Issue #4907 has been updated by mame (Yusuke Endoh).

Priority changed from Normal to Low
Target version set to next minor

This will not work for an enumerator created from IO.

-- 
Yusuke Endoh <mame / tsg.ne.jp>
----------------------------------------
Feature #4907: enumerable#permutation and combination
https://bugs.ruby-lang.org/issues/4907#change-33240

Author: neleai (Ondrej Bilka)
Status: Assigned
Priority: Low
Assignee: mrkn (Kenta Murata)
Category: 
Target version: next minor


Hello
Methods permutation and combination are defined for array but it make more sense to define them for enumerable. 
Here is sample implementation which for simplicity works only with blocks.
Note that implementation works lazily. 
It changes traversal order but documentation states that order is unspecified.

module Enumerable
 def perm(n)
 comb(n){|ary|
 ary.permutation{|p| yield(p)}
 }
 end
 def comb(n,&m)
 ary=[]
 e=to_enum.with_index
 _comb(e,n-1,1.0/0.0,ary,m)
 end
def _comb(e,n,bound,ary,m)
    e.each{|el,i|
      return if i>=bound
      ary[n]=el
      if n==0
        m.call(ary)
      else
        _comb(e,n-1,i,ary,m)
      end
    }
  end

end
[1,2,4,3,5].comb(2){|a| puts a.inspect}
(1..4).comb(2){|a| puts a.inspect}
(1..4).perm(2){|a| puts a.inspect}



-- 
http://bugs.ruby-lang.org/