Question

So if I understand correctly Object#tap uses yield to produce a temporary object to work with during the execution of a process or method. From what I think I know about yield, it does something like, yield takes (thing) and gives (thing).dup to the block attached to the method it’s being used in?
But when I do this:

class Klass
attr_accessor :hash
  def initialize
   @hash={'key' => 'value'}
  end
end

instance=Klass.new
instance.instance_variable_get('@hash')[key] # => 'value', as it should

instance.instance_variable_get('@hash').tap {|pipe| pipe['key']=newvalue}

instance.instance_variable_get('@hash')[key] # => new value... wut?

I was under the impression that yield -> new_obj. I don’t know how correct this is though, I tried to look it up on ruby-doc, but Enumerator::yielder is empty, yield(proc) isn’t there, and the fiber version… I don’t have any fibers, in fact, doesn’t Ruby actually explicitly require include ‘fiber’ to use them?

So what ought have been a read method on the instance variable and a write on the temp is instead a read/write on the instance variable… which is cool, because that’s what I was trying to do and accidentally found when I was looking up a way to deal with hashes as instance variables (for some larger-than-I’m-used-to tables for named arrays of variables), but now I’m slightly confused, and I can’t find a description of the mechanism that’s making this happen.

Answer 1

Object#tap couldn’t be simpler:

VALUE
rb_obj_tap(VALUE obj)
{
    rb_yield(obj);
    return obj;
}

(from the documentation). It just yields and then returns the receiver. A quick check in IRB shows that yield yields the object itself rather than a new object.

def foo
  x = {}
  yield x
  x
end

foo { |y| y['key'] = :new_value }
# => {"key" => :new_value }

So the behavior of tap is consistent with yield, as we would hope.