Sorbet: Does not allow ->() to be used in a module

[YunZhi246]

Input

→ View on sorbet.run

# typed: strict
module Bar
  extend(T::Sig)

  sig { returns(T.proc.params(a: T.untyped, b: T.untyped).void) }
  def hello
    -> (a,b) { a + b }
  end
end

Observed output

editor.rb:7: Method lambda does not exist on Bar https://srb.help/7003
     7 |    -> (a,b) { a + b }
            ^^^^^^^^^^^^^^^^^^
  Did you mean to `include Kernel` in this module?
    https://github.com/sorbet/sorbet/tree/master/rbi/core/kernel.rbi#L1715: Did you mean: Kernel#lambda?
    1715 |  def lambda(&blk); end
            ^^^^^^^^^^^^^^^^
Errors: 1

Expected behaviour

No issues to be displayed.

[jez]

This is expected behavior. See https://github.com/sorbet/sorbet/issues/2236

[aisamanra]

This is something we could model more precisely, but it's going to be a lot of work for comparatively little benefit. You can already work around this problem by using include Kernel in the body of your module, like the error message suggests.

[paracycle]

@jez I think this particular case is a little different, though, since this is actually valid Ruby code being flagged as invalid by Sorbet.

For example:

module Bar
  def hello
    -> (a,b) { a + b }
  end
end

class Foo < BasicObject
  include ::Bar
end

Foo.new.hello # is ok
Foo.new.hello.(1, 2) #=> 3

I think the problem is that Sorbet desugars -> to lambda but lambda happens to be a Kernel method, thus causing the error in Sorbet. However, at runtime -> has slightly different semantics than Kernel#lambda, so the code is actually valid.

But I see the point @aisamanra is making, that it might be way more hassle to model this precisely. I just wanted to point out that this is not exactly the same case as #2236, though.

[Morriar]

What about desugaring -> as Kernel.lambda?

module Bar
  def hello
    Kernel.lambda { |a, b| a + b }
  end
end

class Foo < BasicObject
  include ::Bar
end

puts Foo.new.hello # is ok
puts Foo.new.hello.(1, 2) #=> 3

And it typechecks.

[DarkDimius]

Nice idea! Would you be up to make a PR with it?