Commit 9e429c46 authored by Robbert Krebbers's avatar Robbert Krebbers

Make sure that the fixpoints are non-expansive.

In order to do that, we need to quantify over non-expansive
predicates instead of arbitrary predicates.
parent c01717fb
......@@ -5,20 +5,26 @@ Import uPred.
(** Least and greatest fixpoint of a monotone function, defined entirely inside
the logic. *)
Class BIMonoPred {M A} (F : (A uPred M) (A uPred M)) :=
bi_mono_pred Φ Ψ : (( x, Φ x - Ψ x) x, F Φ x - F Ψ x)%I.
Class BIMonoPred {M} {A : ofeT} (F : (A uPred M) (A uPred M)) := {
bi_mono_pred Φ Ψ : (( x, Φ x - Ψ x) x, F Φ x - F Ψ x)%I;
bi_mono_pred_ne Φ : NonExpansive Φ NonExpansive (F Φ)
}.
Arguments bi_mono_pred {_ _ _ _} _ _.
Local Existing Instance bi_mono_pred_ne.
Definition uPred_least_fixpoint {M A} (F : (A uPred M) (A uPred M))
(x : A) : uPred M :=
( Φ, ( x, F Φ x Φ x) Φ x)%I.
Definition uPred_least_fixpoint {M} {A : ofeT}
(F : (A uPred M) (A uPred M)) (x : A) : uPred M :=
( Φ : A -n> uPredC M, ( x, F Φ x Φ x) Φ x)%I.
Definition uPred_greatest_fixpoint {M A} (F : (A uPred M) (A uPred M))
(x : A) : uPred M :=
( Φ, ( x, Φ x F Φ x) Φ x)%I.
Definition uPred_greatest_fixpoint {M} {A : ofeT}
(F : (A uPred M) (A uPred M)) (x : A) : uPred M :=
( Φ : A -n> uPredC M, ( x, Φ x F Φ x) Φ x)%I.
Section least.
Context {M A} (F : (A uPred M) (A uPred M)) `{!BIMonoPred F}.
Context {M} {A : ofeT} (F : (A uPred M) (A uPred M)) `{!BIMonoPred F}.
Global Instance least_fixpoint_ne : NonExpansive (uPred_least_fixpoint F).
Proof. solve_proper. Qed.
Lemma least_fixpoint_unfold_2 x : F (uPred_least_fixpoint F) x uPred_least_fixpoint F x.
Proof.
......@@ -30,8 +36,8 @@ Section least.
Lemma least_fixpoint_unfold_1 x :
uPred_least_fixpoint F x F (uPred_least_fixpoint F) x.
Proof.
iIntros "HF". iApply "HF". iIntros "!#" (y) "Hy".
iApply bi_mono_pred; last done. iIntros "!#" (z) "?".
iIntros "HF". iApply ("HF" $! (CofeMor (F (uPred_least_fixpoint F))) with "[#]").
iIntros "!#" (y) "Hy". iApply bi_mono_pred; last done. iIntros "!#" (z) "?".
by iApply least_fixpoint_unfold_2.
Qed.
......@@ -41,13 +47,18 @@ Section least.
apply (anti_symm _); auto using least_fixpoint_unfold_1, least_fixpoint_unfold_2.
Qed.
Lemma least_fixpoint_ind (Φ : A uPred M) :
Lemma least_fixpoint_ind (Φ : A uPred M) `{!NonExpansive Φ} :
( y, F Φ y Φ y) x, uPred_least_fixpoint F x Φ x.
Proof. iIntros "#HΦ" (x) "HF". iApply "HF". done. Qed.
Proof.
iIntros "#HΦ" (x) "HF". by iApply ("HF" $! (CofeMor Φ) with "[#]").
Qed.
End least.
Section greatest.
Context {M A} (F : (A uPred M) (A uPred M)) `{!BIMonoPred F}.
Context {M} {A : ofeT} (F : (A uPred M) (A uPred M)) `{!BIMonoPred F}.
Global Instance greatest_fixpoint_ne : NonExpansive (uPred_greatest_fixpoint F).
Proof. solve_proper. Qed.
Lemma greatest_fixpoint_unfold_1 x :
uPred_greatest_fixpoint F x F (uPred_greatest_fixpoint F) x.
......@@ -61,7 +72,7 @@ Section greatest.
Lemma greatest_fixpoint_unfold_2 x :
F (uPred_greatest_fixpoint F) x uPred_greatest_fixpoint F x.
Proof.
iIntros "HF". iExists (F (uPred_greatest_fixpoint F)).
iIntros "HF". iExists (CofeMor (F (uPred_greatest_fixpoint F))).
iIntros "{$HF} !#" (y) "Hy". iApply (bi_mono_pred with "[] Hy").
iIntros "!#" (z) "?". by iApply greatest_fixpoint_unfold_1.
Qed.
......@@ -72,7 +83,7 @@ Section greatest.
apply (anti_symm _); auto using greatest_fixpoint_unfold_1, greatest_fixpoint_unfold_2.
Qed.
Lemma greatest_fixpoint_coind (Φ : A uPred M) :
Lemma greatest_fixpoint_coind (Φ : A uPred M) `{!NonExpansive Φ} :
( y, Φ y F Φ y) x, Φ x uPred_greatest_fixpoint F x.
Proof. iIntros "#HΦ" (x) "Hx". iExists Φ. by iIntros "{$Hx} !#". Qed.
Proof. iIntros "#HΦ" (x) "Hx". iExists (CofeMor Φ). by iIntros "{$Hx} !#". Qed.
End greatest.
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