f# 折返 : Does accumulator need to be the same as list type?

Question

我有以下代码。我花了很多时间试图弄清楚为什么它提示我的 List.foldback 函数的第二个参数。它提示说它希望“acc”为 (char * bool * (Direction -> int * int) * int)。这对我来说没有任何意义，因为 documentation表明它只匹配“状态”。在这种情况下，我试图使“状态”成为“运动列表”。

有关我正在做的事情的完整描述，请访问 the Code golf voronoi problem

type Direction = Left=0 | Up=1 | Right=2 | Down=3
type Movement = (char * (int * int) * Direction)

let rec masterClaims (items:Movement list) =
    items

    // Execute these methods in the order in which they are passed. 
    |> List.map (fun (ch, coord, dir) -> 
        (
            ch,
            // This function does the claiming. This has to be 
            // done in the order of the input list. 
            claimCells ch coord,
            getCoordinate coord,
            int dir
        ))

    // Create next items and fold them into the final results for the layer. 
    // Use foldback so that they are pre-pended to the final list in the correct
    // order.
    |> List.foldBack (fun (ch, wasClaimed, getCoordinate, directionInt) (acc:Movement list) ->           
        if (wasClaimed) then

            // Create a list of next nodes to inspect
            // [counter-clockwise; forward; clockwise]
            [(directionInt+3)%4;directionInt;(directionInt+1)%4]
            |> List.map enum<Direction>
            |> List.iter (fun direction ->
                (
                    ch, 
                    getCoordinate direction, 
                    direction
                ) :: acc |> ignore)

        acc // should be  Movement list
        ) List.empty<Movement>


    // The theory here is that we will execute the fold
    // for every item before we pass the entire collection
    // to the recursive call. 
    |> masterClaims

Tomas 修复后的代码

let inline foldBackNormal f s input = List.foldBack f input s 

type Direction = Left=0 | Up=1 | Right=2 | Down=3
type Movement = (char * (int * int) * Direction)

let rec masterClaims (items:Movement list) =
    items

    // Execute these methods in the order in which they are passed. 
    |> List.map (fun (ch, coord, dir) -> 
        (
            ch,
            // This function does the claiming. This has to be 
            // done in the order of the input list. 
            claimCells ch coord,
            getCoordinate coord,
            int dir
        ))

    // Create next items and fold them into the final results for the layer. 
    // Use foldback so that they are pre-pended to the final list in the correct
    // order.
    |> foldBackNormal (fun (ch, wasClaimed, getCoordinate, directionInt) (acc:Movement list) ->           
        if (wasClaimed) then

            // Create a list of next nodes to inspect
            // [counter-clockwise; forward; clockwise]
            [(directionInt+3)%4;directionInt;(directionInt+1)%4]
            |> List.map enum<Direction>
            |> List.iter (fun direction ->
                (
                    ch, 
                    getCoordinate direction, 
                    direction
                ) :: acc |> ignore)

        acc // should be  Movement list
        ) List.empty<Movement>


    // The theory here is that we will execute the fold
    // for every item before we pass the entire collection
    // to the recursive call. 
    |> masterClaims

Answer 1

List.foldBack 函数有点奇怪，因为它将输入列表作为参数 before 最后一个和初始状态作为 last 论点。因此，切换最后两个参数的顺序应该可以解决问题。或者，您可以定义一个具有切换参数顺序的助手并使用它:

let foldBackNormal f s input = List.foldBack f input s

这是出于历史原因(与 OCaml 的兼容性)，但我相信您不是唯一觉得这令人困惑的人 :-)。