I absolutely love it when we can take advantage of Go's type system and add additional traits and behaviors to existing types like this. That said, I noticed s

I absolutely love it when we can take advantage of Go's type system and add additional traits and behaviors to existing types like this. That said, I noticed something odd here. In order for a module like this to really shine, I think all these operations need to be functionally pure. Right now, some of these mutate the iterator's `iter` method mid-stream, which is about as side-effect-ful as you can get. ``` func (i Iterator[V]) Map(f func(V) V) Iterator[V] { cpy := i.iter i.iter = func(yield func(V) bool) { for v := range cpy { v = f(v) if !yield(v) { return } } } return i } ``` Unless I'm misreading that, `i.iter` has new behavior after this call. A better way would be to return a new _iterator_ with the custom iter behavior instead. ``` func (i Iterator[V]) Map(f func(V) V) Iterator[V] { // create a fresh iterator around a custom closure (NewIterator() is hypothetical in this case) return NewIterator(func(yield func(V) bool) { for v := range i.iter { v = f(v) if !yield(v) { return } } }) } ```