Re: [Haskell-cafe] How to improve below code?

9 Jun 2009

      On Tue, Jun 9, 2009 at 7:21 AM, Neil Brown 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

Re: [Haskell-cafe] How to improve below code?

Ryan Ingram