有一个懒惰的地图吗？ [英] Is there a lazy mapM?

问题描述

乍一看，我认为这两个功能是一样的：

  firstM _ [] = return Nothing 
 firstM p（x：xs）= px>> = \ r  - >如果r然后返回（只是x）else firstM p xs 
 
 firstM'p xs = fmap listToMaybe（mapM p xs）
  

但他们没有。特别是，只要第一个 p x 为真， firstM 就会停止。但是 firstM'，因为 mapM ，需要评估整个列表。

是否有一个lazy mapM 启用第二个定义，或者至少有一个不需要显式递归？

解决方案

一个解决方案是使用 ListT ，列表monad变换器。这种类型交错副作用和结果，所以你可以偷看最初的元素，而无需首先运行整个计算。

下面是一个使用 ListT ：

  import Control.Monad 
导入限定ListT 
 
 firstM :: Monad m => （a  - > Bool） - > [a]  - > m（也许a）
 firstM p = ListT.head。 mfilter p。 ListT.fromFoldable 
  

At first glance I thought these two functions would work the same:

firstM _ []     = return Nothing
firstM p (x:xs) = p x >>= \r -> if r then return (Just x) else firstM p xs

firstM' p xs    = fmap listToMaybe (mapM p xs)

But they don't. In particular, firstM stops as soon as the first p x is true. But firstM', because of mapM, needs the evaluate the whole list.

Is there a "lazy mapM" that enables the second definition, or at least one that doesn't require explicit recursion?

解决方案

One solution is to use ListT, the list monad transformer. This type interleaves side effects and results, so you can peek at the initial element without running the whole computation first.

Here's an example using ListT:

import Control.Monad
import qualified ListT

firstM :: Monad m => (a -> Bool) -> [a] -> m (Maybe a)
firstM p = ListT.head . mfilter p . ListT.fromFoldable

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