在理解 F# 中的 class 构造函数时遇到问题

Question

I have the following code:

module ExchangeSocket =

type Socket(url, id, key) =

    let Communicator = new TestCommunicator(url)
    let Client = new WebsocketClient(Communicator)

    do
        Communicator.Name <- "test"
        Communicator.ReconnectTimeoutMs <- int (TimeSpan.FromSeconds(30.).TotalMilliseconds)

if we look at the last two lines, the C# usage is like that:

Communicator = new WebsocketCommunicator(wsUrl) { Name = "tst", ReconnectTimeoutMs = (int) TimeSpan.FromSeconds(30).TotalMilliseconds };

Now I read that to make a class constructor, I have to use the 'new' keyword; so I make the fields members and do a 'new' section:

    member this.communicator : TestCommunicator
    member this.client : WebsocketClient

    new() =
        this.communicator <- new TestCommunicator(url)
        this.client <- new WebsocketClient(this.communicator)

but this doesn't work (line 15 is the top line in this example)

Socket.fs(15, 64): [FS0010] Incomplete structured construct at or before this point in member definition. Expected 'with', '=' or other token.

my questions are:

• How to make this work?
• What does 'new' bring that 'do' doesn't?

Answer 1

Now I read that to make a class constructor, I have to use the 'new' keyword

To make additional class constructors you should use the new keyword. The primary constructor is sufficient for you here.

In member this.communicator: TestCommunicator , while you have specified the type of the property communicator , you have not specified how to get (or set) this property. It is missing =... or with get/set as the error message says.

new() =
    this.communicator <- new TestCommunicator(url)
    this.client <- new WebsocketClient(this.communicator)

When you fix the previous error, you will get another error here because 1. new() should return a Socket , and 2. it references this and url , which are not in scope.

Probably what you want is:

type Socket(url, id, key) =
    let communicator =
        new TestCommunicator(url,
            Name = "test",
            ReconnectTimeoutMs = int (TimeSpan.FromSeconds(30.).TotalMilliseconds))
    let client = new WebsocketClient(communicator)
    member _.Communicator = communicator
    member _.Client = client

While this answer does what you asked for, better would be to read the spec for classes ( See https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/classes and https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/members/ ) and some examples of usage. Your code suggests you were guessing which is not the right approach.