From: "zeke (Zeke Gabrielse)" <noreply@...>
Date: 2022-08-10T16:46:26+00:00
Subject: [ruby-core:109465] [Ruby master Feature#18961] Introduce support for pattern matching all elements of an array

Issue #18961 has been reported by zeke (Zeke Gabrielse).

----------------------------------------
Feature #18961: Introduce support for pattern matching all elements of an array
https://bugs.ruby-lang.org/issues/18961

* Author: zeke (Zeke Gabrielse)
* Status: Open
* Priority: Normal
----------------------------------------
When pattern matching arrays, often times we want to assert that a given array's elements all match a pattern.

For example:

```ruby
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:

```ruby
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:

```ruby
[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:

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



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