f# - 在 F# 中折叠列表

Question

我有一个非常琐碎的任务，但我不知道如何使解决方案更漂亮。
目标是采用 List 并根据它们是否通过谓词返回结果。结果应该分组。这是一个简化的示例:

谓词:isEven
输入:[2; 4; 3; 7; 6; 10; 4; 5]
输出:[[^^^^]......[^^^^^^^^]..]

这是我目前的代码:

let f p ls =
    List.foldBack
        (fun el (xs, ys) -> if p el then (el::xs, ys) else ([], xs::ys))
        ls ([], [])
    |> List.Cons // (1)
    |> List.filter (not << List.isEmpty) // (2)

let even x = x % 2 = 0

let ret =
    [2; 4; 3; 7; 6; 10; 4; 5]
    |> f even
// expected [[2; 4]; [6; 10; 4]]

这段代码看起来可读性不是很好。另外，我不喜欢第 (1) 和 (2) 行。有没有更好的解决方案？

Answer 1

这是我的看法。你首先需要一些辅助函数:

// active pattern to choose between even and odd intengers
let (|Even|Odd|) x = if (x % 2) = 0 then Even x else Odd x

// fold function to generate a state tupple of current values and accumulated values
let folder (current, result) x =
    match x, current with
    | Even x, _ -> x::current, result // even members a added to current list
    | Odd x, [] -> current, result    // odd members are ignored when current is empty
    | Odd x, _ -> [], current::result // odd members starts a new current

// test on data
[2; 4; 3; 7; 6; 10; 4; 5]
    |> List.rev                             // reverse list since numbers are added to start of current
    |> List.fold folder ([], [])            // perform fold over list
    |> function | [],x -> x | y,x -> y::x   // check that current is List.empty, otherwise add to result