From: "marcandre (Marc-Andre Lafortune)" Date: 2013-01-14T10:32:02+09:00 Subject: [ruby-core:51412] [ruby-trunk - Bug #7690] Enumerable::Lazy#flat_map should not call each Issue #7690 has been updated by marcandre (Marc-Andre Lafortune). Priority changed from Normal to High shugo (Shugo Maeda) wrote: > [1].lazy.flat_map{|i| [i].lazy } should flatten nested lazy enumerators, because Enumerable::Lazy is a monad and flat_map is the monad's bind operator. Thanks for the explanation. The idea is neat. The problem is that: 1) This is documented nowhere 2) Most people think of flat_map as a shortcut to map.flatten(1), but flatten doesn't flatten Lazy enumerators (or Enumerables in general) 3) As Matz stated [ruby-core:26301], flat_map is "taken from flatMap from Scala or concatMap from Haskell". I'm not familiar with either, but I read that Scala's flatMap is not a monadic bind, right? 4) The argument about flat_map being a monadic bind applies only to monads (i.e. lazy enumerators). It should only flatten those, not arbitrary Enumerables > Do you have any use case of [1].flat_map{|i| {i => i} }? It's not just hashes, it could be a Range, or any Enumerable, or even any class that implements #each, even if it doesn't include Enumerable! So yes, I can think of many use cases, but instead of inventing them, here's one in Rails: https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/query_methods.rb#L841 In summary, I see the following 2 possibilities: 1) Lazy#flat_map only flattens arrays, or 2) Lazy#flat_map flattens Array and Enumerator::Lazy (using `is_a? Enumerator::Lazy` instead of `respond_to? :each`) and the documentation reflects this If (1), maybe a new method can be introduced instead, say "bind"? If (2), shouldn't Enumerable#flat_map also flatten lazy enumerators? ---------------------------------------- Bug #7690: Enumerable::Lazy#flat_map should not call each https://bugs.ruby-lang.org/issues/7690#change-35391 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 } # => [#] This is caused by Lazy#flat_map calls each (while Enumerable#flat_map only looks for Arrays/object responding to to_ary). -- http://bugs.ruby-lang.org/