Mutual well-founded recursion using dependent pattern-matching

We present a simple encoding of mutual recursion through the use of dependent pattern-matching on a GADT-like representation of the functions prototypes or just using strong elimination on an enumerated type. We use a simple toy measure here to justify termination, but more elaborate well-founded relations can be used as well.

Set Equations Transparent.

Import Sigma_Notations.

We first declare the prototypes ouf our mutual definitions.
Set Universe Polymorphism.
Inductive ty : (A : Type) (P : A Type), Set :=
| ty0 : ty nat (fun _nat)
| ty1 : ty (list nat) (fun _bool).
Derive Signature NoConfusion for ty.

Our measure is simple, just the natural number or length of the list argument.
Equations measure : (Σ A P (_ : A), ty A P) nat :=
  measure (_, _, a, ty0)a;
  measure (_, _, a, ty1)length a.

Definition rel := Program.Wf.MR lt measure.

#[local] Instance: WellFounded rel.
  red. apply Wf.measure_wf. apply Wf_nat.lt_wf.

Definition pack {A} {P} (x : A) (t : ty A P) := (A, P, x, t) : (Σ A P (_ : A), ty A P).

We define the function by recursion on the abstract packed argument. Using dependent pattern matching, the clauses for ty0 refine the argument and return type to nat and similarly for ty1, we can hence do pattern-matching as usual on each separate definition.
Equations? double_fn {A} {P} (t : ty A P) (x : A) : P x by wf (pack x t) rel :=
  double_fn ty0 n := n + 0;
  double_fn ty1 nil := true;
  double_fn ty1 (x :: xs) := 0 <? length xs + double_fn ty0 (length xs).
It is easily shown terminating in this case.
Proof. red. red. cbn. auto with arith. Qed.

We can define auxilliary definition to select the functions we want and express unfolding lemmas in terms of these abbreviations.
Definition fn0 := double_fn ty0.
Definition fn1 := double_fn ty1.

Lemma fn0_unfold n : fn0 n = n + 0.
  unfold fn0. now simp double_fn.

Lemma fn1_unfold l : fn1 l = match l with niltrue | x :: xs ⇒ 0 <? length xs + fn0 (length xs) end.
  unfold fn1; simp double_fn. destruct l; now simp double_fn.

Well-founded nested recursion

The following example uses just dependent elimination on a finite type (booleans) and shows that this also applies to nested recursive definitions.
We first define list_size and rose trees

Section list_size.
  Context {A : Type} (f : A nat).
  Equations list_size (l : list A) : nat :=
  list_size nil := 0;
  list_size (cons x xs) := S (f x + list_size xs).

End list_size.
Transparent list_size.

Section RoseMut.
  Context {A : Set}.

  Inductive t : Set :=
  | leaf (a : A) : t
  | node (l : list t) : t.
  Derive NoConfusion for t.

  Equations size (r : t) : nat :=
  size (leaf _) := 0;
  size (node l) := S (list_size size l).

An alternative way to define mutual definitions on nested types
  Equations mutmeasure (b : bool) (arg : if b then t else list t) : nat :=
  mutmeasure true t := size t;
  mutmeasure false lt := list_size size lt.

The argument and return type depend on the function label (true or false here) and any well-founded recursive call is allowed.
  Equations? elements (b : bool) (x : if b then t else list t) : if b then list A else list A
    by wf (mutmeasure b x) lt :=
  elements true (leaf a) := [a];
  elements true (node l) := elements false l;
  elements false nil := nil;
  elements false (cons t ts) := elements true t ++ elements false ts.
  Proof. all:lia. Qed.

Dependent return types are trickier but possible:
  Equations? elements_dep (b : bool) (x : if b then t else list t) :
    (if b as b' return (if b' then t else list t) Set then fun x : tlist A else fun x : list tlist A) x
    by wf (mutmeasure b x) lt :=
  elements_dep true (leaf a) := [a];
  elements_dep true (node l) := elements_dep false l;
  elements_dep false nil := nil;
  elements_dep false (cons t ts) := elements_dep true t ++ elements_dep false ts.
  Proof. all:lia. Qed.

End RoseMut.

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