Feature #16769

closed, immutable: true)

Added by k0kubun (Takashi Kokubun) about 4 years ago. Updated about 4 years ago.

Target version:



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].


Post =, :name, immutable: true)

post =, "hello world") = 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 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.

Related issues 1 (0 open1 closed)

Related to Ruby master - Feature #16122: Data: simple immutable value objectClosedzverok (Victor Shepelev)Actions
Actions #1

Updated by k0kubun (Takashi Kokubun) about 4 years ago

  • Tracker changed from Bug to Feature
  • Backport deleted (2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN)
Actions #2

Updated by k0kubun (Takashi Kokubun) about 4 years ago

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

Updated by Eregon (Benoit Daloze) about 4 years ago

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

Updated by mame (Yusuke Endoh) about 4 years ago

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

Post =, :name, freeze: true)

post =, "hello world")
p post.frozen? #=> true = 2 #=> FrozenError

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

Updated by nobu (Nobuyoshi Nakada) about 4 years ago

How about:

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

Updated by Eregon (Benoit Daloze) about 4 years ago

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

Updated by matz (Yukihiro Matsumoto) about 4 years ago

  • 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.


Updated by Eregon (Benoit Daloze) about 4 years ago

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.


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