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

22 May 2006

      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.
Here is a gross hack that compiles:

createMonoMultiFont elements =
      liftIO . block $ do
          let
              (onei,_,_):_ = (undefined, undefined, undefined) : elements
              upper :: Bounded a => a -> a
              upper _ = maxBound
              last = upper onei

          mmfRaw <- duma_MonoMultiFont_create (fromEnum last)
          -- adding of elements to mmfRaw ommitted for clarity
          mmfPtr <- newForeignPtr duma_MonoMultiFont_Release mmfRaw
          return $ MonoMultiFont mmfPtr

Still, it would be nice to be able to just write (maxBound::i) since the 
dictionary for i is present in the result.

Brian.