Add DefFuture

[ronanM]

DefFuture

In Scala, Future is not "referentially transparent" because:

• Future is eagerly evaluated.
• Future is memoized (the result is cached).

1. The execution of this code:

for {
  _ <- Future(println("Start"))
  _ <- Future(println("foo"))
  _ <- Future(println("foo"))
} yield ()

will produce on the stdout: ✅

start
foo
foo

2. The execution of this simple refactored code:

val fut = Future(println("foo"))

for {
  _ <- Future(println("Start"))
  _ <- fut
  _ <- fut
} yield ()

will produce on the stdout: ❌

foo
start

This is a "surprising" buggy behavior for Scala beginners 🤯.

To return to the situation in 1), the code must be written like this:

def fut = Future(println("foo")) // <--- Note the `def` instead of `val` here.

for {
  _ <- Future(println("Start"))
  _ <- fut
  _ <- fut
} yield ()

will produce again on the stdout: ✅

start
foo
foo

[ronanM]

@xuwei-k Is it possible to run the CI please ?