我想将我的 F# OOP 版本的 Tagless Final 转换为典型的 FP 方法,并且我正在考虑使用来自 OO的类型类的静态解析类型参数。
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))
以及它们的具体实现
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) }
那种作品如下
[<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
问题是我收到警告
构造导致代码比类型注释所指示的更通用
对于两者cacheImpl1
,dataSourceImpl1
所以我不能再requestData
用于其他情况。有没有办法绕过这个问题?