f# - 是否可以强制记录尊重某些不变量？

Question

假设我想创建一个代表可接受的最小/最大界限的 Record 类型：

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

有没有办法强制 Min < Max？编写一个 validateBounds 函数很容易，我只是想知道是否有更好的方法来做到这一点。

编辑：我意识到对于这个特定的例子，我可能会暴露两个属性并重新排序参数，所以假设我们正在尝试做

type Person = { Name: string }

和 Name 至少需要一个字符。

Answer 1

这是另一个基于保护级别的解决方案：

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

Answer 2

我认为您最好的选择是静态成员：

type Bounds = { Min: float; Max: float }
    with
        static member Create(min: float, max:float) =
            if min >= max then
                invalidArg "min" "min must be less than max"

            {Min=min; Max=max}

并像使用它一样

> Bounds.Create(3.1, 2.1);;
System.ArgumentException: min must be less than max
Parameter name: min
   at FSI_0003.Bounds.Create(Double min, Double max) in C:\Users\Stephen\Documents\Visual Studio 2010\Projects\FsOverflow\FsOverflow\Script2.fsx:line 5
   at <StartupCode$FSI_0005>.$FSI_0005.main@()
Stopped due to error
> Bounds.Create(1.1, 2.1);;
val it : Bounds = {Min = 1.1;
                   Max = 2.1;}

但是，正如您所指出的，这种方法的最大缺点是没有什么可以直接阻止构建“无效”记录。如果这是一个主要问题，请考虑使用类类型来保证您的不变量：

type Bounds(min:float, max:float) = 
    do
        if min >= max then
            invalidArg "min" "min must be less than max"

    with
        member __.Min = min
        member __.Max = max

为了方便起见，与您从记录中获得的类似的活动模式一起使用（特别是关于模式匹配）：

let (|Bounds|) (x:Bounds) =
    (x.Min, x.Max)

全部一起：

> let bounds = Bounds(2.3, 1.3);;
System.ArgumentException: min must be less than max
Parameter name: min
   at FSI_0002.Bounds..ctor(Double min, Double max) in C:\Users\Stephen\Documents\Visual Studio 2010\Projects\FsOverflow\FsOverflow\Script2.fsx:line 4
   at <StartupCode$FSI_0003>.$FSI_0003.main@()
Stopped due to error
> let bounds = Bounds(1.3, 2.3);;

val bounds : Bounds

> let isMatch = match bounds with Bounds(1.3, 2.3) -> "yes!" | _ -> "no";;

val isMatch : string = "yes!"

> let isMatch = match bounds with Bounds(0.3, 2.3) -> "yes!" | _ -> "no";;

val isMatch : string = "no"

Answer 3

字符串示例的一个狡猾的解决方案 - 使用 DU

type cleverstring = |S of char * string

这将强制字符串至少包含一个字符。然后你可以在你的记录中使用cleverstring而不是string，尽管你可能想写一些包装函数让它看起来像一个字符串。