| > data Wonky f | > = Wonky | > | Manky (f (Wonky f)) | > deriving Show | | The trouble is that when I ask either hugs -98 or ghci | -fglasgow-exts to | | show (Wonky :: Wonky Copy) | | the poor compiler's brain explodes. I fixed this a few weeks ago. GHC (5.03) now says: Foo.hs:3: No instance for `Show (f (Wonky f))' When deriving the `Show' instance for type `Wonky' | I tried to guess the type of the show instance derived for | Wonky. Being a naive sort of chap, I thought it might be | | show :: (forall a. Show a => Show (f a)) => Wonky f -> String Not naive. That's exactly the right type. See Section 7 of "Derivable type classes". http://research.microsoft.com/~simonpj/Papers/derive.htm Havn't implemented this, yet, alas. | It's clear that with typing problems, inference becomes | unsustainable pretty soon after you leave the safe harbours | of the Hindley-Milner system. However, lots of lovely | programs have more interesting types: it would be very | frustrating if Haskell forbade these programs just because | their types were not inferrable---not least since, for these | jobs, we usually do think of the type first and the code | afterwards. Sensibly, Haskell allows us to write these types | down, so the machine's task is merely checking. This hybrid | approach preserves type inference for `old' code, whilst | promoting more adventurous programming by allowing us to say | what we mean when the machine is too dim to guess. I agree wholeheartedly with this; it's exactly the approach I'm trying to take with GHC. One obvious extension is to let the user specify the context for a derived instance decl, but still let the compiler generate the code. Havn't done this either! Simon