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

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