Map.tryFind返回null而不是None

Question

我有以下脚本.

type SetCookie = {
    NameValue : string * string;
    Domain : string option;
    Path : string option;
    Expires : string option;
    MaxAge : string option;
    Secure : bool; // ? no associated value, anything better than bool
    HttpOnly : bool; // ? no associated value, anything better than bool
    }

let GetCookie str =
    let allkeyvalues =
        str.Split ';'
        |> Array.map (fun str ->
            match str.Split '=' with
            | [| key |] -> (key.Trim(), "")
            | [| key; value |] -> (key.Trim(), value)
            | _ -> failwith "there's more than one '='; the format is incorrect.")
    let namevalue = allkeyvalues.[0]
    let attributes =
        Seq.skip 1 allkeyvalues
        |> Seq.map (fun (key, value) ->
            (key.ToLower(), value)) // attribute names are case-insensitive
        |> Map.ofSeq
    {
        NameValue = namevalue
        Domain = Map.tryFind "domain" attributes
        Path = Map.tryFind "path" attributes
        Expires = Map.tryFind "expires" attributes
        MaxAge = Map.tryFind "maxage" attributes
        Secure = Map.containsKey "secure" attributes
        HttpOnly = Map.containsKey "httponly" attributes
    }

但是,函数调用

GetCookie "XXX=YYY; path=/" 

返回null for Domain,Expires....和所有其他选项类型而不是None.根据文档,如果没有找到它应该返回None？http://msdn.microsoft.com/en-us/library/ee353538.aspx

  {NameValue =
    ("XXX", "YYY");
   Domain = null;
   Path = Some "/";
   Expires = null;
   MaxAge = null;
   Secure = false;
   HttpOnly = false;}

Answer 1

F#None值实际上由.NET表示null.在这种情况下,这会以通用方式打印出来,但在其他方面只是一个表示细节.实际上,您可以通过注释没有参数的联合案例来为您自己的歧视联盟获取此行为CompilationRepresentationFlags.UseNullAsTrueValue.