'selective' map 的 Haskell 习语

Question

假设有人想要映射一个集合，但仅在映射值满足特定条件时才收集映射函数的结果。我目前正在这样做:

func = foldl (\acc x, ->  (maybeGrab x):acc) []


maybeGrab a
    | a > 5 = [someFunc a]
    | otherwise = []

虽然这有效，但我确信有一种更惯用的“正确/常见/更容易识别”的方式来做到这一点。

Answer 1

 mapMaybe :: (a -> Maybe b) -> [a] -> [b]

Data.Maybe 包中的

mapMaybe 看起来可以完成这项工作。文档说:

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it just Just b, then b is included in the result list.