Question

是否找到了在清单中首次出现建筑商的明智办法,而没有在下文样本中进行明确的重新检查?

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

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

增编:使用空记录模式(库多斯哈马尔)更妥善、更不受未来影响:

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

然而,值得注意的是,这只是为了配对建筑商而做的。对于一般特性,find为:

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

Answer 2

http://haskell.org/ghc/docs/latest/html/libraries/base/Data-List.html#v%3afind>Data.List.find。

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

Answer 3

You could use find:

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

友情链接