F＃成员对元组的约束

本文介绍了F＃成员对元组的约束的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我通常有一个哦是的时刻写F＃，当我意识到我需要一个额外的价值。这通常通过向被传递的元组添加另一个值来容易地完成。然而，这意味着各种地图/排序/收集/等。需要更新，特别是函数fst / snd只处理长度为2的元组。

这不是一个大问题，但在探索式开发过程中我会写一个帮助来缓解烦恼：

let inline get2（t：^ a）=成员get_Item2：unit - > string）（t，（））） let inline get2a（t：^ a）=（^ a :( member Item2：string）t）

但是，两个版本都不工作。第一个， get2 ，不会编译，用期望1表达式，得到2。第二个， get2a ，将编译，但随后不能在元组上使用：类型'（int *字符串）'不支持任何名为'get_Item2' 。

有没有办法做到这一点，不涉及大量的重载？

因为 ItemX 静态约束对F＃元组不起作用的原因是因为 System.Tuple< ; _，...，_> 只是元组的编码形式，而不是编译器使用的静态表示形式。请参阅规范中的 6.3.2元组表达式。

但是，通过一些工作，您可以获得给定元组的运行时编码，如下所示：

open System //像get2a，但通用返回类型 let inline get2b（t：^ a）=（^ a：（member Item2：'b）t） let x =（1,2） let y =（1,2,3） get2b（box x：？> Tuple& int>） get2b（box y：？> Tuple< int，int，int>）

I commonly have a "oh yeah" moment writing F# when I realize I need an extra value somewhere. This is generally easily done by adding another value to the tuple being passed around. However, this means that various maps/sorts/collects/etc. need updating, and in particular the functions fst/snd only work on tuples of length 2.

It's not a huge issue, but it's annoying enough during exploratory development that I though I'd write a helper to alleviate the annoyance:

let inline get2 (t:^a) = (^a : (member get_Item2 : unit -> string) (t, ())) let inline get2a (t:^a) = (^a : (member Item2 : string) t)

However, both versions do not work. The first, get2, won't compile, with "Expected 1 expressions, got 2". The second, get2a, will compile, but subsequently can't be used on tuples: "The type '(int * string)' does not support any operators named 'get_Item2'".

Is there any way of doing this that doesn't involve lots of overloads? with noisy OverloadID annotations (annotations not required in F# 2.0)

解决方案

The reason why ItemX static constraints on F# tuples do not work is because System.Tuple<_,...,_> is only the encoded form of tuples, and not the static representation used by the compiler. See 6.3.2 Tuple Expressions in the specification.

However, with a little work, you can obtain the runtime encoding of a given tuple like so:

open System //like get2a but generic return type let inline get2b (t:^a) = (^a : (member Item2 : 'b) t) let x = (1,2) let y = (1,2,3) get2b (box x :?> Tuple<int,int>) get2b (box y :?> Tuple<int,int,int>)