在 F# 中是否有一种方法可以在不指定实例类型的情况下针对泛型类型进行类型测试? [英] Is there a way in F# to type-test against a generic type without specifying the instance type?

问题描述

我正在尝试对一些我关心的 SQL 生成类型进行模式匹配.理想情况下，我想这样做:

I'm trying to pattern match against a few types that I care about for SQL generation. Ideally I'd like to do this:

let rec getSafeValue record (prop: PropertyInfo) = 
    match prop.GetValue(record, null) with
    | :? string as str -> "'" + str + "'"
    | :? Option<_> as opt -> 
        match opt with
        | Some v -> getSafeValue v prop
        | None -> "null"
    | _ as v -> v.ToString()

问题是，在这里，Option<_> 的类型参数得到了与 record 匹配的约束，最终只是 obj.

The problem is that here, the type parameter to Option<_> gets constraint to match that of record, which ends up being just obj.

我知道我可以做一些基于反射的后台检查(检查它是否是泛型类型以及它是基于名称的选项类型)，但如果可能的话，我宁愿避免这种情况.

I know I can do some pain-in-the-behind reflection-based check (check that it's a generic type and that it's an option type based on the name), but I'd rather avoid that if at all possible.

推荐答案

不，没有使用 F# 的内置结构来做到这一点的好方法.但是，您可以为此类事情构建自己的可重用活动模式:

No, there's no good way to do this using F#'s built-in constructs. However, you could build your own reusable active pattern for this sort of thing:

open Microsoft.FSharp.Reflection
open Microsoft.FSharp.Quotations
open Microsoft.FSharp.Quotations.DerivedPatterns
open Microsoft.FSharp.Quotations.Patterns

let (|UC|_|) e o =
  match e with
  | Lambdas(_,NewUnionCase(uc,_)) | NewUnionCase(uc,[]) ->
      if (box o = null) then
        // Need special case logic in case null is a valid value (e.g. Option.None)
        let attrs = uc.DeclaringType.GetCustomAttributes(typeof<CompilationRepresentationAttribute>, false)
        if attrs.Length = 1
           && (attrs.[0] :?> CompilationRepresentationAttribute).Flags &&& CompilationRepresentationFlags.UseNullAsTrueValue <> enum 0
           && uc.GetFields().Length = 0
        then Some []
        else None
      else 
        let t = o.GetType()
        if FSharpType.IsUnion t then
          let uc2, fields = FSharpValue.GetUnionFields(o,t)
          let getGenType (t:System.Type) = if t.IsGenericType then t.GetGenericTypeDefinition() else t
          if uc2.Tag = uc.Tag && getGenType (uc2.DeclaringType) = getGenType (uc.DeclaringType) then
            Some(fields |> List.ofArray)
          else None
        else None
  | _ -> failwith "The UC pattern can only be used against simple union cases"

现在您的函数可能如下所示:

Now your function might look something like this:

let rec getSafeValue (item:obj) = 
    match item with
    | :? string as str -> "'" + str + "'"
    | UC <@ Some @> [v] -> getSafeValue v
    | UC <@ None @> [] -> "null"
    | _ as v -> v.ToString()

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