Feature #16769, immutable: true)

Added by k0kubun (Takashi Kokubun) 8 months ago. Updated 8 months 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

Related to Ruby master - Feature #16122: Struct::Value: simple immutable value objectFeedbackActions

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