Library Coq.Lists.ListDec

Decidability results about lists

Require Import List Decidable.
Set Implicit Arguments.

Definition decidable_eq A := forall x y:A, decidable (x=y).

Section Dec_in_Prop.
Variables (A:Type)(dec:decidable_eq A).

Lemma In_decidable x (l:list A) : decidable (In x l).

Lemma incl_decidable (l l':list A) : decidable (incl l l').

Lemma NoDup_decidable (l:list A) : decidable (NoDup l).

End Dec_in_Prop.

Section Dec_in_Type.
Variables (A:Type)(dec : forall x y:A, {x=y}+{x<>y}).

Definition In_dec := List.In_dec dec.
Lemma incl_dec (l l':list A) : {incl l l'}+{~incl l l'}.

Lemma NoDup_dec (l:list A) : {NoDup l}+{~NoDup l}.

End Dec_in_Type.

An extra result: thanks to decidability, a list can be purged from redundancies.

Lemma uniquify_map A B (d:decidable_eq B)(f:A->B)(l:list A) :
 exists l', NoDup (map f l') /\ incl (map f l) (map f l').

Lemma uniquify A (d:decidable_eq A)(l:list A) :
 exists l', NoDup l' /\ incl l l'.