Feature #16769: Struct.new(..., immutable: true)

• Status: Rejected
• Assignee: -
• Target version: -

[k0kubun (Takashi Kokubun)]

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

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_readers 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_readers 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.

[k0kubun (Takashi Kokubun)]

• Tracker changed from Bug to Feature
• Backport deleted (2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN)

[k0kubun (Takashi Kokubun)]

• Related to Feature #16122: Data: simple immutable value object added

[shevegen (Robert A. Heiler)]

Makes sense.

[Eregon (Benoit Daloze)]

Agreed, and @ioquatix (Samuel Williams) and @headius (Charles Nutter) seemed positive too in some recent discussion.

[mame (Yusuke Endoh)]

It would be good to reuse an existing "freeze" mechanism.

Post = Struct.new(:id, :name, freeze: true)

post = Post.new(1, "hello world")
p post.frozen? #=> true
post.id = 2 #=> FrozenError

I'd like to avoid the word immutable because it is a new terminology and a negative form.

[nobu (Nobuyoshi Nakada)]

How about:

Freezing = ->*{def initialize(...) super; freeze; end}
Post = Struct.new(:id, :name, &Freezing)

[Eregon (Benoit Daloze)]

@nobu (Nobuyoshi Nakada) setter methods shouldn't be defined, so just .freeze is not enough.

[matz (Yukihiro Matsumoto)]

• Status changed from Open to Rejected

I don't like the keyword argument that changes the fundamental behavior. I prefer #16122 to this proposal.
Let's discuss there.

Matz.

[Eregon (Benoit Daloze)]

Sad to see this rejected as there was a lot of agreement here, and I think #16122 might take a lot longer before anything gets implemented.