What does <- mean in scala with yield?

Question

I'm pretty new to Scala and came up with the following construction:

val value=
  for {
    p1 <- getList()
    p2 <- parser.parse(p1)  //parser.parse(String) Returns some useful value
  } yield p2
value.asJava

Where

def getList(): List[String] = {
   //compiled code
}

I don't quite understand what's going on in the first piece of code. Searching for scala left arrow operator did't shed the light on this. Can't you explain it?

I guess I really don't understand why someone would start to learn scala and not have a (online or ebook or dead-tree) tutorial to hand. Every one of those would explain `for... yield`. — The Archetypal Paul, Jun 23 '16 at 10:28

airudah · Answer 1 · 2016-06-24T10:12:40.600

2

for {
    p1 <- getList()
    p2 <- parser.parse(p1)
  } yield p2

is equivalent to (psudocode, not tested):

var result: List = Nil
val value = {
    foreach(p1 in getList()){
     foreach(p2 in parser.parse(p1)){
      result ::: p2
     }
    }
    result
   }

But like others said, you could have easily found this by reading up on Scala's for comprehension.

edited Jun 24 '16 at 10:12

answered Jun 23 '16 at 10:39

airudah

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1

That is a lot of psuedocode for something which is essentially just `getList().flatMap(parser.parse)`. – Peter Neyens Jun 23 '16 at 10:52
1

I know @Peter. I was attempting to demonstrate it in an OO way. – airudah Jun 23 '16 at 11:50

What does <- mean in scala with yield?

1 Answers1