From: ko1@...
Date: 2014-10-24T04:52:21+00:00
Subject: [ruby-core:65888] [ruby-trunk - Bug #9605] [Open] Chaining "each_with_index.detect &lambda" raises ArgumentError

Issue #9605 has been updated by Koichi Sasada.

Status changed from Rejected to Open

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Bug #9605: Chaining "each_with_index.detect &lambda" raises ArgumentError
https://bugs.ruby-lang.org/issues/9605#change-49627

* Author: Alex Rothenberg
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* Category: core
* Target version: current: 2.2.0
* ruby -v: 2.1.1
* Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN, 2.1: UNKNOWN
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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.




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