| -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Brian Hulley | Sent: 30 March 2006 13:14 | To: John Meacham; glasgow-haskell-users@haskell.org | Subject: Re: Possible runtime overhead of wrapping the IO monad? | | John Meacham wrote: | > On Thu, Mar 30, 2006 at 03:50:06AM +0100, Brian Hulley wrote: | >> where the intention is that the callback will take the width and | >> height of the window and return a RenderM action, the problem is | >> that because the FFI does not allow RenderM to appear in a foreign | >> type. | > | > it should, the types in foreign declarations should "see through" | > newtypes. | | Unfortunately GHC does not seem to support this: | | foreign import ccall duma_clear :: Word.Word32 -> RenderM () | | Unacceptable result type in foreign declaration: RenderM () | When checking declaration: | foreign import ccall safe "static &duma_clear" | duma_clear :: GHC.Word.Word32 -> RenderM () | | even though the FFI spec agrees with you (Section 3.2): | | The argument types ati produced by fatype must be | marshallable foreign types; that is, each ati is either (1) | a basic foreign type or (2) a type synonym or renamed | datatype of a marshallable foreign type. Moreover, the | result type rt produced by frtype must be a | marshallable foreign result type; that is, | it is either a marshallable foreign type, ... GHC does unwrap newtype result types, as in foreign import foo :: Int -> IO Boogle newtype Boogle = B Int which is what the manual meant. I'd never thought of newtyping the IO monad itself. Why are you doing that, incidentally? I guess it wouldn't be too hard to unwrap the monad type too. If it's important to you, please submit a feature request. Thanks Simon