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Feature #17355

open

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

Added by decuplet (Nikita Shilnikov) over 1 year ago. Updated 10 months ago.

Status:
Assigned
Priority:
Normal
Target version:
-
[ruby-core:101143]

Description

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's thoughts on this.

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