Issue #13795 has been updated by davidarnold (David Arnold).


I did some research to explore the idea that *more* Enumerable classes should implement #select (and even #collect, #drop, etc) to return their own type, since that sounds appealing on the surface.

While classes like Array, Hash, and Set work pretty cleanly, other less thought about classes like Range or -- even worse -- IO, could not possibly return their own type.  So, it seems more logical to me to just standardize on returning Array from Enumerable methods as documented than try to spread the behavior further.

Also, as a side note, it seems that Hash#select and #reject really are the only two methods that behave this way.  And even inside Hash, methods like #collect, #collect_concat, #drop, #drop_while, #grep, #max, #max_by, #min, #min_by, #sort, #sort_by, #take, and #take_while are all still returning Array.

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Bug #13795: Hash#select return type does not match Hash#find_all
https://bugs.ruby-lang.org/issues/13795#change-66129

* Author: davidarnold (David Arnold)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-darwin16]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Enumerable#select and Enumerable#find_all are aliases.  Hash is_a Enumerable, yet only Hash#select was overridden to return a Hash, with Hash#find_all still returning an Array.  This is confusing since the message is that you can use select and find_all interchangeably for Enumerable, yet when you get to Hash, there are warnings that it is no longer true.  

Also any code that expects to call select on an Enumerable and get an array back (as documented) could break, but only for Hash#select.

Example:

~~~ ruby
def select_many(*enumerables, &block)
  result = []
  enumerables.each do |e|
    result.concat e.select(&block)
  end
  result
end

select_many([1, 2], [3, 4]) { |x| x % 2 == 0 } #=> [2, 4]

select_many({ 1 => 2 }, { 3 => 4 }) { |k, v| k < 2 } #=> TypeError: no implicit conversion of Hash into Array
~~~

Should Hash#find_all also return a Hash for consistency?  Or, given the fact that calling #to_h on the resulting Array is so easy, should Hash#select revert to the Enumerable behavior of returning an Array?

Proposal 1:

~~~ ruby
h = { "a" => 100, "b" => 200, "c" => 300 }
h.find_all {|k,v| k > "a"}  #=> {"b" => 200, "c" => 300}
~~~

Proposal 2:

~~~ ruby
h = { "a" => 100, "b" => 200, "c" => 300 }
h.select {|k,v| k > "a"} #=> [["b", 200], ["c", 300]]
h.select {|k,v| k > "a"}.to_h #=> {"b" => 200, "c" => 300}
~~~



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