[ruby-core:71564] [Ruby trunk - Feature #11710] [PATCH] Replace Set#merge with Set#merge! and make Set#merge non-mutating.

Issue #11710 has been updated by Tieg Zaharia.

File non_mutating_set_merge_method.2.diff added

(updating patch with a better change to the | method)

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Feature #11710: [PATCH] Replace Set#merge with Set#merge! and make Set#merge non-mutating.
https://bugs.ruby-lang.org/issues/11710#change-54946

* Author: Tieg Zaharia
* Status: Open
* Priority: Normal
* Assignee: Akinori MUSHA
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The Set#merge method currently mutates its caller. I propose  changing its behavior to non-mutating, and replace its current behavior with a mutating Set#merge! method.

For example, the current behavior:

```
> s = Set.new [1,2,3]  # => #<Set: {1, 2, 3}>
> s.object_id          # => 70125370250380
> s.merge([4,5,6])     # => #<Set: {1, 2, 3, 4, 5, 6}>
> s                    # => #<Set: {1, 2, 3, 4, 5, 6}>
> s.object_id          # => 70125370250380
```

Set describes itself as a hybrid of Array and Hash, but Hash#merge does not mutate its caller, and Set is implemented on top of Hash as well. Hash has a merge! method that can mutate instead:

```
> h = {a: 1, b: 2}            # => {:a=>1, :b=>2}
> h.object_id                 # => 70125369896320
> h.merge({c: 3})             # => {:a=>1, :b=>2, :c=>3}
> h                           # => {:a=>1, :b=>2}
irb(main):015:0> h.object_id  # => 70125369896320
```

We were taken by surprise with the existing behavior of Set#merge, especially since Set follows the bang pattern of mutating/non-mutating method names (e.g. collect!, reject!, select!, flatten!)

I noticed this has been [suggested before](https://bugs.ruby-lang.org/issues/5185), but was hoping it might be possible as a breaking change for 2.3.0?


---Files--------------------------------
non_mutating_set_merge_method.diff (1.47 KB)
non_mutating_set_merge_method.2.diff (1.47 KB)


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