Issue #5694 has been updated by Marc-Andre Lafortune.


Matz, what do you want us to do about ->(a=1){}.arity, in the 1.9.3 release and in the 2.0.0 version?

Marc-André wrote in [ruby-core:41708]:
> Hi,
>  
>  On Sun, Dec 11, 2011 at 10:51 PM, Yukihiro Matsumoto <matz / ruby-lang.org> wrote:
>  > Hi,
>  >
>  > In message "Re: [ruby-core:41600] [ruby-trunk - Bug #5694] Proc#arity doesn't take optional arguments into account."
>  >  on Mon, 12 Dec 2011 11:30:26 +0900, Marc-Andre Lafortune <ruby-core / marc-andre.ca> writes:
>  >
>  > |Yukihiro Matsumoto wrote:
>  > |> We are not going to add incompatible changes to trunk, ...
>  > |
>  > |Could you please explain to me what difference you see between an "incompatible change" and any "bug fix"?
>  >
>  > Good point. It's whether the change makes difference from intention
>  > of the original designer. ...
>  
>  So do we agree that my patch is not an incompatible change?
>  
>  >First I thought to keep the original arity
>  > behavior, but after investigating the behavior, both Method#arity and
>  > Proc#arity have weird corner cases.
>  
>  Apart from the one I'm pointing out (which is only present in Ruby
>  1.9), what other corner cases are there? In particular, is there any
>  corner case in the 1.8 line?
>  
>  > My idea for new behavior is:
>  >
>  > * arity ignores all optional arguments
>  > * arity returns -n-1 if there's rest argument
>  > * where n is number of mandatory arguments
>  >
>  > Any opinion?
>  
>  Can you explain what would be gained by this new behavior, i.e. how is
>  this change more helpful than the current behavior?
>  
>  Also, can you explain why you consider that breaking the previous
>  behavior would be a good idea?
>  
>  A quick check for uses of `arity` in Rails reveal that the typical use
>  looks like "does this method use only 1 parameter or can it use more
>  than that?"
>  
>  Among other things, this change could break many Rails app.
>  
>  Moreover, Rails and any other gem using `arity` would have to jump
>  through hoops to maintain a compatible version with Ruby 1.8.7 (which
>  doesn't have the `parameters` method) and Ruby 1.9.2+.
>  
>  Finally, assuming you decide to go forward with this feature change
>  for Ruby 2.0, shouldn't the 1.9 line still be fixed with my patch to
>  be consistent?
>  
>  Thanks


----------------------------------------
Bug #5694: Proc#arity doesn't take optional arguments into account. 
https://bugs.ruby-lang.org/issues/5694

Author: Marc-Andre Lafortune
Status: Open
Priority: Normal
Assignee: Yukihiro Matsumoto
Category: core
Target version: 
ruby -v: -


Currently:

    ->(foo = 42){}.arity # => 0, should be -1

This is contrary to the documentation and to what we should expect from the equivalent method definition.

Fixed in trunk, requesting backport for the 1.9 line.



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