[Haskell-cafe] Why this exception occurs ?

[Albert Y. C. Lai]

Donald Bruce Stewart wrote:
> instance Num String anyone? Mwhaha
>   
addString xs ys = add 0 xs ys
    where
      m = fromEnum (maxBound :: Char) + 1
      alu c x y =
          let s = c + fromEnum x + fromEnum y
          in if s >= m then (1, s-m) else (0, s)
      add c (x:xs) (y:ys) = case alu c x y of (c', s') -> toEnum s' : add c' xs ys
      add c [] (y:ys) = case alu c 0 y of (0, s') -> toEnum s' : ys
                                          (1, s') -> toEnum s' : add 1 [] ys
      add c xs@(_:_) [] = add c [] xs
      add c [] [] = if c==0 then [] else [toEnum c]