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

• Status: Open
• Priority: Normal
• Assignee: matz (Yukihiro Matsumoto)
• Target version: Next Major

[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