[ruby-core:109048] [Ruby master Feature#18773] deconstruct to receive a range

Issue #18773 has been updated by ktsj (Kazuki Tsujimoto).


As a designer of pattern matching, I also understand your motivation.
However, I have the following concerns in addition to the ones mame pointed out.

* In the current implementation, when a pattern match fails in one-line pattern matching, the reason for the failure is displayed as an error message. I think it difficult to do the same thing by your proposal. (This feature is not a must, but it would be nice to have.)

```
$ ruby -e ‘[0] => []’
-e:1:in `<main>’: [0]: [0] length mismatch (given 1, expected 0) (NoMatchingPatternError)
```

* It might have a negative impact on performance. The current implementation caches the return value of `deconstruct` method, but this approach will no longer be available.


----------------------------------------
Feature #18773: deconstruct to receive a range
https://bugs.ruby-lang.org/issues/18773#change-98198

* Author: kddeisz (Kevin Newton)
* Status: Assigned
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
----------------------------------------
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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