Chris Kuklewicz wrote:
See "7.4.10.3 Declaration type signatures" in the manual: http://haskell.org/ghc/docs/6.4.2/html/users_guide/type-extensions.html#decl...
At the moment, the "i" in the type declaration and in the body are not the same "i".
To get the "i" from the type declaration for createMonoMultiFont into the scope of the definition, the trick is to add the otherwise useless/forbidden "forall i m." to the type declaration:
createMonoMultiFont :: forall i m.(MonadException m, Enum i, Bounded i) => [(i, Font, Colour)] -> m (MonoMultiFont i) createMonoMultiFont elements = liftIO . block $ do mmfRaw <- duma_MonoMultiFont_create (fromEnum (maxBound::i)) mmfPtr <- newForeignPtr duma_MonoMultiFont_Release mmfRaw return $ MonoMultiFont mmfPtr
The above is now accepted by ghci on my GHC 6.4.1 installation on OS X.
Thanks - I'd completely forgotten about the need to add an explicit forall, because although I read the section a while ago, the need for the forall got clouded over in my mind by the fact that the forall is unnecessary for the type signature itself, so the need for the forall seems to be just a good way to slot this extra functionality into the language while remaining backwards compatible with H98. I wonder if Haskell' will automatically make type variables scope over the function body (without an explicit forall)? Is there any reason why they shouldn't always do so? Thanks again, Brian.