data Lit i = Lit i;
data Succ t = Succ t;
data IsZero t = IsZero t;
data If tc tt te = If tc tt te;

class Term a where {
  type Result a;
}

instance Term Bool where {
  type Result Bool = Bool;
}

instance Term (Lit Int) where {
  type Result (Lit Int) = Int;
}

instance Term a, Result a = Int => Term (Succ a) where {
  type Result (Succ a) = Int; 
}

instance Term a, Result a = Int => Term (IsZero a) where {
  type Result (IsZero a) = Bool;
}

instance Term a, Term b, Term c,
         Result a = Result b, Result c = Bool 
         => Term (If c a b) where {
  type Result (If c a b) = Result a;
}

class Term a => Eval a where {
  eval :: a -> Result a;
}

instance Eval Bool where {
  eval b = b;
}

instance Eval (Lit Int) where {
  eval x = case x of Lit i -> i;;
}

instance Eval a, Result a = Int => Eval (Succ a) where {
  eval x = case x of Succ i -> eval i + 1;;
}

instance Eval a, Result a = Int => Eval (IsZero a) where {
  eval x = case x of IsZero i -> eval i == 0;;
}

instance Eval a, Eval b, Eval c, 
         Result a = Result b, Result c = Bool 
         => Eval (If c a b) where {
  eval x = case x of If c t1 t2 -> if eval c then eval t1 else eval t2;;
}

foo n = eval (If (IsZero (Succ n)) (Lit 42) n);
main = foo (Lit (-1));