Mike's corner of the web.

Polymorphism and reimplementing integers

Tuesday 18 December 2012 20:39

While taking part in the Global Day of Coderetreat, one of the sessions had us implementing Conway's Game of Life without using any "if" statements to force us to use polymorphism instead. Anything that was an "if" in spirit, such as a switch statement or storing functions in a dictionary, was also forbidden. For most of the code, this was fairly straightforward, but the interesting problem was the code that decided whether a cell lived or died based on how many neighbours it had:

if numberOfNeighbours in [2, 3]:
    return CellState.LIVE
else:
    return CellState.DEAD

Polymorphism allows different code to be executed depending on the type of a value. In this particular case, we need to execute different code depending on which value we have. It follows that each number has to have a different type so we can give it different behaviour:

class Zero(object):
    def increment(self):
        return One()
        
    def live_cell_next_generation():
        return CellState.DEAD
        
class One(object):
    def increment(self):
        return Two()
        
    def live_cell_next_generation():
        return CellState.DEAD
        
class Two(object):
    def increment(self):
        return Three()
        
    def live_cell_next_generation():
        return CellState.LIVE
        
class Three(object):
    def increment(self):
        return FourOrMore()
        
    def live_cell_next_generation():
        return CellState.LIVE

class FourOrMore(object):
    def increment(self):
        return FourOrMore()
        
    def live_cell_next_generation():
        return CellState.DEAD

In the code that counts the number of neighbours, we use our new number system by starting with Zero and incrementing when we find a neighbour. To choose the next state of the cell, rather than inspecting the number of neighbours, we ask the number of neighbours for the next state directly:

numberOfNeighbours.live_cell_next_generation()

And now we have no "if"s! It's possible to move the logic for choosing the next cell out of the number classes, for instance using the visitor pattern, which might feel a bit more natural. I suspect that reimplementing the natural numbers is still going to feel about the same amount of crazy though.

Topics: Software design