Hello,
There's a prelude function called asTypeOf which might help here:
asTypeOf :: a -> a -> a
asTypeOf = const
You can use maxBound `asTypeOf` (fst3 . head $ elements)
Another option is GHC's scoped type variables, which you use by adding
a type signature to your pattern. See:
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html...
- Cale
On 22/05/06, Brian Hulley
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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