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#scoped-type-variables
- Cale
On 22/05/06, Brian Hulley <[EMAIL PROTECTED]> wrote:
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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