[ruby-core:109305] [Ruby master Feature#18773] deconstruct to receive a range

Issue #18773 has been updated by kddeisz (Kevin Newton).


@nevans I think that's a _really_ good idea. It has the advantage of being backward-compatible as well. @mame, @ktsj what do you think about allowing Enumerable to be returned from deconstruct and using that to match instead? 

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Feature #18773: deconstruct to receive a range
https://bugs.ruby-lang.org/issues/18773#change-98441

* Author: kddeisz (Kevin Newton)
* Status: Assigned
* Priority: Normal
* 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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