F#元组的扩展方法

Question

是否可以为F#元组编写扩展方法？例如,要添加实例方法.Item1和.Item2(如System.Tuple),它们相当于为2元组调用fst和snd？

Answer 1

在System.Tuple<'T1, 'T2>该内部表示式(2元件)在F#元组实际上已经具有特性Item1和Item2,但这些是由F#编译器隐藏.将扩展成员添加到元组的一种显而易见的方法不起作用,所以我不希望这会起作用(但可能有一些我不知道的解决方法).

一般来说,我认为模式匹配优选成员,如Item1,Item2等.(和C#3.0程序员元组:-)工作时经常请模式匹配的支持).

原因是模式匹配迫使您命名.比较这两个代码段:

let (width, height) = tuple
width * height

和使用属性的版本:

tuple.Item1 * tuple.Item2

第二个是更短,但绝对不太可读.

Answer 2

不完美，但我正在使用这个。（我借用了http://www.fssnip.net/6V的原始代码并添加了一些小修改。）

[<AutoOpen>]
module TupleExtensions =
  type System.Tuple with
    static member Item1(t) = let (x,_) = t in x
    static member Item1(t) = let (x,_,_) = t in x
    static member Item1(t) = let (x,_,_,_) = t in x
    static member Item1(t) = let (x,_,_,_,_) = t in x
    static member Item1(t) = let (x,_,_,_,_,_) = t in x
    static member Item1(t) = let (x,_,_,_,_,_,_) = t in x

    static member Item2(t) = let (_,x) = t in x
    static member Item2(t) = let (_,x,_) = t in x
    static member Item2(t) = let (_,x,_,_) = t in x
    static member Item2(t) = let (_,x,_,_,_) = t in x
    static member Item2(t) = let (_,x,_,_,_,_) = t in x
    static member Item2(t) = let (_,x,_,_,_,_,_) = t in x

    static member Item3(t) = let (_,_,x) = t in x
    static member Item3(t) = let (_,_,x,_) = t in x
    static member Item3(t) = let (_,_,x,_,_) = t in x
    static member Item3(t) = let (_,_,x,_,_,_) = t in x
    static member Item3(t) = let (_,_,x,_,_,_,_) = t in x

    static member Item4(t) = let (_,_,_,x) = t in x
    static member Item4(t) = let (_,_,_,x,_) = t in x
    static member Item4(t) = let (_,_,_,x,_,_) = t in x
    static member Item4(t) = let (_,_,_,x,_,_,_) = t in x

    static member Item5(t) = let (_,_,_,_,x) = t in x
    static member Item5(t) = let (_,_,_,_,x,_) = t in x
    static member Item5(t) = let (_,_,_,_,x,_,_) = t in x

    static member Item6(t) = let (_,_,_,_,_,x) = t in x
    static member Item6(t) = let (_,_,_,_,_,x,_) = t in x

    static member Item7(t) = let (_,_,_,_,_,_,x) = t in x  

如何使用它：

let t = (1, 2, 3)
let item1 = Tuple.Item1(t)

这里定义的 Tuple.Item1 比 fst 有优势：它对于项目数量是多态的。一旦我们使用这些扩展方法编写了使用 n 元组的函数，我们就可以将其扩展为 n+1 元组，而无需修改函数体。相反，我们必须修改参数类型声明。更省力。