Feature #10499: Eliminate implicit magic in Proc.new and Kernel#proc

• Status: Closed
• Priority: Normal
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[headius (Charles Nutter)]

Proc.new and Kernel#proc have a little known feature: if called without a block, they capture whatever block was passed to the current method.

I propose that this feature should be removed, finally, since it:

• Doesn't enhance readability (where is this block coming from?)
• Doesn't reflect any other behavior in Ruby
• Can lead to bugs (call either without a block accidentally and you aren't sure what you'll get)

I believe this was an implementation artifact in MRI, since the most recently-pushed block would still be on global stacks, which is where the logic for proc and Proc.new looked for it.

All argument syntaxes now support &block, which I believe is the correct way to clearly, explicitly capture the incoming block into an object.

Thoughts?

[marcandre (Marc-Andre Lafortune)]

• Category set to core
• Assignee set to matz (Yukihiro Matsumoto)

I agree.

Deprecate first (2.2?), remove afterwards.

This would also simplify things if and when we want to warn/raise on unused blocks when calling user methods.

[headius (Charles Nutter)]

Adding a deprecation warning would be easy if we can get buy-in from matz.

matz: ball's in your court, I think!

[k0kubun (Takashi Kokubun)]

• Copied to Feature #15554: warn/error passing a block to a method which never use a block added

[naruse (Yui NARUSE)]

• Target version deleted (3.0)

[dazuma (Daniel Azuma)]

Was this change implemented in Ruby 3.0.0?

It looks to me like it was. (And this is causing test failures on libraries that depend on this behavior.)

Ruby 2.7.2:

def foo
  Proc.new
end

foo          # => ArgumentError
foo { :hi }  # => Proc

Ruby 3.0.0:

def foo
  Proc.new
end

foo          # => ArgumentError
foo { :hi }  # => ArgumentError

[dazuma (Daniel Azuma)]

Note that the documentation still specifies the old behavior in 3.0.0.

Creates a new Proc object, bound to the current context. ::new may be called without a block only within a method with an attached block, in which case that block is converted to the Proc object.

https://ruby-doc.org/core-3.0.0/Proc.html#method-c-new
https://github.com/ruby/ruby/blob/95aff214687a5e12c3eb57d056665741e734c188/proc.c#L800-L813

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset git|8da7f4abc779c6549833d718336d75aac98494b8.

---

[DOC] Update Proc.new without a block [ci skip]

[Feature #10499]
[Feature #15554]

[nobu (Nobuyoshi Nakada)]

Thank you, updated.