Question

有办法做到这一点吗？

type EntryDetails = 
| ADetails of TypeADetails
| BDetails of TypeBDetails
| ...


type Entry = { A, B, C, ... Details:EntryDetails}

let filter (list:list<Entry>) myType = List.filter (fun x -> x.Details is myType)

请注意我希望myType是一个参数，而不是硬编码类型。

我试过这个，但显然不起作用：

let filterDetails (entry:Entry) detailType = match entry.Details with 
                                               | detailType -> true 
                                               | _ -> false

Answer 1

这不是你的问题的确切解决方案，但它非常接近且易于实现 - 如何使用List.choose部分活动模式，如下所示：

type EntryDetails =
    | ADetails of int
    | BDetails of byte
    | CDetails of string

type Entry = { Foo : unit; Bar : unit; Details : EntryDetails; }

module Patterns =
  let (|ADetails|_|) x =
      match x.Details with
      | ADetails _ -> Some x
      | _ -> None

  let (|BDetails|_|) x =
      match x.Details with
      | BDetails _ -> Some x
      | _ -> None

  let (|CDetails|_|) x =
      match x.Details with
      | CDetails _ -> Some x
      | _ -> None

module internal Test =
    let private testData =
        let baseData = { Foo = (); Bar = (); Details = ADetails 0; }
        [   { baseData with Details = ADetails 10; };
            { baseData with Details = BDetails 7uy; };
            { baseData with Details = BDetails 92uy; };
            { baseData with Details = ADetails 32; };
            { baseData with Details = CDetails "foo"; };
            { baseData with Details = BDetails 2uy; };
            { baseData with Details = ADetails 66; };
            { baseData with Details = CDetails "bar"; };
            { baseData with Details = CDetails "baz"; }; ]

    let results =
        testData
        |> List.choose Patterns.(|ADetails|_|)

如果您将该代码粘贴到fsi，则应获得以下输出：

(* Snip ... removed irrelevant type signatures *)
module internal Test = begin
  val private testData : Entry list =
    [{Foo = null;
      Bar = null;
      Details = ADetails 10;}; {Foo = null;
                                Bar = null;
                                Details = BDetails 7uy;};
     {Foo = null;
      Bar = null;
      Details = BDetails 92uy;}; {Foo = null;
                                  Bar = null;
                                  Details = ADetails 32;};
     {Foo = null;
      Bar = null;
      Details = CDetails "foo";}; {Foo = null;
                                   Bar = null;
                                   Details = BDetails 2uy;};
     {Foo = null;
      Bar = null;
      Details = ADetails 66;}; {Foo = null;
                                Bar = null;
                                Details = CDetails "bar";};
     {Foo = null;
      Bar = null;
      Details = CDetails "baz";}]

  val results : Entry list =
    [{Foo = null;
      Bar = null;
      Details = ADetails 10;}; {Foo = null;
                                Bar = null;
                                Details = ADetails 32;};
     {Foo = null;
      Bar = null;
      Details = ADetails 66;}]
end

正如您所看到的，Test.results列表已被过滤，因此它只包含Entry字段类型为Details的{{1}}项。

根据成员基础类型筛选包含区别联合成员的记录列表

1 个答案: