废弃 f# 中的样板

发布于 2024-09-16 10:36:57 字数 192 浏览 11 评论 0原文

我使用了废弃你的样板并且Haskell 编程语言中的 Uniplate 库，我会发现基于可区分联合的泛型编程形式非常有用。 f# 编程语言中是否有等效的库？

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水中月 2024-09-23 10:36:57

据我所知，没有；如果没有语言/编译器内置的支持，我希望唯一的选择是基于反射的版本。（我不知道 Uniplate 是如何实现的 - 你知道吗？）

这是代码基于原始演示文稿中的示例的基于反射的版本。我没有深入思考它的局限性，但这比我想象的要简单得多。

type Company = C of Dept list
and Dept = D of Name * Manager * SubUnit list
and SubUnit = | PU of Employee | DU of Dept
and Employee = E of Person * Salary
and Person = P of Name * Address
and Salary = S of float
and Manager = Employee
and Name = string
and Address = string

let data = C [D("Research",E(P("Fred","123 Rose"),S 10.0),
                  [PU(E(P("Bill","15 Oak"),S 5.0))])]
printfn "%A" data

open Microsoft.FSharp.Reflection 
let everywhere<'a,'b>(f:'a->'a, src:'b) =   // '
    let ft = typeof<'a>             // '
    let rec traverse (o:obj) =
        let ot = o.GetType()
        if ft = ot then
            f (o :?> 'a) |> box    // '
        elif FSharpType.IsUnion(ot) then
            let info,vals = FSharpValue.GetUnionFields(o, ot)
            FSharpValue.MakeUnion(info, vals |> Array.map traverse)
        else 
            o
    traverse src :?> 'b       // '

let incS (S x) = S(x+1.0) 

let newData = everywhere(incS, data)
printfn "%A" newData

everywhere 函数遍历任意 DU 的整个结构，并将函数 f 应用于 f 所处理类型的每个节点，留下所有其他节点按原样。

Not that I know of; without support built-in to the language/compiler, I expect the only alternative is a reflection-based version. (I don't know how Uniplate is implemented - do you?)

Here's the code for a reflection-based version based on the example from the original presentation. I have not thought deeply about its limitations, but this was much simpler to write than I would have guessed.

type Company = C of Dept list
and Dept = D of Name * Manager * SubUnit list
and SubUnit = | PU of Employee | DU of Dept
and Employee = E of Person * Salary
and Person = P of Name * Address
and Salary = S of float
and Manager = Employee
and Name = string
and Address = string

let data = C [D("Research",E(P("Fred","123 Rose"),S 10.0),
                  [PU(E(P("Bill","15 Oak"),S 5.0))])]
printfn "%A" data

open Microsoft.FSharp.Reflection 
let everywhere<'a,'b>(f:'a->'a, src:'b) =   // '
    let ft = typeof<'a>             // '
    let rec traverse (o:obj) =
        let ot = o.GetType()
        if ft = ot then
            f (o :?> 'a) |> box    // '
        elif FSharpType.IsUnion(ot) then
            let info,vals = FSharpValue.GetUnionFields(o, ot)
            FSharpValue.MakeUnion(info, vals |> Array.map traverse)
        else 
            o
    traverse src :?> 'b       // '

let incS (S x) = S(x+1.0) 

let newData = everywhere(incS, data)
printfn "%A" newData

The everywhere function traverses the entire structure of an arbitrary DU and applies the function f to each node that is the type that f works on, leaving all other nodes as-is.

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