Issue #6688 has been reported by prijutme4ty (Ilya Vorontsov).

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Feature #6688: Object#replace
https://bugs.ruby-lang.org/issues/6688

Author: prijutme4ty (Ilya Vorontsov)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


I suggest that #replace works not only on Enumerables but on any Object. It can make use the same object in different places more consistent. It makes it possible to write
class Egg; end
class Hen; end
class HenHouse; attr_accessor :species; end
class Incubator; def incubate(egg) Hen.new; end

# Here it is!
class IncubatorWithReplace; 
  def incubate(egg) 
    egg.replace(Hen.new)
  end
end

e1,e2,e3 = Egg.new, Egg.new, Egg.new
h1, h2 = HenHouse.new, HenHouse.new

# One egg is shared between hen houses
h1.species = [e1, e2]
h2.species = [e1, e3]
p h1 # ==> <HenHouse @species = [#<Egg 1>,#<Egg 2>]
p h2 # ==> <HenHouse @species = [#<Egg 1>,#<Egg 3>]


 # First option. It's bad choise because it makes two "data structures" HenHouse inconsistent: 
 #   they have different object while must have the same
h1[0] = Incubator.new.incubate(h1[0])
p h1 # ==> <HenHouse @species = [#<Hen>,#<Egg 2>]
p h2 # ==> <HenHouse @species = [#<Egg 1>,#<Egg 3>]

 # Second option is ok - now both shared objects're changed.
IncubatorWithReplace.new.incubate(h1[0])
h1 # ==> <HenHouse @species = [#<Hen>,#<Egg 2>]
h2 # ==> <HenHouse @species = [#<Hen>,#<Egg 3>]  

 # Third option is bad - it wouldn't affect HenHouses at all
e1 = Incubator.new.incubate(e1)
p h1 # ==> <HenHouse @species = [#<Egg 1>,#<Egg 2>]
p h2 # ==> <HenHouse @species = [#<Egg 1>,#<Egg 3>]

 # while Fourth option is ok and works as second do
IncubatorWithReplace.new.incubate(e1) ## would affect both HenHouses
p h1 # ==> <HenHouse @species = [#<Hen>,#<Egg 2>]
p h2 # ==> <HenHouse @species = [#<Egg 1>,#<Egg 3>]


I can't imagine how it'd be realized, it looks like some dark magic with ObjectSpace needed to replace one object at a reference with another at the same reference. But I didn't found a solution.

About ret-value. I think it should be two forms:
Object#replace(obj, retain = false)
If retain is false #replace should return a reference to a new object (in fact the same reference as to old object but with other content)
If retain is true, old object should be moved at another place and new reference to it is returned, so:
e1 # ==> <Egg id:1>
e1.replace( Hen.new, true ) # ==> <Egg id:2>
e1 # ==> <Hen id:1>


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