Issue #13795 has been updated by adp90 (Alexander Patrick).


Changing the behavior of Hash#select/reject broke a lot of files, some in [very strange ways](https://travis-ci.org/ruby/ruby/builds/263508096?utm_source=github_status&utm_medium=notification).

This makes me think that such a change won't be popular. Consistency with previous versions of Ruby and with legacy code might outweigh being consistent within Enumerable.

Also, with select! and reject! not actually being Enumerable methods, changing select and reject to return Arrays makes them inconsistent with Hash#select! and Hash#reject!

The best solution to this might be to update the documentation to point out the difference between select and find_all.

I've closed the pull request for the time being, as this issue likely warrants further discussion.

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Bug #13795: Hash#select return type does not match Hash#find_all
https://bugs.ruby-lang.org/issues/13795#change-66160

* 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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