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

[Magicloud Magiclouds]

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