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，而下面的示例中没有显式的递归？

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，未来使用空记录模式（kudos hammar）：

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

但请注意，这仅适用于匹配构造函数。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

Answer 2

您可以使用Data.List.find。

getA = find isA
   where isA (A {}) = True
         isA _ = False

Answer 3

您可以使用find：

data Elem = A Int | B Char deriving Show
getA elements = find (\x->case x of (A _) -> True; _ -> False) elements

从列表中获取特定构造函数的第一个匹配项的惯用方法

3 个答案: