[ruby-core:51416] [ruby-trunk - Bug #7690] Enumerable::Lazy#flat_map should not call each

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


shugo (Shugo Maeda) wrote:
> > 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?
> 
> Where did you read that?  I guess Scala's flatMap is also bind.

"Scala's flatMap is indeed not a monadic bind" here http://igstan.ro/posts/2012-08-23-scala-s-flatmap-is-not-haskell-s.html but I only scanned this quickly and I'm don't know if that's correct.

> Ruby's Enumerable#flat_map is also bind.
> Because Enumerable#flat_map returns an Array, Enumerable#flat_map takes a block which returns an Array.
> Because Enumerator::Lazy#flat_map returns an Enumerator::Lazy, Enumerator::Lazy#flat_map takes a block which returns an Enumerator::Lazy.
> They are consistent in that sense.

Right. Except that both also accept straight objects.

> > 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
> 
> I feel difficulty about it because duck typing is preferred in Ruby.

Right, but the core of Ruby relies more on conversions than pure duck typing.

In particular, Enumerable#flat_map uses `to_ary`. For the lazy flat_map, there is no "to_lazy" or similar...

> Technically, this code should be written as follows:
> ...

It indeed could be written with [{...}], but it does not have to, as confirmed by Matz [ruby-core:43365].

> > 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
> 
> I prefer 2), but am not sure whether `is_a? Enumerable::Lazy' is a neat solution.
> However, if I don't come up with a better solution, I will fix Lazy#flat_map using it.

Sounds good.

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Bug #7690: Enumerable::Lazy#flat_map should not call each
https://bugs.ruby-lang.org/issues/7690#change-35395

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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