You have a couple problems here.
The first is that GHC has no idea what particular type 'w' widgetList
has, because the empty list is polymorphic.
The second is that it looks like you probably want a heterogeneous
list of widgets -- that is, possibly different types of widget as long
as they all conform to Widget. To do this you'll need
ExistentialQuantification (or GADTs I guess?).
For example:
{-# LANGUAGE ExistentialQuantification #-}
class Widget w where
widgetRun :: w -> IO ()
data SomeWidget = forall w. Widget w => SomeWidget w
widgetList :: [(Integer, Integer, SomeWidget)]
widgetList = []
main = mapM aux widgetList
aux (x, y, sw) =
case sw of
SomeWidget w -> widgetRun w
Note that the type variable for widgetList 'w' has disappeared.
Before, with the type variable 'w', all elements of the widgetList had
to be of the same type (lists being homogeneous). By wrapping up the
type variable 'w' inside SomeWidget, you can now have whatever types
of widgets in that SomeWidget, e.g.
data Button = Button (IO ())
instance Widget Button where widgetRun = ...
data Label = Label (String -> IO ())
instance Widget Label where widgetRun = ...
widgetList:: [(Integer, Integer, SomeWidget)]
widgetList =
[ SomeWidget (Button $ putStrLn "ding!")
, SomeWidget (Label $ putStrLn . ("entered: " ++)) ]
Before, without existential quantification, you had to have all the
same type of widget (e.g. all Button or all Label)
Hope this makes it more clear.
-Ross
On Jul 3, 2009, at 12:00 AM, Magicloud Magiclouds wrote:
Hi,
I thought "class" was for this purpose. But it turns out not.
Code as following could not compiled.
1 main = do
2 mapM_ (\(x, y, widget) -> do
3 a <- widgetRun widget
4 putStrLn $ show a
5 ) widgetList
6
7 widgetList :: (Widget w) => [(Integer, Integer, w)]
8 widgetList = []
9
10 class Widget w where
11 widgetRun :: w -> IO ()
---
% ghc --make tmp/test.hs
[1 of 1] Compiling Main ( tmp/test.hs, /tmp/Main.o )
tmp/test.hs:3:16:
Ambiguous type variable `t' in the constraint:
`Widget t' arising from a use of `widgetRun' at tmp/test.hs:
3:16-31
Probable fix: add a type signature that fixes these type
variable(s)
--
竹密岂妨流水过
山高哪阻野云飞
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
