F# alternate constructor assigning values to (mutable) let bindings

Question

Suppose I have this class:

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

I want to be able to create a new Pet based on some serialized data, which I represent with this record:

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

(TLDR: I can't figure out the syntax to make a constructor that'll take a PetData to populate the let bindings. My various attempts follow.)

So I make a new Pet constructor that'll assign values to the let bindings. I try using the class initializer syntax:

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

Hmm, nope: 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.

I check to make sure I've got all the syntax: no missing commas, correct "assignment" (cough) operator, correct indentation.

Okay the, I'll try the record initializer syntax.

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

Error: The type 'Pet' does not contain a field 'name'

Okay, so I need to call the main constructor. I guess there are probably two places I can put it, so let's try both:

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

Nope: Invalid object, sequence or record expression

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

And nope: 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.

I didn't want to have to do this, but maybe since the fields are mutable anyway, I can just assign values to the object after initializing it:

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'

Lol, what??

Okay, let's try this:

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

Right, so it can't access let bindings from outside.

Let's try a member then:

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.

Okay... let's try this whole thing differently then, using explicit fields:

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

And that's when I punch my elderly cat in the face.

Any other ideas? These error messages are throwing me off. I can't even find half of them on Google.

gradbot gradbot · Accepted Answer · 2011-12-17T03:00:59

You could do this.

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# has its own style which looks like this.

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)

Edit

Added an event used in do per comment request.

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)

F# alternate constructor assigning values to (mutable) let bindings

2 Answers