[anonymoushn]

I make an online card game. The rules are pretty simple, about as complex as Duel of Champions. There are no ongoing effects, no instants, no stack, no replacement effects, and so on. What the game does have is literal thousands of triggered abilities.

When combat happens, the attacker's triggered abilities will happen, then the defender's triggered abilities, then the attacker will attack the defender. Lots of triggered abilities interact with the attacker or defender, and some may cause it to die (it dies immediately, not at the end of combat). Subsequent triggered abilities might crash if they depend on the attacker or defender existing.

In my implementation, these triggered abilities are functions with signature

  triggered_ability(player: Player, my_idx: number, my_card: Card,
    skill_idx: number, other_idx: number, other_card: Card?)

Having the type system enforce null checks on uses of other_card (and also on all uses of cards from other zones, like the opponent's battlefield or the player's hand) would have saved me a lot of trouble. As it is, I use a fuzzer to play lots of games between AI players and look for errors, but most of the errors I look for could have been caught by a compiler instead.