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.
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