Mapping an update function over a list of entities is a form of concurrency (note: not parallelism)... If you ever write a game engine from scratch, there are s

Mapping an update function over a list of entities is a form of concurrency (note: not parallelism)... If you ever write a game engine from scratch, there are several tricky issues you must deal with because of this -- for example, the death and birth of new entities caused by traversing the list. Consider what happens if your game character's arrow kills a monster while you are iterating over the list of entities. Do you remove it from the list right then? If other objects are depending on e.g. the number of monsters present, then you have a race condition where some objects will be updated before the monster was removed and some objects will be updated after the monster was removed and will thus see inconsistent views of the current state of the world... this can cause all kinds of fun, intermittent bugs!