"***"怎么理解?

sw2wolf

import Control.Arrow ((***))

splitVerticallyBy,splitHorizontallyBy :: RealFrac r => r -> Rectangle -> (Rectangle, Rectangle)
splitVerticallyBy f = (mirrorRect *** mirrorRect) . splitHorizontallyBy f . mirrorRect

mirrorRect :: Rectangle -> Rectangle
mirrorRect (Rectangle rx ry rw rh) = (Rectangle ry rx rh rw)

Prelude> :m Control.Arrow
Prelude Control.Arrow> :i ***
class (Control.Category.Category a) => Arrow a where
  ...
  (***) :: a b c -> a b' c' -> a (b, b') (c, c')
  ...
          -- Defined in Control.Arrow
infixr 3 ***

不明白(mirrorRect *** mirrorRect) . splitHorizontallyBy f . mirrorRect 中, "***"的用法!

sw2wolf

谢谢!  但我不明白如下定义中, 能解释下吗?

(***) :: a b c -> a b' c' -> a (b, b') (c, c')

sw2wolf

原帖由 MMMIX 于 2009-6-15 14:48 发表

看不懂 (***) 的类型？对比看看下面两个：

class Arrow a where
    ...
    (***) :: a b c -> a b' c' -> a (b, b') (c, c')
    ...


instance Arrow (->) where
    ...

对于 instance Arr ...

比较清楚了，谢谢！