F# 缺少类型约束

Question

在下面的代码中，注意 get_Zero 的类型约束:

type Wrapper<'t> = { Data : 't[] }

let compute<'t
    when 't : (static member get_Zero : unit -> 't)
    and 't : (static member (~-) : 't -> 't)
    and 't : (static member (+) : 't * 't -> 't)>
        (wrapper : Wrapper<'t>) =
    wrapper.Data
        |> Seq.mapi (fun i value -> (i, value))
        |> Seq.sumBy (fun (i, value) ->
            if i % 2 = 0 then value
            else -value)

即使我已经有了明确的类型约束，我仍然在调用 Seq.sumBy 时遇到以下编译器错误:

A type parameter is missing a constraint 'when ^t : (static member get_Zero : -> ^t)'

有人知道这里发生了什么吗？谢谢。

Answer 1

尝试使下游静态成员约束显式化可能会令人沮丧，幸运的是，很少有必要这样做。只需将函数标记为 inline 并让它们被推断。

let inline compute (wrapper : Wrapper<_>) =
    wrapper.Data
    |> Seq.mapi (fun i value -> (i, value))
    |> Seq.sumBy (fun (i, value) ->
        if i % 2 = 0 then value
        else -value)

正确的签名是:

let inline compute<'t
            when 't : (static member Zero : 't)
            and 't : (static member (~-) : 't -> 't)
            and 't : (static member (+) : 't * 't -> 't)>

(您会注意到错误消息中的签名甚至不是有效语法:when ^t : (static member get_Zero : -> ^t)。这是我的部分意思令人沮丧。)