F#用`id<XXX>`函数前置标注类型

F#核心库有个不起眼的函数id，它非常简单，位于模块module FSharp.Core.Operators，签名是：

val id: 't->'t

The identity function

更具体的实现代码是：

let id<'t> (x:'t) = x

I sometimes use id<_> as a convenient way to do type annotation as prefix or without parens. It's a bit weird but it works nicely:

type Point = {x:int;y:int}
type Vector = {x:int;y:int}

let p = id<Point> {x=1;y=2}