Feature #15357
closedProc#parameters returns incomplete type information
Description
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.
Files
Updated by larskanis (Lars Kanis) almost 6 years ago
- Subject changed from Unify proc{} and lambda{}.parameters to Proc#parameters returns incomplete type information
- Description updated (diff)
Updated by jeremyevans0 (Jeremy Evans) about 5 years ago
- 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.
Updated by larskanis (Lars Kanis) almost 5 years ago
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?
Updated by Eregon (Benoit Daloze) almost 5 years ago
- Related to Feature #16499: define_method(non_lambda) should not change the semantics of the given Proc added
Updated by Eregon (Benoit Daloze) almost 5 years ago
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?
.
Updated by jeremyevans0 (Jeremy Evans) over 4 years ago
@matz (Yukihiro Matsumoto) considered this during the March 2020 developers meeting, but has not yet made a decision on it.
Updated by matz (Yukihiro Matsumoto) over 2 years ago
Hmm, I accept lambda:
keyword argument (default false
). There might be a better candidate than lambda
but it's acceptable at least.
Matz.
Updated by Eregon (Benoit Daloze) over 2 years ago
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.
Updated by jeremyevans (Jeremy Evans) over 2 years ago
- Status changed from Open to Closed
Applied in changeset git|b6804d62f822237e136e698e006c913df9990ec2.
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]