but if f is exported, that's probably not what you want. And if you give a type sig to f, f:: forall a. C a => a -> Maybe a the type sig will say it's polymorphic, while the improvement rule will say that a must be Int. Simon | -----Original Message----- | From: André Pang [mailto:ozone@algorithm.com.au] | Sent: 12 August 2004 14:52 | To: Simon Peyton-Jones | Cc: MR K P SCHUPKE; haskell-cafe@haskell.org; qrczak@knm.org.pl | Subject: Re: [Haskell-cafe] closed classes [was: Re: exceptions vs. Either] | | On 12/08/2004, at 11:05 PM, Simon Peyton-Jones wrote: | | > module M where | > | > class C a where | > op :: a -> a | > | > instance C Int where | > op x = x+1 | > | > f x = Just (op x) | > | > Under your proposal, I'd infer f :: Int -> Maybe Int, on the grounds | > that C is closed and there is only one instance. | | If I'm reading Keean's posts right, that's exactly his point: if you | only have one instance of class C, then it's valid to improve f's type | to :: Int -> Maybe Int, right? | | If, on the other hand, you had another instance (e.g. instance C Bool), | then the signature of f would have to remain polymorphic. | | | -- | % Andre Pang : trust.in.love.to.save