From: "shugo (Shugo Maeda)" <redmine@...>
Date: 2012-03-19T16:57:19+09:00
Subject: [ruby-core:43463] [ruby-trunk - Bug #6155] Enumerable::Lazy#flat_map raises an exception when an element does not respond to #each


Issue #6155 has been updated by shugo (Shugo Maeda).


I'll fix lazy flat_map respecting Matz's opinion, but let me clarify one point.

dkubb (Dan Kubb) wrote:
> > >   [1, 2, 3].flat_map { |n| n }.to_a
> > 
> > I doubt that this behavior of Enumerable#flat_map is reasonable.
> 
> I was writing rubyspec for Enumerable::Lazy#flat_map had the same behaviour as Enumerable#flat_map (besides the obvious differences in return value). Here is what I was using as a basis: https://github.com/rubyspec/rubyspec/blob/master/core/enumerable/shared/collect_concat.rb
> 
> The last example doesn't apply, since it should raise an exception when no block is provided (according to the current implementation), but the first 3 specs fail because they have mixed data, they are equivalent to:
> 
>     [1, [2, 3], [4, [5, 6]], {:foo => :bar}].lazy.flat_map { |e| e }.to_a
>     [1, [], 2].lazy.flat_map { |e| e }.to_a
>     [[:foo], Object.new.tap { |o| class << o; def to_a() end end }].lazy.flat_map { |e| e }.to_a
> 
> All of the above examples work when the enumerable is not lazy.

Why the last example defines to_a?  flat_map doesn't call to_a, but to_ary.

  p [Object.new.tap { |o|
    class << o
      def to_a; [:to_a] end
    end
  }].flat_map { |e| e } #=> [#<Object:0x21aae8a0>]
  p [Object.new.tap { |o|
    class << o
      def to_ary; [:to_ary] end
    end
  }].flat_map { |e| e } #=> [:to_ary]

----------------------------------------
Bug #6155: Enumerable::Lazy#flat_map raises an exception when an element does not respond to #each
https://bugs.ruby-lang.org/issues/6155#change-24932

Author: dkubb (Dan Kubb)
Status: Assigned
Priority: Normal
Assignee: shugo (Shugo Maeda)
Category: core
Target version: 2.0.0
ruby -v: ruby 2.0.0dev (2012-03-15 trunk 35028) [x86_64-darwin11.3.0]


The following statement will raise "NoMethodError: undefined method `each' for 1:Fixnum":

  [1, 2, 3].lazy.flat_map { |n| n }.to_a

It appears as if Enumerable::Lazy#flat_map is calling #each on every element, regardless of whether it can work or not.

As a reference, the equivalent statement using Enumerable#flat_map works:

  [1, 2, 3].flat_map { |n| n }.to_a


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