Poking around in parent's internal state

Question

tl:dr how does decoupling work? could need some little example

I'm reading Programming Ruby - The Pragmatic Programmer's Guide. (http://ruby-doc.org/docs/ProgrammingRuby/html/tut_classes.html)

There is an example on how to implement to_s for subclass KaraokeSong of Song.

class KaraokeSong < Song
# ...
def to_s
"KS: #{@name}--#{@artist} (#{@duration}) [#{@lyrics}]"
end
end
aSong = KaraokeSong.new("My Way", "Sinatra", 225, "And now, the...")
aSong.to_s    »  "KS: My Way--Sinatra (225) [And now, the...]"

Now they say its a bad way to do it:

Say we decided to change Song to store the duration in milliseconds. Suddenly, KaraokeSong would start reporting ridiculous values. The idea of a karaoke version of ``My Way'' that lasts for 3750 minutes is just too frightening to consider.

Instead you should define to_s with super:

def to_s
super + " [#{@lyrics}]"
end

Now when the @duration variable still stores the song duration in miliseconds, how does the new to_s which just calls the parent's method solve the problem? It still returns 3750 minutes, doesn't it?

I think i dont really understand the difference between those 2.

Answer 1

It's assumed that Song will take care of the proper output of its @duration.

And if we will decide to change Song to store the duration in milliseconds, we will just change Song#to_s method and won't have to change to_s of all its descendants.