灵活的类型标注在F＃的目的是什么？ [英] What is the purpose of flexible type annotation in F#?

问题描述

我学习F＃，我不明白灵活的类型，或者更好的目的，我不明白写这之间的区别：

I'm studying F# and I don't understand the purpose of flexible types, or better, I can't understand the difference between writing this:

set TextOfControl (c : Control) s = c.Text <- s

和写的：

set TextOfControl (c : 'T when 'T :> Control) s = c.Text <- s

其中，控制是 System.Windows.Forms.Control的类。

推荐答案

。在你的例子没有什么区别。如果返回类型的限制，你开始看到的区别：

There is no difference in your example. If return types are constrained, you start seeing the difference:

let setText (c: Control) s = c.Text <- s; c
let setTextGeneric (c: #Control) s = c.Text <- s; c

let c = setText (TreeView()) "" // return a Control object
let tv = setTextGeneric (TreeView()) "" // return a TreeView object

注意 #Control 是的快捷键T时，'T：＆GT;控制。 类型约束是很重要的子类型创建通用的功能。

Note that #Control is a shortcut of 'T when 'T :> Control. Type constraints are important to create generic functions for subtypes.

例如，

let create (f: _ -> Control) = f()

let c = create (fun () -> Control()) // works
let tv = create (fun () -> TreeView()) // fails

VS

let create (f: _ -> #Control) = f()

let c = create (fun () -> Control()) // works
let tv = create (fun () -> TreeView()) // works

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