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TypesAndMutation: a diverging term

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Adam Chlipala 2016-03-24 11:24:14 -04:00
parent ff42602069
commit f76a1055d8
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63
TypesAndMutation.v
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@ -208,6 +208,69 @@ Module References.
Hint Constructors value plug step0 step hasty heapty.
(* Perhaps surprisingly, this language admits well-typed, nonterminating
* programs! Here's an example. *)
Definition let_ (x : var) (e1 e2 : exp) :=
App (Abs x e2) e1.
Example loopy :=
let_ "r" (New (Abs "x" (Var "x")))
(let_ "_" (Write (Var "r") (Abs "x" (App (Read (Var "r")) (Var "x"))))
(App (Read (Var "r")) (Const 0))).
Theorem loopy_hasty : hasty $0 $0 loopy Nat.
Proof.
repeat (econstructor; simplify).
Qed.
Hint Resolve lookup_add_eq.
Ltac loopy := propositional; subst; simplify;
repeat match goal with
| [ x : _ * _ |- _ ] => cases x; simplify
end;
propositional; subst;
repeat match goal with
| [ H : ex _ |- _ ] => invert H; propositional; subst
end;
try match goal with
| [ H : step _ _ |- _ ] => invert H
end;
repeat match goal with
| [ H : plug _ _ _ |- _ ] => invert H
| [ H : step0 _ _ |- _ ] => invert1 H
| [ H : value _ |- _ ] => invert1 H
| [ H : ?X = Some _, H' : ?X = Some _ |- _ ] => rewrite H in H'; invert H'
end; eauto 7.
Theorem loopy_diverge : invariantFor (trsys_of loopy) (fun he => ~value (snd he)).
Proof.
(* We prove divergence (unreachability of a value) by strengthening to an
* invariant that enumerates all reachable expressions. It isn't quite a
* finite set. We need to quantify existentially over the location chosen
* for "r". *)
apply invariant_weaken with (invariant1 := fun he =>
snd he = loopy
\/ exists l,
(fst he $? l = Some (Abs "x" (Var "x")))
/\ (snd he = let_ "r" (Loc l)
(let_ "_" (Write (Var "r") (Abs "x" (App (Read (Var "r")) (Var "x"))))
(App (Read (Var "r")) (Const 0)))
\/ snd he = let_ "_" (Write (Loc l) (Abs "x" (App (Read (Loc l)) (Var "x"))))
(App (Read (Loc l)) (Const 0)))
\/ (fst he $? l = Some (Abs "x" (App (Read (Loc l)) (Var "x")))
/\ (snd he = let_ "_" (Abs "x" (App (Read (Loc l)) (Var "x")))
(App (Read (Loc l)) (Const 0))
\/ snd he = App (Read (Loc l)) (Const 0)
\/ snd he = App (Abs "x" (App (Read (Loc l)) (Var "x"))) (Const 0)))).
apply invariant_induction; simplify.
loopy.
loopy.
loopy.
Qed.
(** * Type soundness *)
Definition unstuck (he : heap * exp) := value (snd he)