Init-only & required properties support

[smoothdeveloper]

This is initialising only, the required discussion about F# compiler to honour C# types that bear init and required modifiers on properties.

• RFC
• PR

[smoothdeveloper]

Assuming C# required properties works with the "initializer" syntax (I gather that is the point of the feature); there will be need for F# to distinguish the property setter that are accepted into any method call, versus those that are for a constructor call:

public class WithRequired {
   public required int Foo {get; set;}
   public required int Bar {get; init;}
}

var ok = new WithRequired {Foo=42, Bar =42}

type WithRequiredFactory =
  static member MakeDefault() = WithRequired(Foo=42, Bar=42)
  static member MakeWithFoo(foo) = WithRequired(Foo=foo, Bar=42)

let notOk = WithRequiredFactory.MakeDefault(Bar=42) // can't set because `Bar` is init-only
let notOk2 = WithRequiredFactory.MakeWithFoo(1, Bar=42) // can't set because `Bar` is init-only
let ok = WithRequiredFactory.MakeDefault(Foo=43) // an extra call to Foo setter is appended before returning the expression
let ok2 = WithRequired(Foo=42, Bar=42)

This may have weird interaction with inline member.

Initially, I always seen initializer syntax in C# as "syntax sugar" only, but now it seems to evolve into "checks on the caller/initializer".

[NinoFloris]

Hahah what is this syntax even. I'm extremely surprised this is valid today. Was it a deliberate choice for this to work outside constructor calls @dsyme?

[vzarytovskii]

Hahah what is this syntax even. I'm extremely surprised this is valid today. Was it a deliberate choice for this to work outside constructor calls @dsyme?

Yeah, it's pretty much a property initializer syntax outside of constructor. I have learned it today after chatting with @dsyme :D

[vzarytovskii]

Assuming C# required properties works with the "initializer" syntax (I gather that is the point of the feature); there will be need for F# to distinguish the property setter that are accepted into any method call, versus those that are for a constructor call:

public class WithRequired {
   public required int Foo {get; set;}
   public required int Bar {get; init;}
}

var ok = new WithRequired {Foo=42, Bar =42}

type WithRequiredFactory =
  static member MakeDefault() = WithRequired(Foo=42, Bar=42)
  static member MakeWithFoo(foo) = WithRequired(Foo=foo, Bar=42)

let notOk = WithRequiredFactory.MakeDefault(Bar=42) // can't set because `Bar` is init-only
let notOk2 = WithRequiredFactory.MakeWithFoo(1, Bar=42) // can't set because `Bar` is init-only
let ok = WithRequiredFactory.MakeDefault(Foo=43) // an extra call to Foo setter is appended before returning the expression
let ok2 = WithRequired(Foo=42, Bar=42)

This may have weird interaction with inline member.

Initially, I always seen initializer syntax in C# as "syntax sugar" only, but now it seems to evolve into "checks on the caller/initializer".

Yeah, this is one of the points in the "TODO" list on the PR, to make sure we only allow to call init-only setter from the constructor. I've learned of this possibility today :D

[smoothdeveloper]

Syntax is great syntactical sugar when doing "object programming", I hope it doesn't disappear, albeit, I wish it used : rather than =.

In the F# 2.0 language specification:

14.4 Method Application Resolution
Resolves post-hoc property assignments.

There is also similar note in 6.4.1 Basic Application Expressions

[64J0]

Hey, is it possible to annotate an F#'s property value with this required modifier? Similar to that C# snippet mentioned at this discussion:

public class WithRequired {
   public required int Foo {get; set;}
   public required int Bar {get; init;}
}

var ok = new WithRequired {Foo=42, Bar =42}

[vzarytovskii]

I don't think it is.

[64J0]

Got it, thanks for the clarification @vzarytovskii