F# 缺少类型约束

在以下代码中，请注意 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)

尽管我已经有了显式类型约束，但在调用 Seq.sumBy 时仍然收到以下编译器错误：

类型参数缺少约束 'when ^t : (static member get_Zero : -> ^t)'

有人知道这是怎么回事吗？谢谢。

尝试使下游静态成员约束显式化可能会令人沮丧，而且幸运的是，这很少有必要。只需标记功能即可inline并让它们被推断出来。

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的语法：when ^t : (static member get_Zero : -> ^t)。这就是我所说的令人沮丧的一部分。）