haskell - 从列表中获取特定构造函数的第一次出现的惯用方法

Question

有没有一种很好的方法可以在列表中找到第一次出现的构造函数，而无需下面示例中的显式递归？

data Elem = A Int | B Char deriving Show

getA :: [Elem] -> Maybe Elem
getA [] = Nothing
getA (e:es) = 
    case e of 
        A a -> Just (A a)
        _   -> getA es

Answer 1

简单地

import Data.Maybe (listToMaybe)
getA xs = listToMaybe [e | e@(A _) <- xs]

附录:更好，使用空记录模式(kudos hammar)面向 future :

getA xs = listToMaybe [e | e@(A{}) <- xs]

但是请注意，这只适用于匹配的构造函数。对于一般属性，find更好:

get prop xs = listToMaybe [e | e <- xs, prop e]
get prop xs = listToMaybe (filter prop xs)
get prop xs = find prop xs