[Haskell-cafe] Problem trying to get class Bounded to work

[Chris Kuklewicz]

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.

-- 
Chris


Brian Hulley wrote:
> Hi -
> I've got the following function which doesn't compile:
> 
>  createMonoMultiFont
>      :: (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 problem is that ghc complains that there is no instance for Bounded
> i even though I've written "Bounded i" in the type declaration for the
> function.
> 
> I was expecting that I could use the same type variables in the function
> body as in the type signature so that I could use maxBound::i to find
> the max bound of whatever type is instantiated to i.
> 
> What am I doing wrong here, and is it possible to create such a  function?
> 
> Thanks, Brian.
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