[ruby-core:47002] [ruby-trunk - Feature #6688] Object#replace

Issue #6688 has been updated by alexeymuranov (Alexey Muranov).


=begin
Ilya, according to how Ruby works (as far as i understand), there should be no difference, as far as exceptions concerned, between

 1.become(2)

and

 a = 1
 a.become(2)

so this should be forbidden, i think.

However, i was thinking recently about other problems associated with OO programming, and in my opinion there should be a clear distinction between mutable and immutable objects, maybe even the latter in addition to a class should also have a "type".

To me, (({Object#become})) would make sense for mutable objects, which have a ((*state*)) not determined by their ((*identity*)). I can imagine that it might require changes in the current implementation, to allow all properties of every mutable object be replaced.  Then i can imagine type conversion for mutable objects done like this:

 class Object
   def to(new_class)
     become(new_class[self]) # raises an exception if `self` is immutable
   end
   def class=(new_class)
     to(new_class)
     self.class
   end
 end

 a = "abc".to(Array) # => ["a", "b", "c"]

Of course calling (({become})) on objects that are already referenced by other objects can break many things, (({like with String.become("")})).
----------------------------------------
Feature #6688: Object#replace
https://bugs.ruby-lang.org/issues/6688#change-28659

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>


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