Feature #18961: Introduce support for pattern matching all elements of an array

• Status: Open
• Assignee: -
• Target version: -

[zeke (Zeke Gabrielse)]

When pattern matching arrays, often times we want to assert that a given array's elements all match a pattern.

For example:

class Robot; end
class User; end

case [User.new, User.new]
in [User, *] => u if u.all? { _1 in User }
  puts 'array of users!'
end
# => array of users!

case [User.new, User.new, Robot.new]
in [User, *] => u if u.all? { _1 in User }
  puts 'array of users!'
end
# => Error: guard clause does not return true (NoMatchingPatternError)

I propose an improved syntax for matching an array's elements:

case [User.new, User.new]
in [User*]
  puts 'array of users!'
end

Putting the * on the right-hand side differentiates the syntax from the array Splat operator. Another option would be **, or perhaps using a range User...

Not only is the pattern more readable, it also allows these constraints to be expressed in single line pattern matching:

[User.new, User.new] => [User*] => arr
# => [#<User:0x00000001074f5828>, #<User:0x00000001074f56e8>]

[User.new, Robot.new] => [User*] => arr
# => [#<User:0x00000001074f56e8>, #<Robot:0x00000001074f5580>] does not return true (NoMatchingPatternError)

[User.new, User.new, Robot.new] => [User*, Robot] => arr
# => [#<User:0x00000001074f5828>, #<User:0x00000001074f56e8>, #<Robot:0x00000001074f5580>]

[User.new, User.new] in [User*]
# => true

[User.new, Robot.new] in [User*]
# => false

It allows single line pattern matching to express similar constraints to case in, since guard clauses are not available for single line pattern matching:

[User.new, User.new] in [User, *] => u if u.all? { _1 in User }
# => undefined method `all?' for nil:NilClass (NoMethodError)