The callback mechanism we have shown is sufficient to chain future results with subsequent computations.

We therefore create another future purchase which makes a decision to buy only if it’s profitable to do so, and then sends a request.

We would have to repeat this pattern within the onSuccess callback, making the code overly indented, bulky and hard to reason about.

For these two reasons, futures provide combinators which allow a more straightforward composition.

One of the basic combinators is map, which, given a future and a mapping function for the value of the future, produces a new future that is completed with the mapped value once the original future is successfully completed.

You can reason about mapping futures in the same way you reason about mapping collections.

This exception propagating semantics is present in the rest of the combinators, as well.