[ruby-core:118241] [Ruby master Feature#18773] deconstruct to receive a range

Issue #18773 has been updated by Eregon (Benoit Daloze).


FWIW I think CRuby currently support subclassing Array and redefining `[]` and pattern matching will call that on CRuby, maybe it can be used for this purpose.
I don't think it's a good idea to rely on this behavior though, it might change. For example that doesn't work on TruffleRuby, which currently always uses standard `Array#[]` from pattern matching.

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Feature #18773: deconstruct to receive a range
https://bugs.ruby-lang.org/issues/18773#change-108740

* Author: kddnewton (Kevin Newton)
* Status: Rejected
* Assignee: ktsj (Kazuki Tsujimoto)
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Currently when you're pattern matching against a hash pattern, `deconstruct_keys` receives the keys that are being matched. This is really useful for computing expensive hashes.

However, when you're pattern matching against an array pattern, you don't receive any information. So if the array is expensive to compute (for instance loading an array of database records), you have no way to bail out. It would be useful to receive a range signifying how many records the pattern is specifying. It would be used like the following:

```ruby
class ActiveRecord::Relation
  def deconstruct(range)
    (loaded? || range.cover?(count)) ? records : nil
  end
end
```

It needs to be a range and not just a number to handle cases where `*` is used. You would use it like:

```ruby
case Person.all
in []
  "No records"
in [person]
  "Only #{person.name}"
else
  "Multiple people"
end
```

In this way, you wouldn't have to load the whole thing into memory to check if it pattern matched. The patch is here: https://github.com/ruby/ruby/pull/5905.



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