Also inspired by Ralf Hinze's post, I thought of removing GADTs from that code. The result is Haskell98! code, which works well in Hugs. The code seems to be a bit simpler too. Like the original code, the function 'parseAny' correctly discriminates between the list of characters (i.e., strings) and the list of other things. {-- Haskell98! --} class Type a where parse :: ReadS a newtype Str = Str{unStr :: String} instance Type Char where parse = reads instance Type Int where parse = reads instance Type Str where parse = Str <$> reads instance Type a => Type [a] where parse = parseList parse instance Type Value where parse = parseAny data Value = ValChar Char | ValInt Int | ValString String | ValList [Value] deriving (Show) parseAny = ValChar <$> parse <+> ValInt <$> parse <+> ValString <$> (unStr <$> parse) <+> ValList <$> parse -- The following is Ralf Hinze's code verbatim parseList parsea = readParen False (\ s -> [ xs | ("[", t) <- lex s, xs <- parsel t ]) where parsel s = [ ([], t) | ("]", t) <- lex s ] ++ [ (x : xs, u) | (x, t) <- parsea s, (xs, u) <- parsel' t ] parsel' s = [ ([], t) | ("]", t) <- lex s ] ++ [ (x : xs, v) | (",", t) <- lex s, (x, u) <- parsea t, (xs, v) <- parsel' u] infix 8 <$> infixr 6 <+> (f <$> p) s = [ (f a, t) | (a, t) <- p s ] (p <+> q) s = p s ++ q s Tests:
hugs /tmp/h.hs Haskell 98 mode: Restart with command line option -98 to enable extensions
Main> parseAny "4711" [(ValInt 4711,"")] Main> parseAny "\"4711\"" [(ValString "4711","")] Main> parseAny "[4711, 0]" [(ValList [ValInt 4711,ValInt 0],"")] Main> parseAny "[4711, 'a']" [(ValList [ValInt 4711,ValChar 'a'],"")] Main> parseAny "[\"hello world\"] x" [(ValList [ValString "hello world"]," x")]