# Oddity with block argument passing **URL:** \ **Category:** Ruby\ **Created:** [December 20, 2005, 9:42pm UTC](https://www.ruby-forum.com/t/oddity-with-block-argument-passing/50754 "2005-12-20T21:42:15Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![glimjaur](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/3be4f8/32.png) [@glimjaur](https://www.ruby-forum.com/u/glimjaur)\ **Post date:** [December 20, 2005, 9:42pm UTC](https://www.ruby-forum.com/t/oddity-with-block-argument-passing/50754/1 "2005-12-20T21:42:15Z")
In my latest project, I was a bit surprised by the way arguments to Proc.new were handled. Somehow an array which i passed in was mysteriously lost and presented as a nil within the block I had created. If this is to be expected, I would have liked to see a mention of it in the documentation. After reading the documentation more carefully, I realized that if i would use Kernel.proc instead of Proc.new it should do the same “except the resulting Proc objects check the number of parameters passed”, which evidentially in my case also means that the parameter actually goes through. Wondering whether this was a bug or not, I tried out the latest CVS version to see whether the behavior had changed or not. Indeed the behavior was changed in the CVS version, but in a direction quite opposite to what I had hoped for, since now both Proc.new and Kernel.proc performs in the same way. (Eating up my array argument) Kernel.lambda seems to work the way I want for all cases though and thus using this will solve my problems, but I was curious whether this difference between lambda and proc is intended, and also whether the change of semantics from current to CVS was intended. Below is an example which isn’t especially meaningful, but serves it’s purpose to highlight the differences between the semantics of the different approaches in different versions of ruby. Constructs = [ lambda {|x, \*y| [x, y]}, proc {|x, \*y| [x, y]}, Proc.new {|x, \*y| [x, y]} ] puts “ruby #{RUBY\_VERSION} (#{RUBY\_RELEASE\_DATE}) [#{RUBY\_PLATFORM}]” Constructs.each{ |x| puts x[[]].inspect } Below is the output of this script from the two versions of ruby where I’ve tried it. $ ruby proc\_test.rb ruby 1.8.3 (2005-09-21) [i686-linux] [[], []] [[], []] [nil, []] $ ruby-cvs proc\_test.rb ruby 1.9.0 (2005-12-20) [i686-linux] [[], []] [nil, []] [nil, []] Is this to be expected? !g ---
**Author:** ![glimjaur](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/3be4f8/32.png) [@glimjaur](https://www.ruby-forum.com/u/glimjaur)\ **Post date:** [December 23, 2005, 3:31am UTC](https://www.ruby-forum.com/t/oddity-with-block-argument-passing/50754/2 "2005-12-23T03:31:31Z")
[Gustav M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), 2005-12-20 21.40 CET] […] > I’ve tried it. > [nil, []] > [nil, []] Compare: def a x, \*y [x, y] end a [] # =\> [[], []] x, \*y = [] [x, y] # =\> [nil, []] In one case, values are assigned as in normal assignment; in others, as in method parameter assignment. The “normal variable assignment” is to be expected in normal blocks; the ones created with lambda use parameter-like assignment. > Is this to be expected? Many people expect different things here, and you can find a lot of discussions about this topic in the list archives. HTH. ---
**Author:** ![glimjaur](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/3be4f8/32.png) [@glimjaur](https://www.ruby-forum.com/u/glimjaur)\ **Post date:** [December 26, 2005, 12:42pm UTC](https://www.ruby-forum.com/t/oddity-with-block-argument-passing/50754/3 "2005-12-26T12:42:16Z")
angus wrote: > > Constructs.each{ |x| puts x[[]].inspect } > > $ ruby-cvs proc\_test.rb > > end > > assignment. thanks for this explanation, now i really understand why there’s a difference between the parameter-like assignment and normal variable assignment. they way you say it, it sounds as if the difference should be between lambda and proc, but in 1.8.3 the difference is between Proc.new and proc, does this mean that there is an issue with Kernel.proc in 1.8.3? and i would also suggest changing the api-documation, allthough i don’t know exactly i shall proceed to propose such a change. for Kernel.lambda it currently states: Equivalent to Proc.new, except the resulting Proc objects check the number of parameters passed when called. a better phrasing i think would be: Equivalent to Proc.new, except the resulting Proc objects have method-like-parameter-passing instead of the normal expression-like-parameter-passing. or something along those lines. personally i would think it’d be nice with something more explaining the differences of the two, but the api-documentation might not be the proper place for making this clear. !g ---
**Author:** ![glimjaur](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/3be4f8/32.png) [@glimjaur](https://www.ruby-forum.com/u/glimjaur)\ **Post date:** [December 26, 2005, 1:57pm UTC](https://www.ruby-forum.com/t/oddity-with-block-argument-passing/50754/4 "2005-12-26T13:57:31Z")
Hi – On Mon, 26 Dec 2005, Gustav M. wrote: > thanks for this explanation, now i really understand why there’s a difference > between the parameter-like assignment and normal variable assignment. > > they way you say it, it sounds as if the difference should be between lambda > and proc, but in 1.8.3 the difference is between Proc.new and proc, does this > mean that there is an issue with Kernel.proc in 1.8.3? proc and lambda are synonyms. Because it’s confusing for proc and Proc.new to mean different things, Matz has said that proc is (will be?) deprecated, so it will just be Proc.new and lambda. David – David A. Black removed\_email\_address@domain.invalid “Ruby for Rails”, from Manning Publications, coming April 2006! > **[Ruby for Rails](https://www.manning.com/books/ruby-for-rails)** > > NEWER EDITION AVAILABLE > > The Well-Grounded Rubyist, Second Edition is now available. An eBook of the previous edition, The Well-Grounded Rubyist is included at no additional cost when you buy the revised edition! > > > Ruby for Rails helps Rails...