f# - 类型推断 : functions vs types

Question

我正在学习 F#，但我不明白类型推断和泛型在这种语言中是如何工作的。例如，我可以声明一个通用的 min 函数并将其与不同类型的参数一起使用:

let min a b = if a < b then a else b

let smallestInt = min 3 5
let smallestFloat = min 3.0 5.0

但是如果我用一个类型尝试同样的事情，它就不起作用了:

type Point2D(x, y) = 
    member this.X = x
    member this.Y = y

let float32Point = new Point2D(0.0f, 1.0f)
let intPoint = new Point2D(0, 1) // This expression was expected to have type 
                                 // float32 but here has type int

所以，我有几个问题:

• 为什么我可以为不同的类型重用泛型函数定义而不是类型定义？
• 该函数是否像 C# 泛型一样在运行时专用于每种类型？或者像 C++ 模板一样在编译时？或者是否执行装箱以将每个参数都视为 IComparable？

谢谢。

Answer 1

类需要明确的类型参数。这有效:

type Point2D<'T>(x:'T, y:'T) = 
    member this.X = x
    member this.Y = y

let float32Point = Point2D(0.0f, 1.0f)
let intPoint = Point2D(0, 1)

要回答您的第二个问题，您对 min 的定义有签名'a -> 'a -> 'a (requires comparison) . comparison约束只存在于编译时(运行时签名是相同的，减去约束)。

<替换为对 GenericLessThanIntrinsic 的调用, 有约束。约束仅传播给调用者。

此外，来自规范的第 14.6.7 节:

Generalization is the process of inferring a generic type for a definition where possible, thereby making the construct reusable with multiple different types. Generalization is applied by default at all function, value, and member definitions, except where listed later in this section. Generalization also applies to member definitions that implement generic virtual methods in object expressions.

(强调)

注意，列表中缺少类。我想它没有给出基本原理，但是设计的。