定义数据类型

Question

要使这些类声明起作用，我需要哪种数据类型？

class YesNo a where
   yesno :: a -> Bool

instance YesNo Int where
   yesno 0 = False
   yesno _ = True

instance YesNo [a] where
   yesno [] = False
   yesno _ = True

instance YesNo Bool where
   yesno = id

instance YesNo (Maybe a) where
   yesno (Just _) = True
   yesno Nothing = False

目前*Main> :t yesno产生<interactive>:1:1: error: Variable not in scope: yesno，大概是因为我需要定义适当的数据类型。

建议我将约束(YesNo a) =>添加到yesno的类型似乎不起作用（在询问<interactive>:14:1: error: Variable not in scope类型时我仍然得到yesno ：

  *Main> :l experiment
  [1 of 3] Compiling Geometry         ( Geometry.hs, interpreted )
  [2 of 3] Compiling Shapes           ( Shapes.hs, interpreted )
  [3 of 3] Compiling Main             ( experiment.hs, interpreted )
  Ok, three modules loaded.
  *Main> :t yesno

<interactive>:1:1: error: Variable not in scope: yesno