Can anyone spot the fault in the derived instance of Read produced by DrIFT?
Regards, Malcolm
data List a = Nil | Cons { hd :: a, tl :: List a }
{-* Generated by DrIFT-v1.0 : Look, but Don't Touch. *-} instance (Show a) => Show (List a) where showsPrec d (Nil) = showString "Nil" showsPrec d (Cons aa ab) = showParen (d >= 10) (showString "Cons" . showChar '{' . showString "hd" . showChar '=' . showsPrec 10 aa . showChar ',' . showString "tl" . showChar '=' . showsPrec 10 ab . showChar '}')
instance (Read a) => Read (List a) where readsPrec d input = (\ inp -> [((Nil) , rest) | ("Nil" , rest) <- lex inp]) input ++ readParen (d > 9) (\ inp -> [((Cons aa ab) , rest) | ("Cons" , inp) <- lex inp , ("{" , inp) <- lex inp , ("hd" , inp) <- lex inp , ("=" , inp) <- lex inp , (aa , inp) <- readsPrec 10 inp , ("," , inp) <- lex inp , ("tl" , inp) <- lex inp , ("=" , inp) <- lex inp , (ab , inp) <- readsPrec 10 inp , ("}" , rest) <- lex inp]) input
Since the Report states that the derived Show instances only insert parentheses where needed and that the derived Read instances can read the output produced by show, I would suggest to change all occurences of {showsPrec 10,readsPrec 10} to {showsPrec 0,readsPrec 0} in the above code. Best, Andres -- Andres Loeh, Universiteit Utrecht mailto:andres@cs.uu.nl mailto:mail@andres-loeh.de http://www.andres-loeh.de