[ruby-core:97746] Re: [Ruby master Bug#16769] Struct.new(..., immutable: true)

Nice. Also references this Java JEP
https://openjdk.java.net/jeps/359

On Wed, 8 Apr 2020 at 10:01, <[e-mail]> wrote:

> Issue #16769 has been reported by k0kubun (Takashi Kokubun).
>
> ----------------------------------------
> Bug #16769: Struct.new(..., immutable: true)
> https://bugs.ruby-lang.org/issues/16769
>
> * Author: k0kubun (Takashi Kokubun)
> * Status: Open
> * Priority: Normal
> * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
> ----------------------------------------
> ## Background
> We've discussed interface to pass Struct attributes (like `immutable:
> true`, which is actually not added yet) at once. But I believe just adding
> `immutable: true` alone is really helpful in various cases. Thus I've spun
> out this ticket only for `immutable: true` from [Feature #16122].
>
> ## Proposal
>
> ```rb
> Post = Struct.new(:id, :name, immutable: true)
>
> post = Post.new(1, "hello world")
> post.id = 2 # NoMethodError (undefined method `id=' for #<struct Post
> id=1, name="hello world">)
> ```
>
> Given `immutable: true`, an instance returned by `.new` is frozen, and
> writer methods are not defined.
>
> ## Use case
>
> * Allow using Struct's nice features when we need an immutable model,
> instead of defining a normal class with `attr_reader`s and methods to
> support the Struct's features.
>   * If it were a Struct, `to_s`, `inspect`, `==`, and a bunch of other
> methods are nicely defined by default. Deconstructing a Struct on Pattern
> Matching is also available.
>      * This level of support from the entire ecosystem may not be
> available if it's just a third-party library.
>   * We could achieve a similar thing if we call `Post.new(...).freeze` or
> override `#initialize` to call `freeze` inside it, but it is not fun and
> feels like a workaround.
>      * Today I suggested to use Struct for a model class to take advantage
> of the above benefits in a code review, but the implementation stuck with a
> bare class with `attr_reader`s because the author didn't want writer
> methods to be defined (of course we don't want to manually undef them from
> a Struct class either) and calling `freeze` to workaround it seems tricky.
> I strongly desired Ruby's Struct is useful enough to cover this use case.
>
>
>
> --
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