Feature #17355: Using same set of names in or-patterns (pattern matching with Foo(x) | Bar(x))

• Status: Assigned
• Priority: Normal
• Assignee: ktsj (Kazuki Tsujimoto)
• Target version: -

[decuplet (Nikita Shilnikov)]

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

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

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:

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

I'd like to know ktsj (Kazuki Tsujimoto)'s thoughts on this.

[Dan0042 (Daniel DeLorme)]

Or-patterns are supported, just not with variable assignment. I agree with the request but the title of the ticket is a bit misleading.

But I think the user_email example actually makes a rather good case for the usefulness of And-patterns:

def user_email(user)
  case user
  in (User | Admin | Moderator) & {email:} then email
  end
end

[decuplet (Nikita Shilnikov)]

• Subject changed from Or-patterns (pattern matching like Foo(x) | Bar(x)) to Using same set of names in or-patterns (pattern matching with Foo(x) | Bar(x))

Dan0042 (Daniel DeLorme) wrote in #note-1:

Or-patterns are supported, just not with variable assignment. I agree with the request but the title of the ticket is a bit misleading.

Yeah, thanks, I updated the title.

Dan0042 (Daniel DeLorme) wrote in #note-1:

But I think the user_email example actually makes a rather good case for the usefulness of And-patterns:

def user_email(user)
  case user
  in (User | Admin | Moderator) & {email:} then email
  end
end

Out of curiosity, did you see an example of syntax like that in any other language?

[Dan0042 (Daniel DeLorme)]

decuplet (Nikita Shilnikov) wrote in #note-2:

Out of curiosity, did you see an example of syntax like that in any other language?

No I haven't. I briefly mentioned it in #16464#note-2; it just seemed a logical complement to Or-patterns (pun not intended)

[ktsj (Kazuki Tsujimoto)]

• Assignee set to ktsj (Kazuki Tsujimoto)
• Status changed from Open to Assigned

I am pretty positive. I will discuss it with Matz after I finish to implement.