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Feature #18773

open

deconstruct to receive a range

Added by kddnewton (Kevin Newton) almost 2 years ago. Updated over 1 year ago.

Status:
Assigned
Target version:
-
[ruby-core:108511]

Description

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:

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:

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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