Why future has side effects?

Question

I am reading the book FPiS and on the page 107 the author says:

We should note that Future doesn’t have a purely functional interface. This is part

Answer 1

As far as I know, Future runs its computation automatically when it's created. Even if it lacks side-effects in its nested computation, it still breaks flatMap composition rule, because it changes state over time:

someFuture.flatMap(Future(_)) == someFuture // can be false

Equality implementation questions aside, we can have a race condition here: new Future immediately runs for a tiny fraction of time, and its isCompleted can differ from someFuture if it is already done.

In order to be pure w.r.t. effect it represents, Future should defer its computation and run it only when explicitly asked for it, like in the case of Par (or scalaz's Task).