Thanks Lennart -- you are quite right. That comes of sending email before having my morning coffee. Yes you can make the specialised foo', but it'll never be called -- unless you call foo on a *particular* data structure and inlining removes the MkC wrapper etc. GHC is, of course, magical, but not quite that magical. You could do a run-time type test (via class Typeable, making Typeable a superclass of C), and then you *would* be able to call the specialised foo --- but at the cost of the test. Which is, I guess, not unreasonable. Simon | -----Original Message----- | From: Lennart Augustsson [mailto:lennart@augustsson.net] | Sent: 25 November 2003 09:08 | To: Simon Peyton-Jones | Cc: Hal Daume III; GHC Users Mailing List | Subject: Re: specializing on existentially quantified data types | | I don't know how specialised code works in ghc, so I have to ask. | Does this actually work? | The type of x is never known at the foo' call site, so the specialised | version of foo' will only be called if ghc does a run time test on the | type when invoking foo'. Does it do that? | | -- Lennart | | Simon Peyton-Jones wrote: | > | class C a where ... | > | data MkC = forall a . C a => MkC a | > | | > | foo :: MkC -> ... | > | | > | and I want to specialize foo for when the 'a' in the MkC is, say, Int. | > Is | > | this possible? | > | > Not directly. But you can say | > | > foo (MkC x) = foo' x | > | > foo' :: forall a. C a => a -> ... | > foo' x = ... | > {-# SPECIALISE foo' :: Int -> ... #-} | > | > There's no built-in mechanism I'm afraid. | > | > Simon | > | > _______________________________________________ | > Glasgow-haskell-users mailing list | > Glasgow-haskell-users@haskell.org | > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users | > | |