Issue #9605 has been updated by Alex Rothenberg.


A little more digging and I found Rubinius is consistent with the lambda and -> syntax but neither raises the ArgumentError. It seems like they remove the arity check when the lambda is converted to a block

$ ruby -v -e '[1].each(&lambda{p :ng})'
rubinius 2.2.3 (2.1.0 4792e746 2013-12-29 JI) [x86_64-darwin13.0.2]
:ng

$ ruby -v -e '[1].each(&->(){p :ng})'
rubinius 2.2.3 (2.1.0 4792e746 2013-12-29 JI) [x86_64-darwin13.0.2]
:ng

$ ruby -v -e 'def test(l); l.call(1); end;  test(lambda{p :ng})'
rubinius 2.2.3 (2.1.0 4792e746 2013-12-29 JI) [x86_64-darwin13.0.2]
An exception occurred evaluating command line code:

    method '__block__': given 1, expected 0 (ArgumentError)

Backtrace:

                                    Proc#call at kernel/bootstrap/proc.rb:20
                                  Object#test at -e:1
                     { } in Object#__script__ at -e:1
  Rubinius::BlockEnvironment#call_on_instance at kernel/common/block_environment.rb:53
                Kernel(Rubinius::Loader)#eval at kernel/common/eval.rb:176
                       Rubinius::Loader#evals at kernel/loader.rb:616
                        Rubinius::Loader#main at kernel/loader.rb:830

$ ruby -v -e 'def test(l); l.call(1); end;  test(->(){p :ng})' 
rubinius 2.2.3 (2.1.0 4792e746 2013-12-29 JI) [x86_64-darwin13.0.2]
An exception occurred evaluating command line code:

    method '__block__': given 1, expected 0 (ArgumentError)

Backtrace:

                                    Proc#call at kernel/bootstrap/proc.rb:20
                                  Object#test at -e:1
                     { } in Object#__script__ at -e:1
  Rubinius::BlockEnvironment#call_on_instance at kernel/common/block_environment.rb:53
                Kernel(Rubinius::Loader)#eval at kernel/common/eval.rb:176
                       Rubinius::Loader#evals at kernel/loader.rb:616
                        Rubinius::Loader#main at kernel/loader.rb:830



----------------------------------------
Bug #9605: Chaining "each_with_index.detect &lambda" raises ArgumentError
https://bugs.ruby-lang.org/issues/9605#change-45755

* Author: Alex Rothenberg
* Status: Rejected
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
* ruby -v: 2.1.1
* Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
I found an odd edge case where "detect" and "select" behave differently from other methods of Enumerable.

Normally these methods yield a single argument to a block but when you chain them after "each_with_index" they yield two arguments "item" and "index". The problem is when you try passing a lambda instead of a block then they raise an ArgumentError

    $ irb
    2.1.1 :001 > lambda = ->(word, index) { word.length == 3 }
    => #<Proc:0x007ff8848630d8@(irb):1 (lambda)>
    2.1.1 :002 > %w(Hi there how are you).each_with_index.detect &lambda
    ArgumentError: wrong number of arguments (1 for 2)
    from (irb):1:in `block in irb_binding'
    from (irb):2:in `each'
    from (irb):2:in `each_with_index'
    from (irb):2:in `each'
    from (irb):2:in `detect'
    from (irb):2
    from /Users/alex/.rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'

    2.1.1 :003 > %w(Hi there how are you).each_with_index.select &lambda
    ArgumentError: wrong number of arguments (1 for 2)
    from (irb):1:in `block in irb_binding'
    from (irb):3:in `each'
    from (irb):3:in `each_with_index'
    from (irb):3:in `each'
    from (irb):3:in `select'
    from (irb):3
    from /Users/alex/.rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'

Interestingly it works just find when calling other methods like "map"

    2.1.1 :004 > %w(Hi there how are you).each_with_index.map &lambda
    => [false, false, true, true, true]

It also works when you use a proc

    2.1.1 :001 > proc = Proc.new {|word, index| word.length == 3 }
    => #<Proc:0x007fc375a3a558@(irb):1>
    2.1.1 :002 > %w(Hi there how are you).each_with_index.detect &proc
    => ["how", 2]
    2.1.1 :003 > %w(Hi there how are you).each_with_index.map &proc
    => [false, false, true, true, true]

or a block

    2.1.1 :001 > %w(Hi there how are you).each_with_index.detect {|word, index| word.length == 3 }
    => ["how", 2]
    2.1.1 :002 > %w(Hi there how are you).each_with_index.map {|word, index| word.length == 3 }
    => [false, false, true, true, true]

When testing against JRuby or Rubinius none of these scenarios raise an ArgumentError. I'm guessing this is a bug and not intended behavior. If it is intended then both those implementations have a bug in not raising ArgumentError.




-- 
http://bugs.ruby-lang.org/