當成員簽名在F＃中具有類型變量時，成員約束調用表達式出現問題

Question

當成員簽名具有類型變量時，我在使用成員約束調用表達式時遇到麻煩。 具體來說，代碼

[<AutoOpen>]
module Foo

// Preapplied lens
type lens <'i,'j> = { get : 'j; set : 'j -> 'i }

// Record type with lenses 
type foo1 = {
    num_ : int;
    letter_ : char
} with
    member this.num = {
        get = this.num_;
        set = (fun num' -> {this with num_=num'})}
    member this.letter = {
        get = this.letter_;
        set = (fun letter' -> {this with letter_=letter'})}
end

let (foo1:foo1) = {num_ = 1; letter_ = 'a'}

// Another ecord type with lenses 
type foo2 = {
    num_ : int;
    name_ : string
} with
    member this.num = {
        get = this.num_;
        set = (fun num' -> {this with num_=num'})}
    member this.name = {
        get = this.name_;
        set = (fun name' -> {this with name_=name'})}
end

let (foo2:foo2) = {num_ = 2; name_ = "bob"}

// Add two to any record with the num lens
let inline add2 (x : ^t) =
    let num = (^t : (member num : lens<^t,int>) (x)) in
    num.get + 2

給出錯誤：

test05.fsx（39,47）：表達式中錯誤FS0010：意外符號)

問題出在倒數第二行的成員約束調用表達式上。 基本上，我不知道如何在成員約束調用表達式中將類型變量^t和int傳遞給lens類型。 有一個好的方法可以做到這一點嗎？

Answer 1

靜態約束中<和^之間必須有一個空格：

// Add two to any record with the num lens
let inline add2 (x : ^t) =
    let num = (^t : (member num : lens< ^t,int>) (x)) in
    num.get + 2

否則，兩個字符<^將被視為中綴函數的名稱（也稱為“運算符”）。