Feature #16372: Allow pattern matching to bind instance variables

• Status: Rejected
• Assignee: -
• Target version: -

[jnchito (Junichi Ito)]

I tried this code, but got sytax error:

case [1, 2]
in @a, @b
end

syntax error, unexpected instance variable, expecting '}'
    in @a, @b
       ^~

I think it would be more useful if we could bind instance variables with pattern matching.

Use case

I understand this is not Rails way, but we have some codes like this:

def Foo.create_foo(params)
  if valid?(params)
    [:ok, create(params)]
  else
    [:ng, nil]
  end
end

class FooController < ApplicationController
  def create
    result, @foo = Foo.create_foo(foo_params)

    case result
    when :ok
      redirect_to @foo
    when :ng
      # @foo will be used in view
      render :new
    else
      raise "Unknown result: #{result}"
    end
  end
end

I thought I could make it simpler with pattern matching (but impossible):

class FooController < ApplicationController
  def create
    case Foo.create_foo(foo_params)
    in :ok, @foo
      redirect_to @foo
    in :ng, _
      render :new
    end
  end
end

[shevegen (Robert A. Heiler)]

If I understood you correctly then the suggestion is to allow both local and
instance variables become assignable, when before pattern matching would only
be possible to local variables, yes?

If that is the case then I think this might make sense, since we can assign
to local variables and instance variables as-is, so that would seem logical
to have pattern matching work on instance variables as well. It might be
useful to ask the ruby user who suggested pattern matching too about this,
perhaps there was a reason @variables were not considered (but it could
simply be that he did not think about it at all when suggesting the
feature).

[jnchito (Junichi Ito)]

shevegen (Robert A. Heiler) wrote:

If I understood you correctly then the suggestion is to allow both local and
instance variables become assignable, when before pattern matching would only
be possible to local variables, yes?

Yes, correct. And people might want to use @@variables and $variables too (I rarely use them, though) .

[matz (Yukihiro Matsumoto)]

• Status changed from Open to Rejected

Extending pattern variables to non-local variables would make pattern-matching more complex and confusing. I reject the idea (at least for now). There might be a chance to make pinned variables (^a) to pinned expression in the future.

Matz.

[ktsj (Kazuki Tsujimoto)]

• Related to Feature #18408: Allow pattern match to set instance variables added