[ruby-core:51417] [ruby-trunk - Bug #7690] Enumerable::Lazy#flat_map should not call each

Issue #7690 has been updated by marcandre (Marc-Andre Lafortune).


PS: The only duck typing I can think of is `respond_to?(:each) && respond_to?(:force)`; not sure if that's much better though.
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Bug #7690: Enumerable::Lazy#flat_map should not call each
https://bugs.ruby-lang.org/issues/7690#change-35396

Author: marcandre (Marc-Andre Lafortune)
Status: Assigned
Priority: High
Assignee: shugo (Shugo Maeda)
Category: core
Target version: 2.0.0
ruby -v: r38794


I would expect that

    array.flat_map{...} == array.lazy.flat_map{...}.force

This is not always the case:

    [1].flat_map{|i| {i => i} } # => [{1 => 1}], ok
    [1].lazy.flat_map{|i| {i => i} }.force # => [[1, 1]], expected [{1 => 1}]

Note that Matz confirmed that it is acceptable to return straight objects instead of arrays for flat_map [ruby-core:43365]

It looks like this was intended for nested lazy enumerators:

    [1].lazy.flat_map{|i| [i].lazy }.force # => [1]

I don't think that's the correct result, and it is different from a straight flat_map:

    [1].flat_map{|i| [i].lazy } # => [#<Enumerator::Lazy: [1]>]

This is caused by Lazy#flat_map calls each (while Enumerable#flat_map only looks for Arrays/object responding to to_ary).





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