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-type-sigs
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.
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