[gcommer]

It is a weird example. But that transform does not work generally. Consider if you want to stop when insertIntoMouth() returns a particular value. Or if there is some filter() step, so that you don't know that `n` input items will map to `n` output items.

The difference here is push vs pull (also called eager vs lazy). It is often easier to write pull computations that only do the work that is actually needed.