How to make sublists in Coq? - coq

Question

I'm working in Coq and trying to figure out how to do the next thing: If I have a list of natural numbers and a given number n, I want to break my list in what goes before and after each of the n's. To make it clearer, if I have the list [1; 2; 0; 3; 4; 0; 9] and the number n = 0, then I want to have as output the three lists: [1;2], [3;4] and [9]. The main problem I have is that I don't know how to output several elements on a Fixpoint. I think I need to nest Fixpoints but I just don't see how. As a very raw idea with one too many issues I have:
Fixpoint SubLists (A : list nat)(m : nat) :=
match A with
|[] => []
|n::A0 => if n =? m then (SubLists L) else n :: (SubLists L)
end.
I would very much appreciate your input on how to do this, and how to navigate having an output of several elements.

Answer 1

You can do this by combining a few fixpoints:
Require Import Coq.Arith.Arith.
Require Import Coq.Lists.List.
Import ListNotations.
Fixpoint prefix n l :=
match l with
| [] => []
| m :: l' => if beq_nat n m then []
else m :: prefix n l'
end.
Fixpoint suffix n l :=
match l with
| [] => l
| m :: l' => if beq_nat n m then l'
else suffix n l'
end.
Fixpoint split_at n l :=
match l with
| [] => []
| m :: l' => prefix n (m :: l') :: split_at n (suffix n (m :: l'))
end.
Notice that Coq's termination checker accepts the recursive call to split_at, even though it is not done syntactically a subterm of l. The reason for that is that it is able to detect that suffix only outputs subterms of its argument. But in order for this to work, we must return l, and not [] on its first branch (try changing it to see what happens!).

Answer 2

In addition to Arthur's solution, you can use an accumulator, which is typical of Functional Programming style:
Require Import Coq.Arith.Arith.
Require Import Coq.Lists.List.
Import ListNotations.
Definition add_acc m (s : list (list nat)) :=
match s with
| [] => [[m]]
| s :: ss => (m :: s) :: ss
end.
Fixpoint split_seq n l acc :=
match l with
| [] => map (#rev _) (rev acc)
| m :: l' => if beq_nat n m then
split_seq n l' ([] :: acc)
else
split_seq n l' (add_acc m acc)
end.
Compute (split_seq 0 [1; 2; 0; 3; 4; 0; 9] []).
Note that the result is reversed so you need to use rev. A bonus exercise is to improve this.
EDIT: Provided second variant that doesn't add [] for repeated separators.
Definition reset_acc (s : list (list nat)) :=
match s with
| [] :: ss => [] :: ss
| ss => [] :: ss
end.
Fixpoint split_seq_nodup n l acc :=
match l with
| [] => map (#rev _) (rev acc)
| m :: l' => if beq_nat n m then
split_seq_nodup n l' (reset_acc acc)
else
split_seq_nodup n l' (add_acc m acc)
end.
Compute (split_seq_nodup 0 [1; 2; 0; 3; 4; 0; 9] []).

Answer 3

An alternative way to tackle this issue is to formally describe the problem you are trying to solve and then either write a dependently-typed function proving that this problem can indeed be solved or using tactics to slowly build up your proof.
This is, if I am not mistaken, a relation describing the relationship between the outputs n and ns you want to pass your function and the output mss you want to get back.
The (* ------- *) lines are simple comments used to suggest that these constructors should be seen as inference rules: whatever is under one such line is the conclusion one can make based on the assumptions above it.
Inductive SubListsRel (n : nat) : forall (ns : list nat)
(mss : list (list nat)), Prop :=
| base : SubListsRel n nil (nil :: nil)
| consEq : forall ns m mss,
n = m -> SubListsRel n ns mss ->
(* ----------------------------- *)
SubListsRel n (m :: ns) (nil :: mss)
| consNotEq : forall ns m ms mss,
(n <> m) -> SubListsRel n ns (ms :: mss) ->
(* ------------------------------------------------- *)
SubListsRel n (m :: ns) ((m :: ms) :: mss)
.
We can then express your Sublists problem as being, given inputs n and ns, the existence of an output mss such that SubListsRel n ns mss holds:
Definition SubLists (n : nat) (ns : list nat) : Set :=
{ mss | SubListsRel n ns mss }.
Using tactics we can readily generate such Sublists for concrete examples in order to sanity-check our specification. We can for instance take the example you had in your original post:
Example example1 : SubLists 0 (1 :: 2 :: 0 :: 3 :: 4 :: 0 :: 9 :: nil).
Proof.
eexists ; repeat econstructor ; intro Hf; inversion Hf.
Defined.
And check that the output is indeed the list you were expecting:
Check (eq_refl : proj1_sig example1
= ((1 :: 2 :: nil) :: (3 :: 4 :: nil) :: (9 :: nil) :: nil)).
Now comes the main part of this post: the proof that forall n ns, SubLists n ns. Given that the premise of consNotEq assumes that mss is non-empty, we will actually prove a strengthened statement in order to make our life easier:
Definition Strenghtened_SubLists (n : nat) (ns : list nat) : Set :=
{ mss | SubListsRel n ns mss /\ mss <> nil }.
And given that oftentimes we will have goals of the shape something_absurd -> False, I define a simple tactic to handle these things. It introduces the absurd assumption and inverts it immediately to make the goal disappear:
Ltac dismiss := intro Hf; inversion Hf.
We can now prove the main statement by proving the strengthened version by induction and deducing it. I guess that here it's better for you to step through it in Coq rather than me trying to explain what happens. The key steps are the cut (proving a stronger statement), induction and the case analysis on eq_nat_dec.
Lemma subLists : forall n ns, SubLists n ns.
Proof
intros n ns; cut (Strenghtened_SubLists n ns).
- intros [mss [Hmss _]]; eexists; eassumption.
- induction ns.
+ eexists; split; [econstructor | dismiss].
+ destruct IHns as [mss [Hmss mssNotNil]];
destruct (eq_nat_dec n a).
* eexists; split; [eapply consEq ; eassumption| dismiss].
* destruct mss; [apply False_rect, mssNotNil; reflexivity |].
eexists; split; [eapply consNotEq; eassumption| dismiss].
Defined.
Once we have this function, we can come back to our example and generate the appropriate Sublists this time not by calling tactics but by running the function subLists we just defined.
Example example2 : SubLists 0 (1 :: 2 :: 0 :: 3 :: 4 :: 0 :: 9 :: nil) :=
subLists _ _.
And we can Check that the computed list is indeed the same as the one obtained in example1:
Check (eq_refl : proj1_sig example1 = proj1_sig example2).
Nota Bene: It is paramount here that our proofs are ended with Defined rather than Qed in order for them to be unfolded when computing with them (which is what we want to do here: they give us the list (list nat) we are looking for!).
A gist with all the code and the right imports.

Answer 4

Here is another take, based on the standard library function List.fold_left.
It works by maintaining an accumulator, which is a pair of the overall reversed result (a list of lists) and a current sublist (also reversed while accumulating). Once we reach a delimiter, we reverse the current sublist and put it into the resulting list of sublists. After executing fold_left, we reverse the result in the outermost match expression.
Require Import Coq.Arith.Arith.
Require Import Coq.Lists.List.
Import ListNotations.
Definition split_skip_dup_delims (m : nat) (xs : list nat) :=
match fold_left
(fun (acctup: _ * _) x => let (acc, rev_subl) := acctup in
if beq_nat x m
then match rev_subl with (* a delimiter found *)
| [] => (acc, []) (* do not insert empty sublist *)
| _ => (rev rev_subl :: acc, []) end
else (acc, x :: rev_subl)) (* keep adding to the current sublist *)
xs
([],[]) with
| (acc, []) => rev acc (* list ends with a delimiter *)
| (acc, rev_subl) => rev (rev rev_subl :: acc) (* no delimiter at the end *)
end.
Eval compute in split_skip_dup_delims 0 [1; 2; 0; 0; 0; 3; 4; 0; 9].
(* = [[1; 2]; [3; 4]; [9]]
: list (list nat) *)