Learning Functional Programming in Go

Pass by value or reference?

Here's the rule of thumb--if you want to share a state, then pass by reference, that is, use a pointer type; otherwise, pass by value. Since we need to update our duck's strokeSupply type in this Stroke method, we pass it as an int pointer (*int). So, pass a pointer parameter only when absolutely necessary. We should begin to code defensively, assuming that someone may try to run our code concurrently. When we pass our parameters by value, it's safe for concurrent use. When we pass by reference, we may need to add sync.mutex or some channels to coordinate concurrency.

Our duck builds its energy back by eating more bugs that it gets from the pond:

func (Duck) Eat(e EatBehavior, strokeSupply *int, p Pond) {       for  ...