wtf.scala

scala> def foo(a: Int): Future[Int] = memoize { Future { println("Computing..."); a + 1 } }
foo: (a: Int)scala.concurrent.Future[Int]

scala> foo(123)
res0: scala.concurrent.Future[Int] = List()  // <-- WTF

scala> Computing...


scala> foo(123)
res1: scala.concurrent.Future[Int] = List() // <-- WTF

scala> res1
res2: scala.concurrent.Future[Int] = Success(124) // <-- oh ok never mind (?)

scala>

I could also reproduce this more simply:

scala> Future { "wut" }
res11: scala.concurrent.Future[String] = List()

scala> res11
res12: scala.concurrent.Future[String] = Success(wut)

The problem went away with a fresh REPL.

This is pretty interesting. I think I may have traced it down to the default implementation of Future, which is DefaultPromise that extends AbstractPromise.java.

When one of those is initialised, the very first thing that happens is the internal state gets set to Nil using updateState. I think this is a very small window, and I can't reproduce what you've shown, but I'm fairly certain that is the List() that you're seeing in res1 and res11.