[ruby-core:104215] [Ruby master Feature#17942] Add a `initialize(public @a, private @b)` shortcut syntax for defining public/private accessors for instance vars as part of constructor

Issue #17942 has been updated by jeremyevans0 (Jeremy Evans).


You should probably read @matz's response to a previous request for instance variable parameters: https://bugs.ruby-lang.org/issues/8563#note-3 (which he confirmed had not changed as of 2017: https://bugs.ruby-lang.org/issues/8563#note-18). Your proposal is more complex, but doesn't address the complaint @matz has regarding using instance variables as parameters.  A new issue with your proposal is it would turn `public`/`private` into keywords, when they are currently just methods.

If you want really concise class definitions, use `Struct`.  It doesn't get much more concise than:

```ruby
Thing = Struct.new(:a, :b, :c)
```

----------------------------------------
Feature #17942: Add a `initialize(public @a, private @b)` shortcut syntax for defining public/private accessors for instance vars as part of constructor
https://bugs.ruby-lang.org/issues/17942#change-92399

* Author: TylerRick (Tyler Rick)
* Status: Open
* Priority: Normal
----------------------------------------
This proposal builds on the proposed `initialize(@a, @b)` instance var assignment shortcut syntax described in #15192.

1. It allows you to add an *optional* `public`/`protected`/`private` modifier before any instance var parameter. Doing so automatically defines *accessor methods* (with the given access modifier; equivalent to `attr_accessor` inside of a  `public`/`protected`/`private` block) for the instance var it precedes.
2. If the visibility modifier is omitted, then it defaults to automatically _no_ getter/setter methods for that instance var (it _only_ does an assignment of that already-private instance var).

## Parameter properties in TypeScript language

This is inspired by TypeScript's `constructor(public a, private b)` syntax, which allows you to write this ([REPL](https://www.typescriptlang.org/play?#code/MYGwhgzhAEBiD29oG8BQ0PWPAdhALgE4Cuw+8hAFAA7EBGIAlsNGAFw7EC2dApoQBpotBs2h0O3PoOGFGANzD5eWST34BKFOkwBfVPqA)):
```js
class Foo {
    constructor(public a:number, public b:number, private c:number) {
    }
}
```
instead of this:
```js
class Foo {
    constructor(a, b, c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }
}
```

(The `public`/`private` access modifiers actually disappear in the transpiled JavaScript code because it's only the TypeScript compiler that enforces those access modifiers, and it does so at *compile* time rather than at run time.)

Further reading:
- https://www.typescriptlang.org/docs/handbook/2/classes.html#parameter-properties
- https://basarat.gitbook.io/typescript/future-javascript/classes#define-using-constructor
- https://kendaleiv.com/typescript-constructor-assignment-public-and-private-keywords/


## Differences from TypeScript

I propose adding a similar feature to Ruby, but with following differences from TypeScript:

1. Use **`@a`** instead of bare `a`. This makes it *much* clearer that you are assigning directly to instance variables instead of to locals.
    - Rationale: The `@` is actually _part_ of the instance variable name, and is inseparable from it. (This is also consistent with how the `#` is part of the name itself in JavaScript's [(Private instance fields)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields#private_instance_fields).)
    - (`public a` would be a syntax error because there's no such thing as access modifiers for locals. Okay, I guess there's no such thing as access modifiers for instance vars either, which is why...)

1. Make the syntax for ***assigning*** to instance vars (`@a`) (the proposal in #15192) and defining ***accessor methods*** for those instance vars (`public`/`private`) separate/distinct.
    - In other words, rather than make the `public`/`private` keywords a *required* part of the syntax like it is for TypeScript [parameter properties](https://www.typescriptlang.org/docs/handbook/2/classes.html#parameter-properties), you could omit the modifier and it would still do the instance var _assignment*.
    - The `public`/`private` access modifiers be an additional (*optional*) shortcut when you want to add an ***accessor method*** in *addition* to doing an ***assignment*** .
    - Unlike Java and TypeScript where you _can_ add access modifiers to instance variables, in Ruby, `public`/`private` _can't_ be applied to instance variables (direct access is only possible from within the instance). So if we're going to allow a `public`/`private` modifier here at all, They _must_ refer to methods, specifically accessor methods for those instance variables.

1. Keep it **private** by default (which of course `@a` by itself implies—it _is_ private unless you add a public accessor).
    - (Rather than make it `public` by default like it is in TypeScript.)
    - Keeping instance variables completely private is probably what people will want most of the time, and we should optimize the ergonomics for the most common case.
    - Private is a safer default, and should be assumed unless you explicitly ask for a public accessor to be added.
    - I bet TypeScript made the `public` the default mostly to be consistent with JavaScript (which TypeScript compiles to): JavaScript (along with other languages like Java) allows direct access (no getter/setter neede) to instance properties/variables from objects outside the instance. JavaScript doesn't even _have_ a way to make instance variables private (but hopefully will soon with this [proposal](https://github.com/tc39/proposal-private-methods) to add `#a` syntax for private properties).

So this:

```ruby
class Thing
  def initialize(public @a, public @b, @c)
  end
end
```

would be equivalent to this:

```ruby
class Thing
  attr_accessor :a, :b

  def initialize(a, b, c)
    @a = a
    @b = b
    @c = c
  end
```

## How is `initialize(private @a)` different from `initialize(@a)`?

Even though `@a` by itself is already private...
1. This defines a private accessor for that instance var, which lets you write `self.a =` instead of `@a =` (if you want).
2. Having a concise way to do that is helpful, for example if you want to make it a matter of practice/policy to only set an instance variable by going through its *setter method*. (See [discussion here](https://stackoverflow.com/questions/25571642/ruby-private-and-public-accessors).)


Why not just use `initialize(private @a)` to be consistent with TypeScript spec?
- TypeScript's `public`/`private` is not standard JavaScript. In fact, if the [private methods/fields proposal](https://github.com/tc39/proposal-private-methods) had existed when TypeScript added [parameter properties](https://www.typescriptlang.org/docs/handbook/2/classes.html#parameter-properties), I'd like to think that they might have actually *made use* of the new `#b` syntax and gone with a terser syntax like `constructor(public a, #b)` instead of ``constructor(public a, private b)`.


## Upsides of this proposal

1. Removes even more boilerplate (all those `attr_accessor` lines), much of the time

## Downsides of this proposal

1. Only provides a way to define both getter and setter at once. Doesn't provide a way to _just_ define a getter and not a setter, for example.
    - Doesn't seem like a big deal, however. You can just not use this feature and define the getter with `attr_reader :a` instead. Or define private getter/setter with `private @a` and then override with `attr_reader :a` to add a public getter (while keeping the private setter).




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