list - 给定一个 (a * b) 列表，返回一个 (a * b list) 列表

Question

也许是一个时髦的标题，但我遇到了以下问题:

给定一个类型为 (a * b) list 的列表，我想创建一个类型为 (a * b list) list 的新列表。一个例子:

给定列表 let testList = [(1,"c");(2,"a");(1,"b")]，我的函数应该返回 [( 1, ["c";"b"]; (2, ["a"])]。

我有以下内容，但我对如何继续有点困惑:

let rec toRel xs =
    match xs with
    | (a,b)::rest -> (a,[b])::toRel rest
    | _           -> []

Answer 1

可以使用内置函数List.groupBy然后map去除多余的key:

testList |> List.groupBy fst |> List.map (fun (k,v) -> (k, List.map snd v))

// val it : (int * string list) list = [(1, ["c"; "b"]); (2, ["a"])]

否则如果你想继续比赛你可以这样做:

let toRel x = 
    let rec loop acc xs =
        match xs with
        | (k, b) :: rest ->
            let acc =
                match Map.tryFind k acc with
                | Some v -> Map.add k (b::v) acc
                | None   -> Map.add k [b] acc
            loop acc rest
        | _             -> acc
    loop Map.empty x |> Map.toList

或者使用Option.toList你可以这样写:

let toRel x = 
    let rec loop acc xs =
        match xs with
        | (k, b) :: rest ->
            let acc =
                let lst = Map.tryFind k acc |> Option.toList |> List.concat
                Map.add k (b::lst) acc
            loop acc rest
        | _              -> acc
    loop Map.empty x |> Map.toList