On Tue, Jun 9, 2009 at 7:21 AM, Neil Brown<nc...@kent.ac.uk> wrote:
> data Page a =
> Page {pageName :: IORef String
> ,pageId :: Int
> ,pageBuffer :: a
> ,pageBox :: VBox
> }
>
> class PageBuffer a where
> pageBufferClone :: a -> IO (a, VBox)
>
> pageClone :: PageBuffer a => Page a -> IO (Page a)
> pageClone page = do
> -- Get common information for clone page.
> name <- pageGetName page
> let id = pageId page
> pb = pageBuffer page
>
> -- Get clone information for dynamic interface. (pBuffer, pBox) <-
> pageBufferClone pb
>
> -- Return clone page.
> pageNewInternal name id pBuffer pBox
Actually you can avoid the type parameter on "a" using an existential:
> {-# LANGUAGE ExistentialQuantification #-}
> data Page = forall a. PageBuffer a =>
> Page {pageName :: IORef String
> ,pageId :: Int
> ,pageBuffer :: a
> ,pageBox :: VBox
> }
Now you can still use [Page]. You can't do "pageBuffer p", though,
you'll get this fun error message:
Cannot use record selector `pageBuffer' as a function due to
escaped type variables
Probable fix: use pattern-matching syntax instead
Instead you need to do
case p of Page{pageBuffer = x} -> ...something with x...
This will bring the PageBuffer context into scope, inside of the case statement.
-- ryan
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
