是否可以强制某条记录尊重某些不变性? [英] Is it possible to enforce that a Record respects some invariants?

问题描述

假设我想创建一个表示可接受的最小/最大界限的记录类型:

Suppose I wanted to create a Record type that represents acceptable min/max bounds:

type Bounds = { Min: float; Max: float }

有没有一种方法可以强制执行Min<最大限度?编写validateBounds函数很容易，我只是想知道是否有更好的方法可以做到这一点.

Is there a way to enforce that Min < Max? It is easy to write a validateBounds function, I was just wondering if there was a better way to do this.

我意识到对于这个特定的示例，我可能可以避免暴露两个属性并对参数进行重新排序，所以我们可以尝试

I realized that for this specific example I could probably get away with exposing two properties and re-order the arguments, so let's say we were trying to do

type Person = { Name: string }

姓名"必须至少包含一个字符.

and Name needs to have at least one character.

推荐答案

这是基于保护级别的另一种解决方案:

Here's another solution based on protection levels:

module MyModule =
    type Bounds = private { _min: float; _max: float } with
        // define accessors, a bit overhead
        member public this.Min = this._min
        member public this.Max = this._max
        static member public Make(min, max) =
            if min > max then raise (ArgumentException("bad values"))
            {_min=min; _max=max}

    // The following line compiles fine,
    // e.g. within your module you can do "unsafe" initialization
    let myBadBounds = {_min=10.0; _max=5.0}

open MyModule
let b1 = Bounds.Make(10.0, 20.0) // compiles fine
let b1Min = b1.Min
let b2 = Bounds.Make(10.0, 5.0) // throws an exception
// The following line does not compile: the union cases of the type 'Bounds'
// are not accessible from this code location
let b3 = {_min=10.0; _max=20.0}
// The following line takes the "bad" value from the module
let b4 = MyModule.myBadBounds

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