data I f   = I f;
data C f   = C f;
data S f   = S (List Char) f;

class Format fmt where {
  type Sprintf fmt;
  sprintf' :: List Char -> fmt -> Sprintf fmt;
}

instance Format (List Char) where {
  type Sprintf (List Char) = List Char;
  sprintf' prefix str = prefix ++ str;
}

instance Format a => Format (I a) where {
  type Sprintf (I a) = Int -> Sprintf a;
  sprintf' prefix i = case i of I a -> (\j -> sprintf' (prefix ++ show j) a);;
}

instance Format a => Format (C a) where {
  type Sprintf (C a) = Char -> Sprintf a;
  sprintf' prefix c = case c of C a -> (\d -> sprintf' (prefix ++ show d) a);;
}

instance Format a => Format (S a) where {
  type Sprintf (S a) = Sprintf a;
  sprintf' prefix s = case s of S str a -> sprintf' (prefix ++ str) a;;
}

sprintf :: Format fmt => fmt -> Sprintf fmt;
sprintf = sprintf' "";

formatSpec = S "Int: " (I (S ", Char: " (C ".")));

main = sprintf formatSpec 42 'S';