Program execution hangs on recursive functions which should terminate

[jazullo]

Consider this interpreter for logical expressions:

data Val
  = Zero
  | One

data Ast
  = Val Val
  | Not Ast
  | Or Ast Ast
  | And Ast Ast

eval :: Ast -> Val
eval x = case x of
  Val b -> b
  Not e -> 
    case eval e of
      One -> Zero
      Zero -> One
  Or e1 e2 -> 
    case eval e1 of
      One -> One
      Zero -> eval e2
  And e1 e2 -> 
    case eval e1 of
      One -> eval e2
      Zero -> Zero

gibbon_main =
  let
    _ = printPacked (eval (Val Zero))  -- works
    _ = printPacked (eval (Val One))  -- works
    _ = printPacked (eval (Not (Val One)))  -- hangs
    _ = printPacked (eval (And (Val One) (Val One)))  -- hangs
    _ = printPacked (eval (Or (Val One) (Val One)))  -- hangs
  in ()

All expressions which contain a node with an operator will cause the evaluator to hang. Tested on issue_191.

[jazullo]

The interpreter works when Bool is used over Val

data Ast
  = Val Bool
  | Not Ast
  | Or Ast Ast
  | And Ast Ast

eval :: Ast -> Bool
eval x = case x of
  Val b -> b
  Not e -> if eval e then False else True
  Or e1 e2 -> if eval e1 then True else eval e2
  And e1 e2 -> if eval e1 then eval e2 else False

simplify :: Ast -> Ast
simplify x = Val (eval x)

gibbon_main =
  let
    _ = printPacked (simplify (Val False))
    _ = printPacked (simplify (Val True))
    _ = printPacked (simplify (Not (Val True)))
    _ = printPacked (simplify (And (Val True) (Val True)))
    _ = printPacked (simplify (Or (Val True) (Val True)))
  in ()