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// RUN: %dafny /compile:0 /dprint:"%t.dprint" "%s" > "%t"
// RUN: %diff "%s.expect" "%t"
datatype List<X> = Nil | Cons(Node<X>, List<X>);
datatype Node<X> = Element(X) | Nary(List<X>);
function FlattenMain<M>(list: List<M>): List<M>
ensures IsFlat(FlattenMain(list));
{
Flatten(list, Nil)
}
function Flatten<X>(list: List<X>, ext: List<X>): List<X>
requires IsFlat(ext);
ensures IsFlat(Flatten(list, ext));
{
match list
case Nil => ext
case Cons(n, rest) =>
match n
case Element(x) => Cons(n, Flatten(rest, ext))
case Nary(nn) => Flatten(nn, Flatten(rest, ext))
}
function IsFlat<F>(list: List<F>): bool
{
match list
case Nil => true
case Cons(n, rest) =>
match n
case Element(x) => IsFlat(rest)
case Nary(nn) => false
}
function ToSeq<X>(list: List<X>): seq<X>
{
match list
case Nil => []
case Cons(n, rest) =>
match n
case Element(x) => [x] + ToSeq(rest)
case Nary(nn) => ToSeq(nn) + ToSeq(rest)
}
ghost method Theorem<X>(list: List<X>)
ensures ToSeq(list) == ToSeq(FlattenMain(list));
{
Lemma(list, Nil);
}
ghost method Lemma<X>(list: List<X>, ext: List<X>)
requires IsFlat(ext);
ensures ToSeq(list) + ToSeq(ext) == ToSeq(Flatten(list, ext));
{
match (list) {
case Nil =>
case Cons(n, rest) =>
match (n) {
case Element(x) =>
Lemma(rest, ext);
case Nary(nn) =>
Lemma(nn, Flatten(rest, ext));
Lemma(rest, ext);
}
}
}
// ---------------------------------------------
function NegFac(n: int): int
decreases -n;
{
if -1 <= n then -1 else - NegFac(n+1) * n
}
ghost method LemmaAll()
ensures forall n :: NegFac(n) <= -1; // error: induction heuristic does not give a useful well-founded order, and thus this fails to verify
{
}
ghost method LemmaOne(n: int)
ensures NegFac(n) <= -1; // error: induction heuristic does not give a useful well-founded order, and thus this fails to verify
{
}
ghost method LemmaAll_Neg()
ensures forall n :: NegFac(-n) <= -1; // error: fails to verify because of the minus in the trigger
{
}
ghost method LemmaOne_Neg(n: int)
ensures NegFac(-n) <= -1; // error: fails to verify because of the minus in the trigger
{
}
ghost method LemmaOneWithDecreases(n: int)
ensures NegFac(n) <= -1; // here, the programmer gives a good well-founded order, so this verifies
decreases -n;
{
}
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