> 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:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED]
http://www.andres-loeh.de
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