`+` not defined!?

[mbauman]

Welcome! Yes, that’s an unusually bad error message.

@ollie_d

function update(x,y)
x = x + dx,
y = y + dy,
plot!((x,y))
end

The trailing commas there are your problem; they’re mixing up all your assignments in the update function (the mixup is making Julia think you’re defining your own + method). Commas are only used to separate function arguments and list elements, not lines of code.