[ruby-core:51410] [ruby-trunk - Bug #7690][Assigned] Enumerable::Lazy#flat_map should not call each

Issue #7690 has been updated by shugo (Shugo Maeda).

Status changed from Open to Assigned
Assignee set to shugo (Shugo Maeda)

marcandre (Marc-Andre Lafortune) wrote:
> 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}]

I agree that this looks weird.

> 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]>]

[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.
In the monad, [x].lazy is equivalent to Haskell's return and flat_map is equivalent to Haskell's >>= (bind).

  # return :: a -> ma
  [x].lazy

  # (>>=)  :: m a -> (a -> m b) -> m b
  x.flat_map(&f)

Note that f's type is a -> m b, which means that f returns not an Array, but an Enumerable::Lazy.

In fact, [x].lazy and flat_map obey the monad laws.

  # (return x) >>= f == f x
  [x].lazy.flat_map(&f) == f.(x)

  # m >>= return == m
  m.flat_map { |i| [i].lazy } == m 

  # (m >>= f) >>= g == m >>= (\x -> f x >>= g)
  m.flat_map(&f).flat_map(&g) == m.flat_map { |x| f.(x).flat_map(&g) }

That is, flat_map is an operator to compose computations which return an Enumerable::Lazy.

Do you have any use case of [1].flat_map{|i| {i => i} }?

----------------------------------------
Bug #7690: Enumerable::Lazy#flat_map should not call each
https://bugs.ruby-lang.org/issues/7690#change-35388

Author: marcandre (Marc-Andre Lafortune)
Status: Assigned
Priority: Normal
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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