require Import List.


.. coqtop:: all

   Ltac Permut n :=
            match goal with
            | |- (permut _ ?l ?l) => apply permut_refl
            | |- (permut _ (?a :: ?l1) (?a :: ?l2)) =>
                let newn := eval compute in (length l1) in
                (apply permut_cons; Permut newn)
            | |- (permut ?A (?a :: ?l1) ?l2) =>
                match eval compute in n with
                | 1 => fail
                | _ =>
                    let l1' := constr:(l1 ++ a :: nil) in
                    (apply (permut_trans A (a :: l1) l1' l2);
                    [ apply permut_append | compute; Permut (pred n) ])
                end
            end.


.. coqtop:: all

   Ltac PermutProve :=
            match goal with
            | |- (permut _ ?l1 ?l2) =>
                match eval compute in (length l1 = length l2) with
                | (?n = ?n) => Permut n
                end
            end.

Next, we can write naturally the tactic and the result can be seen
above. We can notice that we use two top level definitions
``PermutProve`` and ``Permut``. The function to be called is
``PermutProve`` which computes the lengths of the two lists and calls
``Permut`` with the length if the two lists have the same
length. ``Permut`` works as expected. If the two lists are equal, it
concludes. Otherwise, if the lists have identical first elements, it
applies ``Permut`` on the tail of the lists.  Finally, if the lists
have different first elements, it puts the first element of one of the
lists (here the second one which appears in the permut predicate) at
the end if that is possible, i.e., if the new first element has been
at this place previously. To verify that all rotations have been done
for a list, we use the length of the list as an argument for Permut
and this length is decremented for each rotation down to, but not
including, 1 because for a list of length ``n``, we can make exactly
``n−1`` rotations to generate at most ``n`` distinct lists. Here, it
must be noticed that we use the natural numbers of Coq for the
rotation counter. In :ref:`ltac-syntax`, we can
see that it is possible to use usual natural numbers but they are only
used as arguments for primitive tactics and they cannot be handled, in
particular, we cannot make computations with them. So, a natural
choice is to use Coq data structures so that Coq makes the
computations (reductions) by eval compute in and we can get the terms
back by match.

With ``PermutProve``, we can now prove lemmas as follows:

.. coqtop:: in

   Lemma permut_ex1 : permut nat (1 :: 2 :: 3 :: nil) (3 :: 2 :: 1 :: nil).

.. coqtop:: in

   Proof. PermutProve. Qed.

.. coqtop:: in

   Lemma permut_ex2 : permut nat
            (0 :: 1 :: 2 :: 3 :: 4 :: 5 :: 6 :: 7 :: 8 :: 9 :: nil)
            (0 :: 2 :: 4 :: 6 :: 8 :: 9 :: 7 :: 5 :: 3 :: 1 :: nil).

   Proof. PermutProve. Qed.



Deciding intuitionistic propositional logic
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. _decidingintuitionistic1:

.. coqtop:: all

   Ltac Axioms :=
            match goal with
            | |- True => trivial
            | _:False |- _ => elimtype False; assumption
            | _:?A |- ?A => auto
            end.

.. _decidingintuitionistic2:

.. coqtop:: all

   Ltac DSimplif :=
            repeat
            (intros;
            match goal with
            | id:(~ _) |- _ => red in id
            | id:(_ /\ _) |- _ =>
            elim id; do 2 intro; clear id
            | id:(_ \/ _) |- _ =>
                elim id; intro; clear id
            | id:(?A /\ ?B -> ?C) |- _ =>
                cut (A -> B -> C);
                [ intro | intros; apply id; split; assumption ]
            | id:(?A \/ ?B -> ?C) |- _ =>
                cut (B -> C);
                [ cut (A -> C);
                [ intros; clear id
            | intro; apply id; left; assumption ]
            | intro; apply id; right; assumption ]
            | id0:(?A -> ?B),id1:?A |- _ =>
                cut B; [ intro; clear id0 | apply id0; assumption ]
            | |- (_ /\ _) => split
            | |- (~ _) => red
            end).

.. coqtop:: all

   Ltac TautoProp :=
            DSimplif;
            Axioms ||
            match goal with
            | id:((?A -> ?B) -> ?C) |- _ =>
                cut (B -> C);
                [ intro; cut (A -> B);
                [ intro; cut C;
                [ intro; clear id | apply id; assumption ]
            | clear id ]
            | intro; apply id; intro; assumption ]; TautoProp
            | id:(~ ?A -> ?B) |- _ =>
                cut (False -> B);
                [ intro; cut (A -> False);
                [ intro; cut B;
                [ intro; clear id | apply id; assumption ]
            | clear id ]
            | intro; apply id; red; intro; assumption ]; TautoProp
            | |- (_ \/ _) => (left; TautoProp) || (right; TautoProp)
            end.

The pattern matching on goals allows a complete and so a powerful
backtracking when returning tactic values. An interesting application
is the problem of deciding intuitionistic propositional logic.
Considering the contraction-free sequent calculi LJT* of Roy Dyckhoff
:cite:`Dyc92`, it is quite natural to code such a tactic
using the tactic language as shown on figures: :ref:`Deciding
intuitionistic propositions (1) <decidingintuitionistic1>` and
:ref:`Deciding intuitionistic propositions (2)
<decidingintuitionistic2>`. The tactic ``Axioms`` tries to conclude
using usual axioms. The tactic ``DSimplif`` applies all the reversible
rules of Dyckhoff’s system. Finally, the tactic ``TautoProp`` (the
main tactic to be called) simplifies with ``DSimplif``, tries to
conclude with ``Axioms`` and tries several paths using the
backtracking rules (one of the four Dyckhoff’s rules for the left
implication to get rid of the contraction and the right or).

For example, with ``TautoProp``, we can prove tautologies like those:

.. coqtop:: in

   Lemma tauto_ex1 : forall A B:Prop, A /\ B -> A \/ B.

.. coqtop:: in

   Proof. TautoProp. Qed.

.. coqtop:: in

   Lemma tauto_ex2 :
            forall A B:Prop, (~ ~ B -> B) -> (A -> B) -> ~ ~ A -> B.

.. coqtop:: in

   Proof. TautoProp. Qed.


Deciding type isomorphisms
~~~~~~~~~~~~~~~~~~~~~~~~~~

A more tricky problem is to decide equalities between types and modulo
isomorphisms. Here, we choose to use the isomorphisms of the simply
typed λ-calculus with Cartesian product and unit type (see, for
example, :cite:`RC95`). The axioms of this λ-calculus are given below.

.. coqtop:: in reset

   Open Scope type_scope.

.. coqtop:: in

   Section Iso_axioms.

.. coqtop:: in

   Variables A B C : Set.

.. coqtop:: in

   Axiom Com : A * B = B * A.

   Axiom Ass : A * (B * C) = A * B * C.

   Axiom Cur : (A * B -> C) = (A -> B -> C).

   Axiom Dis : (A -> B * C) = (A -> B) * (A -> C).

   Axiom P_unit : A * unit = A.

   Axiom AR_unit : (A -> unit) = unit.

   Axiom AL_unit : (unit -> A) = A.

.. coqtop:: in

   Lemma Cons : B = C -> A * B = A * C.

   Proof.

   intro Heq; rewrite Heq; reflexivity.

   Qed.

.. coqtop:: in

   End Iso_axioms.



.. _typeisomorphism1:

.. coqtop:: all

   Ltac DSimplif trm :=
            match trm with
            | (?A * ?B * ?C) =>
                rewrite <- (Ass A B C); try MainSimplif
            | (?A * ?B -> ?C) =>
                rewrite (Cur A B C); try MainSimplif
            | (?A -> ?B * ?C) =>
                rewrite (Dis A B C); try MainSimplif
            | (?A * unit) =>
                rewrite (P_unit A); try MainSimplif
            | (unit * ?B) =>
                rewrite (Com unit B); try MainSimplif
            | (?A -> unit) =>
                rewrite (AR_unit A); try MainSimplif
            | (unit -> ?B) =>
                rewrite (AL_unit B); try MainSimplif
            | (?A * ?B) =>
                (DSimplif A; try MainSimplif) || (DSimplif B; try MainSimplif)
            | (?A -> ?B) =>
                (DSimplif A; try MainSimplif) || (DSimplif B; try MainSimplif)
            end
            with MainSimplif :=
                match goal with
                | |- (?A = ?B) => try DSimplif A; try DSimplif B
                end.

.. coqtop:: all

   Ltac Length trm :=
            match trm with
            | (_ * ?B) => let succ := Length B in constr:(S succ)
            | _ => constr:(1)
            end.

.. coqtop:: all

   Ltac assoc := repeat rewrite <- Ass.


.. _typeisomorphism2:

.. coqtop:: all

   Ltac DoCompare n :=
            match goal with
            | [ |- (?A = ?A) ] => reflexivity
            | [ |- (?A * ?B = ?A * ?C) ] =>
                apply Cons; let newn := Length B in
                DoCompare newn
            | [ |- (?A * ?B = ?C) ] =>
                match eval compute in n with
                | 1 => fail
                | _ =>
                    pattern (A * B) at 1; rewrite Com; assoc; DoCompare (pred n)
                end
            end.

.. coqtop:: all

   Ltac CompareStruct :=
            match goal with
            | [ |- (?A = ?B) ] =>
                let l1 := Length A
                with l2 := Length B in
                match eval compute in (l1 = l2) with
                | (?n = ?n) => DoCompare n
                end
            end.

.. coqtop:: all

   Ltac IsoProve := MainSimplif; CompareStruct.


The tactic to judge equalities modulo this axiomatization can be
written as shown on these figures: :ref:`type isomorphism tactic (1)
<typeisomorphism1>` and :ref:`type isomorphism tactic (2)
<typeisomorphism2>`.  The algorithm is quite simple. Types are reduced
using axioms that can be oriented (this done by ``MainSimplif``). The
normal forms are sequences of Cartesian products without Cartesian
product in the left component. These normal forms are then compared
modulo permutation of the components (this is done by
``CompareStruct``). The main tactic to be called and realizing this
algorithm isIsoProve.

Here are examples of what can be solved by ``IsoProve``.

.. coqtop:: in

   Lemma isos_ex1 :
       forall A B:Set, A * unit * B = B * (unit * A).
   Proof.
   intros; IsoProve.
   Qed.

.. coqtop:: in

   Lemma isos_ex2 :
       forall A B C:Set,
         (A * unit -> B * (C * unit)) = (A * unit -> (C -> unit) * C) * (unit -> A -> B).
   Proof.
   intros; IsoProve.
   Qed.