From mininix Require Export res lambda.operational_props.
From stdpp Require Import options.

Module Import lambda.
Export lambda.

Inductive thunk :=
  Thunk { thunk_env : gmap string thunk; thunk_expr : expr }.
Add Printing Constructor thunk.
Notation env := (gmap string thunk).

Inductive val :=
  | VString (s : string)
  | VClo (x : string) (E : env) (e : expr).

Global Instance maybe_VClo : Maybe3 VClo := λ v,
  if v is VClo x E e then Some (x, E, e) else None.

Definition interp1 (interp : env → expr → res val) (E : env) (e : expr) : res val :=
  match e with
  | EString s =>
      mret (VString s)
  | EId x =>
      t ← Res (E !! x);
      interp (thunk_env t) (thunk_expr t)
  | EAbs x e =>
      mret (VClo x E e)
  | EApp e1 e2 =>
      v1 ← interp E e1;
      '(x, E', e') ← Res (maybe3 VClo v1);
      interp (<[x:=Thunk E e2]> E') e'
  end.

Fixpoint interp (n : nat) (E : env) (e : expr) : res val :=
  match n with
  | O => NoFuel
  | S n => interp1 (interp n) E e
  end.

Global Opaque interp.

End lambda.

Add Printing Constructor lambda.thunk.