CODEWITHSUNDEEP
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Attila Karoly
Attila Karoly

Reputation: 1021

Prove properties of lists

My aim is to prove that certain properties of generated lists hold. For instance, a generator function produces a list of 1s, the length of the list is given as an argument; I'd like to prove that the length of the list is what the argument specifies. This is what I have so far:

Require Import List.

Fixpoint list_gen lng acc :=
match lng with
0 => acc
| S lng_1 => list_gen (lng_1) (1::acc)
end.

Lemma lm0 : length(list_gen 0 nil) = 0.
intuition.
Qed.

Lemma lm1 : forall lng:nat, length(list_gen lng nil) = lng.
induction lng.
apply lm0.

Now after applying lm0 the induction step is left:

1 subgoal
lng : nat
IHlng : length (list_gen lng nil) = lng
______________________________________(1/1)
length (list_gen (S lng) nil) = S lng

I was hoping that the proof of this step would be deduced from the code of list_gen but it's most likely a mistaken concept. How can this subgoal be proved?

Upvotes: 0

Views: 139

Answers (1)

Anton Trunov
Anton Trunov

Reputation: 15404

I would go with Daniel's approach, however a bit more general one is to write out a spec of list_gen, e.g. using non-tail-recursive repeat function:

Require Import List Arith.
Import ListNotations.

Lemma list_gen_spec : forall lng acc, list_gen lng acc = repeat 1 lng ++ acc.
Proof.
  induction lng as [| lng IH]; intros xs; simpl; trivial.
  rewrite IH.
  now rewrite app_cons_middle, repeat_singleton, repeat_app, Nat.add_comm.
Qed.

where I had to add a bunch of lemmas about repeat's interaction with some standard list functions.

Lemma repeat_singleton {A} (x : A) :
  [x] = repeat x 1.
Admitted.

Lemma repeat_app {A} (x : A) n m :
  repeat x n ++ repeat x m = repeat x (n + m).
Admitted.

Lemma app_cons {A} (x : A) xs :
  x :: xs = [x] ++ xs.
Admitted.  (* this is a convenience lemma for the next one *)

Lemma app_cons_middle {A} (y : A) xs ys :
  xs ++ y :: ys = (xs ++ [y]) ++ ys.
Admitted.

I'll leave the proofs of these lemmas as an exercise.

After proving the spec, your lemma could be proved with a few rewrites.

Lemma lm1 lng : length(list_gen lng nil) = lng.
Proof.
  now rewrite list_gen_spec, app_nil_r, repeat_length.
Qed.

Upvotes: 2

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