Haskell-Cafe 上一个关于生成 stream 问题的解答

sw2wolf

>>concatMap (\n -> replicateM n ['a'..'z']) [1..]
好理解

>>tail $ concat $ iterate (map (:) ['a'..'z'] <*>) [[]]
不好理解

sw2wolf

原帖由 MMMIX 于 2009-6-21 11:34 发表

我的感觉正好相反，后者的逻辑要简单多了。

能解释一下后者吗？ 谢谢！

sw2wolf

对于concatMap (\n -> replicateM n ['a'..'z']) [1..], 我是这样理解的:(为了好说明, 我将范围定为'a'..'c')

replicateM 1 ['a'..'c'] = ["a","b","c"]
replicateM 2 ['a'..'c'] = ["aa","ab","ac","ba","bb","bc","ca","cb","cc"]
replicateM 3 ['a'..'c'] = ["aaa","aab","aac","aba","abb","abc","aca","acb","acc","baa","bab","bac","bba","bbb","bbc","bca","bcb","bcc","caa","cab","cac","cba","cbb","cbc","cca","ccb","ccc"]
......
然后再将这些list合并就可以了.

BTW,
:m Control.Applicative
Prelude Control.Applicative> :i <*>
class (Functor f) => Applicative f where
  ...
  (<*>) :: f (a -> b) -> f a -> f b
          -- Defined in Control.Applicative
infixl 4 <*>

主要对<*>不太明白 !