juvix/tests/positive/issue3064.juvix

module issue3064;

import Stdlib.Prelude open;

type Expression :=
  -- Put both of these into a Const type
  | Const Nat
  | Str String
  | Var String
  | Lam String Expression
  | App Expression Expression;

axiom undefined : {A : Type} -> A;

Environment : Type := List (Pair String Expression) ;

type Return :=
  | RNatural Nat
  | RString String;

terminating eval (env : Environment) : Expression -> Maybe Return
  | (Const x) := RNatural x          |> just
  | (Str str) := RString str         |> just
  | (App f x) := eval-lookup env f x
  | (Var var) := lookup-var env var >>= eval env
  | _ := undefined;

eval-lookup (env : Environment) (f : Expression) (x : Expression) : Maybe Return :=
  let recursive : _ -- Expression -> Return
      | (Lam variable body) := eval ((variable , x) :: env) body
      | _ := undefined;
  in recursive f;

lookup-var (env : Environment) (to-lookup : String) : Maybe Expression
  := (snd <$> find \{ x := fst x == to-lookup } env);