Scala Language Futures Combine Multiple Futures – For Comprehension
Example
The for comprehension is a compact way to run a block of code that depends on the successful result of multiple futures.
With f1, f2, f3 three Future[String]'s that will contain the strings one, two, three respectively,
val fCombined =
for {
s1 <- f1
s2 <- f2
s3 <- f3
} yield (s"$s1 - $s2 - $s3")
fCombined will be a Future[String] containing the string one - two - three once all the futures have completed successfully.
Note that an implicit ExectionContext is assumed here.
Also, keep in mind that for comprehension is just a syntactic sugar for a flatMap method, so Future objects construction inside for body would eliminate concurrent execution of code-blocks enclosed by futures and lead to sequential code. You see it on example:
val result1 = for {
first <- Future {
Thread.sleep(2000)
System.currentTimeMillis()
}
second <- Future {
Thread.sleep(1000)
System.currentTimeMillis()
}
} yield first - second
val fut1 = Future {
Thread.sleep(2000)
System.currentTimeMillis()
}
val fut2 = Future {
Thread.sleep(1000)
System.currentTimeMillis()
}
val result2 = for {
first <- fut1
second <- fut2
} yield first - second
Value enclosed by result1 object would be always negative while
result2 would be positive.
For more details about the for comprehension and yield in general, see http://docs.scala-lang.org/tutorials/FAQ/yield.html
