From: fg@...
Date: 2020-11-29T19:41:29+00:00
Subject: [ruby-core:101143] [Ruby master Feature#17355] Or-patterns (pattern matching like Foo(x) | Bar(x))

Issue #17355 has been reported by decuplet (Nikita Shilnikov).

----------------------------------------
Feature #17355: Or-patterns (pattern matching like Foo(x) | Bar(x))
https://bugs.ruby-lang.org/issues/17355

* Author: decuplet (Nikita Shilnikov)
* Status: Open
* Priority: Normal
----------------------------------------
Given pattern matching is officially supported in Ruby 3, I have an idea about making it more flexible.

Currently, this piece of code produces a syntax error
```ruby
case [1, 2]
in [1, a] | [a, 3] => a then a
end # duplicated variable name
```
Duplications don't seem to be a problem here, semantically-wise. We just need to check if all patterns have the same set of names. It's supported in OCaml (also here's an RFC in Rust https://github.com/rust-lang/rust/issues/54883) so I think it can work in Ruby too.

I've been using pattern matching in Ruby since day 1 and it worked great so far. Since I use OCaml daily too I miss this feature every once in a while :) 
A more practical example: imagine you have code like this

```ruby
def user_email(user)
  case user
  in User(email:) then email
  in Admin(email:) then email
  in Moderator(email:) then email
  end
end
```

Clearly, it could be simplified if or-patterns were supported:

```ruby
def user_email(user)
  case user
  in User(email:) | Admin(email:) | Moderator(email:) then email
  end
end
```

I'd like to know @ktsj's thoughts on this.



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