Scala Futures in For Comprehension -

i'm trying wrap head around scala , i'm wondering following does:

val ffuture: future[int] = future { println("f called"); 3 }                                                                                                                     val gfuture: future[int] = future { println("g called"); 4 }                                                                                                                      {     f <- ffuture     g <- gfuture } yield f   

the futures executed inside of comprehension, correct? f in yield statement? mean it's available? considered return value if inside of function?

the futures starting execute here:

val ffuture: future[int] = future { println("f called"); 3 }                                                                                                                     val gfuture: future[int] = future { println("g called"); 4 }  

so both executions starting in parallel. if accidentally put future{...} inside for-comprehension - gonna executed sequentially.

for-comprehension subscribes , merges 2 results 1 future. however, in case seems second future's result ignored, doesn't makes sense. code makes sense:

for {     f <- ffuture     g <- gfuture } yield f + g 

this code returns future[int](same code in example). if extract value future - 3 + 4 = 7. it's still not best approach computations independent , probability of developer's mistake (stated above) makes them sequential still high, recommended approach independent computations is:

(ffuture zip gfuture) map {   case (f, g) => f + g } 

this code referentially transparent in meaning if replace ffuture future{...} - still behaves same (in case of future -they're going executed in prallel, might different other concurrency primitives)

where for-comprehension make sense? here:

for {     f <- future{... 9}     g <- if (f > 0) future{...} else future{...} } yield g 

as g depends on f here - there no way run in parallel, for provides non-blocking way of composing several futures