F# 备用构造函数为(可变的)let 绑定(bind)赋值

Question

假设我有这个类:

type Pet (name:string) as this =
    let mutable age = 5
    let mutable animal = "dog"

我希望能够创建一个新的 Pet基于一些序列化的数据，我用这条记录表示:

type PetData = {
    name : string
    age : int
    animal : string
}

(TLDR:我无法弄清楚创建一个构造函数的语法，该构造函数将使用 PetData 来填充 let 绑定(bind)。​​接下来是我的各种尝试。)

所以我做了一个新的 Pet将值分配给 let 绑定(bind)的构造函数。我尝试使用类初始化语法:

new (data:PetData) =
    Pet(name,
        age = data.age,
        animal = data.animal
    )

嗯，不: No accessible member or object constructor named 'Pet' takes 1 arguments. The named argument 'age' doesn't correspond to any argument or settable return property for any overload.
我检查以确保我掌握了所有语法:没有缺少逗号，正确的“赋值”(咳嗽)运算符，正确的缩进。

好的，我将尝试记录初始值设定项语法。

new (data:PetData) =
    {
        name = data.name;
        age = data.age;
        animal = data.name
    }

错误: The type 'Pet' does not contain a field 'name'
好的，所以我需要调用主构造函数。我想可能有两个地方可以放，所以让我们两个都试试:

new (data:PetData) =
    {
        Pet(data.name);
        age = data.age;
        animal = data.name
    }

没有: Invalid object, sequence or record expression

new (data:PetData) =
    Pet(data.name)
    {
        age = data.age;
        animal = data.name
    }

不: This is not a valid object construction expression. Explicit object constructors must either call an alternate constructor or initialize all fields of the object and specify a call to a super class constructor.
我不想这样做，但也许因为字段无论如何都是可变的，我可以在初始化对象后为其赋值:

new (data:PetData) =
    let p = Pet(data.name)
    p.age <- data.age
    p.animal <- data.animal
    p

Type constraint mismatch. The type Pet is not compatible with type PetData The type 'Pet' is not compatible with the type 'PetData'
笑什么？？

好的，让我们试试这个:

let assign(data:PetData) =
    this.age <- data.age
    this.animal <- data.animal

new (data:PetData) =
    let p = Pet(data.name)
    p.assign(data)
    p

The field, constructor or member 'assign' is not defined
对，所以它不能从外部访问 let 绑定(bind)。

让我们尝试一个成员:

new (data:PetData) =
    let p = Pet(data.name)
    p.Assign(data)
    p

member x.Assign(data:PetData) =
    this.age <- data.age
    this.animal <- data.animal

This is not a valid object construction expression. Explicit object constructors must either call an alternate constructor or initialize all fields of the object and specify a call to a super class constructor.
好的......让我们尝试这整个事情不同，然后使用显式字段:

type Pet =
    [<DefaultValue>]val mutable private age : int
    [<DefaultValue>]val mutable private animal : string
    val private name : string

    new(name:string) =
        { name = name }

    new(data:PetData) =
        {
            name = data.name;
            age = data.age;
            animal = data.animal
        }

Extraneous fields have been given values
那是我打我老猫的脸的时候。

还有其他想法吗？这些错误消息让我失望。我什至无法在 Google 上找到其中的一半。

Answer 1

你可以这样做。

type Pet =
    val mutable private age : int
    val mutable private animal : string
    val private name : string

    new (name:string) =
        { 
            name = name;
            age = 5; // or age = Unchecked.defaultof<_>;
            animal = "dog"; // or animal = Unchecked.defaultof<_>;
        }

    new (data:PetData) =
        {
            name = data.name;
            age = data.age;
            animal = data.animal;
        }

F# 有自己的风格，看起来像这样。

type Pet(name:string, age:int, animal:string) =
    let mutable age = age
    let mutable animal = animal

    new (name:string) =
        Pet(name, 5, "dog")

    new (data:PetData) =
        Pet(data.name, data.age, data.animal)

编辑

添加了 do 中使用的事件每个评论请求。

type Pet(name:string, age:int, animal:string, start:IEvent<string>) =
    let mutable age = age
    let mutable animal = animal

    // all three constructors will call this code.
    do  start.Add (fun _ -> printf "Pet was started")

    new (name:string, start:IEvent<_>) =
        // an example of different logic per constructor
        // this is called before the `do` code.
        let e = start |> Event.map (fun x -> x + " from 'name constructor'")
        Pet(name, 5, "dog", e)

    new (data:PetData, start:IEvent<_>) =
        Pet(data.name, data.age, data.animal, start)