使用 SRTP 在 F# 中实现无标记最终编码

Question

我想改造我的 F# OOP version of Tagless Final into a typical FP approach and I'm thinking to use Statically Resolved Type Parameters of Type Classes from OO。

我所做的是

open System
open FSharpPlus

type UserName = string
type DataResult<'t> = DataResult of 't with
    static member Map ( x:DataResult<'t>  , f) =
        match x with 
        | DataResult t -> DataResult (f t)

创建我需要的 SRTP

type Cache = 
    static member inline getOfCache cacheImpl data =
        ( ^T : (member getFromCache : 't -> DataResult<'t> option) (cacheImpl, data))
    static member inline storeOfCache cacheImpl data =
        ( ^T : (member storeToCache : 't -> unit) (cacheImpl, data))

type DataSource() =
    static member inline getOfSource dataSourceImpl data =
        ( ^T : (member getFromSource : 't -> DataResult<'t>) (dataSourceImpl, data))
    static member inline storeOfSource dataSourceImpl data =
        ( ^T : (member storeToSource : 't -> unit) (dataSourceImpl, data))

及其具体实现

type CacheNotInCache() = 
        member this.getFromCache _ = None
        member this.storeCache _ = () 

type CacheInCache() =
        member this.getFromCache user = monad { 
           return! DataResult user |> Some}
        member this.storeCache _ = () 

type  DataSourceNotInCache() = 
          member this.getFromSource user = monad { 
               return! DataResult user } 

type  DataSourceInCache()  =
          member this.getFromSource _  = 
              raise (NotImplementedException())

通过它我可以定义一个无标签的最终 DSL

let requestData (cacheImpl: ^Cache) (dataSourceImpl: ^DataSource) (userName:UserName) = monad {
    match Cache.getOfCache cacheImpl userName with
    | Some dataResult -> 
            return! map ((+) "cache: ") dataResult
    | None -> 
            return! map ((+) "source: ") (DataSource.getOfSource dataSourceImpl userName) }

还有这样的作品

[<EntryPoint>]
let main argv =
    let cacheImpl1 = CacheInCache() 
    let dataSourceImpl1 = DataSourceInCache()
    let cacheImpl2 = CacheNotInCache() 
    let dataSourceImpl2 = DataSourceNotInCache()
    requestData cacheImpl1 dataSourceImpl1 "john" |> printfn "%A"
    //requestData (cacheImpl2 ) dataSourceImpl2 "john" |> printfn "%A"
    0

问题是我收到了警告

construct causes code to be less generic than indicated by the type annotations

用于 cacheImpl1 和 dataSourceImpl1，所以我不能在另一种情况下重复使用 requestData。有没有办法绕过这个问题？

Answer 1

我不熟悉您要实现的抽象，但查看您的代码，您似乎缺少 inline 修饰符：

let inline requestData (cacheImpl: ^Cache) (dataSourceImpl: ^DataSource) (userName:UserName) = monad {
    match Cache.getOfCache cacheImpl userName with
    | Some dataResult -> 
            return! map ((+) "cache: ") dataResult
    | None -> 
            return! map ((+) "source: ") (DataSource.getOfSource dataSourceImpl userName) }

附带说明一下，您可以像这样简化地图函数：

type DataResult<'t> = DataResult of 't with
    static member Map (DataResult t, f) = DataResult (f t)

Answer 2

我很熟悉 final tagless，但我不确定你为什么要使用 SRTP。 Final tagless 使用 type 类，这些可以用接口模拟（参见 scala emulates type类的方式）。

该方法类似于（基本相同）"object algebra"，可以使用标准 OO 构造来实现。

使用 SRTP 在 F# 中实现无标记最终编码

Implementing Tagless Final Encoding in F# with SRTP

dsl

f#

fsharp-typeclasses

tagless-final

f#+