[ruby-core:108899] [Ruby master Feature#18773] deconstruct to receive a range

Issue #18773 has been updated by mame (Yusuke Endoh).


It would be easier to discuss if you could write a spec of what pattern match will pass what range. I understand as follows by reading your implementation. Right?

* `ary in [1, 2, 3]` will call `ary.deconstruct(3..3)`, which means "the length must be exactly 3"
* `ary in [1, 2, *, 3]` will call `ary.deconstruct(3..)`, which means "the length must be greater than or equal to 3"

I understand your motivation, but I wonder if the spec could be more efficient. In the second calling sequence, the match requires only the first, second and last elements, but `ary.deconstruct(3..)` needs to create an array including all elements because it does not know which elements are required.

Though the current implementation of pattern matching is not so efficient, but I am afraid that the proposed implementation looks very inefficient because it creates a Method object and calls `#arity`.

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Feature #18773: deconstruct to receive a range
https://bugs.ruby-lang.org/issues/18773#change-97979

* 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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