From: duerst <noreply@...>
Date: 2021-09-12T22:54:31+00:00
Subject: [ruby-core:105209] [Ruby master Feature#17355] Using same set of names in or-patterns (pattern matching with Foo(x) | Bar(x))

Issue #17355 has been updated by duerst (Martin D�rst).


I'm confused by this code:
```
case [1, 2]
in [1, a] | [a, 3] => a then a
end # duplicated variable name
```

The variable isn't used twice, it's used four times. We can ignore the last one, because this is just to give a result back, and of course we should be able to give a back if we matched it.

But the third `a` is very confusing. We match `a` to 2 because `[1, 2]` matches `[1, a]` and not `[a, 2]`, but then we match `[1, a]` again to `a`? It looks like an attempt to some kind of recursive unification, which is definitely not possible in Ruby.

So I would be okay to allow
```
case [1, 2]
in [1, a] | [a, 3] => b then a
end # duplicated variable name
```
or of course also
```
case [1, 2]
in [1, a] | [a, 3] => b then b
end # duplicated variable name
```
but anything that comes close to
```
in [1, a] | [a, 3] => a
```
seems confusing and dangerous.

----------------------------------------
Feature #17355: Using same set of names in or-patterns (pattern matching with Foo(x) | Bar(x))
https://bugs.ruby-lang.org/issues/17355#change-93617

* Author: decuplet (Nikita Shilnikov)
* Status: Assigned
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
----------------------------------------
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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