如何精简F#中长度受限字符串的重复代码？

在数据处理场景里，我们经常需要严格控制字符串长度——不管是适配固定宽度的文本输出，还是存入有限长度的nvarchar数据库字段。这时候用强类型代替裸的System.String会更安全，但之前的实现方式太冗余了，比如下面的重复模块：

module String40 = 
    let private MaxLength = 40
    type T = private T of string
    let create (s:string) = checkStringLength MaxLength s |> Result.map T
    let trustCreate (s:string) = checkStringLength MaxLength s |> Result.okVal |> T
    let truncateCreate (s:string) = truncateStringToLength MaxLength s |> T
    let toString (T s) = s
    type T with member this.AsString = this |> toString

module String100 = 
    let private MaxLength = 100
    type T = private T of string
    let create (s:string) = checkStringLength MaxLength s |> Result.map T
    let trustCreate (s:string) = checkStringLength MaxLength s |> Result.okVal |> T
    let truncateCreate (s:string) = truncateStringToLength MaxLength s |> T
    let toString (T s) = s
    type T with member this.AsString = this |> toString

你看，String100和String40几乎一模一样，就模块名和MaxLength值不同，维护起来简直是噩梦。我一直想把它精简成type String40 = LengthLimitedString<40>这种清爽的形式，期间试过几种思路都踩了坑：

• T4代码生成：F#项目不支持，直接行不通；
• Type Provider：复杂度太高，而且没法生成我们需要的模块结构；
• Scott Wlaschin的约束字符串方案：还是避免不了重复代码的问题。

最后我用常规泛型约束实现了一个可行的方案，代码示例如下：

type IMaxLengthProvider = 
    abstract member GetMaxLength: unit -> int

type MaxLength3 () = 
    interface IMaxLengthProvider with 
        member this.GetMaxLength () = 3

type MaxLength4 () = 
    interface IMaxLengthProvider with 
        member this.GetMaxLength () = 4

module LimitedString = 
    type T<'a when 'a :> IMaxLengthProvider> = private T of string

    let create<'t when 't :> IMaxLengthProvider and 't : (new:unit -> 't)> (s:string) = 
        let len = (new 't()).GetMaxLength()
        match checkStringLength len s with
        | Ok s -> 
            let x : T<'t> = s |> T
            x |> Ok
        | Error e -> Error e

    let trustCreate<'t when 't :> IMaxLengthProvider and 't : (new:unit -> 't)> (s:string) = 
        let len = (new 't()).GetMaxLength()
        match checkStringLength len s with
        | Ok s -> 
            let x : T<'t> = s |> T
            x
        | Error e -> 
            let msg = e |> formErrorMessage
            failwith msg

    let truncateCreate<'t when 't :> IMaxLengthProvider and 't : (new:unit -> 't)> (s:string) = 
        let len = (new 't()).GetMaxLength()
        let s = truncateStringToLength len s
        let x : T<'t> = s |> T
        x

    let toString (T s) = s
    type T<'a> with member this.AsString = this |> toString

（注：原代码里toStri是截断内容，这里补全了toString及对应的成员定义）

这个方案虽然需要实例化IMaxLengthProvider的实现类来获取长度，但对使用者来说非常简洁，扩展也容易。最重要的是，它能让我们的记录类型保持强类型区分，比如可以定义：

// 先封装类型别名
type String40 = LimitedString.T<MaxLength40>
type String100 = LimitedString.T<MaxLength100>

// 再定义业务记录
type MyRecord = { FirstName: String40; LastName: String100; }

完全不用担心类型混淆，完美满足强类型的安全需求。

内容的提问来源于stack exchange，提问作者Clyde