Feature #15357: Proc#parameters returns incomplete type information

• Status: Closed
• Assignee: -
• Target version: -

[larskanis (Lars Kanis)]

The current implementation of Proc#parameters differentiate between lambda? true and false.
This is presumably due to the fact, that procs use tricks to apply arguments differently than lambda and methods.

Unfortunately proc{}.parameters states all :req parameters as :opt, so that these both types of parameters are not distinguishable. This information loss leads to the situation that two different proc signatures return an equal parameters list, but behave differently:

pr = proc{|a,b=2| [a,b] }
pr.parameters  # => [[:opt, :a], [:opt, :b]]
pr.call(:a)    # => [:a, 2]  # :a is interpret as the first parameter

pr = proc{|a=1,b| [a,b] }
pr.parameters  # => [[:opt, :a], [:opt, :b]]
pr.call(:a)    # => [1, :a]  # :a is interpret as the second parameter

That means that the current return values of proc{}.parameters are not suitable to build wrapper or proxy objects for a proc. In Eventbox a workaround is used: The proc is passed to define_method and the method parameters are retrieved instead. That way the list of parameters can be retrieved including :req and :opt differentiation, so that a wrapper proc can be built.

The application of argument assignment tricks is a property of the Proc object - not a property of single parameters. Therefore it shouldn't be applied to Proc#parameters in addition - at least not in a way that discards valuable information. The property of the Proc object is already readable through Proc#lambda?, so that there's no need to apply this property to Proc#parameters as well.

My proposal is to unify proc{}.parameters and lambda{}.parameters, so that proc{}.parameters shows positional arguments without default value as type :req as well.

[larskanis (Lars Kanis)]

• Subject changed from Unify proc{} and lambda{}.parameters to Proc#parameters returns incomplete type information
• Description updated (diff)

[jeremyevans0 (Jeremy Evans)]

• File proc-parameters-req-15357.patch proc-parameters-req-15357.patch added
• Tracker changed from Bug to Feature
• ruby -v deleted (ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux])
• Backport deleted (2.4: UNKNOWN, 2.5: UNKNOWN)

I don't think the current behavior is a bug, as the parameters for a non-lambda proc are optional. Also, changing the behavior could be backwards incompatible.

I propose instead a lambda keyword to Proc#parameters, which would return the parameters as if the proc were a lambda. Attached is a patch that implements this.

[larskanis (Lars Kanis)]

Sorry for my late answer! I didn't get a notification mail.

As noted above: The application of argument assignment tricks is a property of the Proc object - not a property of single parameters. Therefore it shouldn't be applied to Proc#parameters in addition to Proc#lambda? . I therefore think it's kind of bug.

Nevertheless your patch looks like a good alternative, which keeps backward compatibility and doesn't discard information when called as proc.parameters(lambda:true). Would you still mind to merge it?

[Eregon (Benoit Daloze)]

• Related to Feature #16499: define_method(non_lambda) should not change the semantics of the given Proc added

[Eregon (Benoit Daloze)]

I agree, and discussed that a bit in #16499.

I think Proc#parameters should expose source-level information, not interpret it according to Kernel#lambda?.

[jeremyevans0 (Jeremy Evans)]

@matz (Yukihiro Matsumoto) considered this during the March 2020 developers meeting, but has not yet made a decision on it.

[matz (Yukihiro Matsumoto)]

Hmm, I accept lambda: keyword argument (default false). There might be a better candidate than lambda but it's acceptable at least.

Matz.

[Eregon (Benoit Daloze)]

A lambda: kwarg when it's meant to be used for non-lambda Procs seems confusing.
How about parameters(from: :source), and the default would be parameters(from: :behavior)?
Could also be parameters(from_source: true/false) or so.

• Status changed from Open to Closed

Applied in changeset git|b6804d62f822237e136e698e006c913df9990ec2.

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Make Proc#parameters support lambda keyword for returning parameters as if lambda

This makes it easier to use Proc#parameters to build wrappers.

Implements [Feature #15357]