[Haskell-cafe] How to present the commonness of some objects?

[Ross Mellgren]

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
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