It's bloat due to 'introns' (useless statements that don't effect the output, like x = x * 1). And yes, just adding a fitness function to shorten program length

It's bloat due to 'introns' (useless statements that don't effect the output, like x = x * 1). And yes, just adding a fitness function to shorten program length isn't optimal. I've found it easier to evolve successful programs (letting the bloat happen) and then keep removing statements from correctly generated programs whilst checking if the output is the same. Probably not optimal either but I feel like it gives better results.