Map and Flatmap Future[JsValue]

0
votes

Im having an issue to extract string value from a Future[JsValue]

I tried Map and Flatmap but i guess im doing somthing wrong...

I have this method that return Future[JsValue]

 val chunk: Future[JsValue] = BusinessLogic.Methods.getJsonValue(url)

If i was map i get a Future[String]

 val next: Future[String] = (chunk.map(_.\("paging").\("next").as[String]))

If i use flat map i get type mismatch 

 val next = chunk.flatMap(x=>x.\("paging").\("next").as[String])

What will be the correct way to extract the value ??

thanks

2
scala
What do you mean extract the value? If you want to unwrap the future you have to block (and depending on what are you trying to do it can be a bad idea). map wraps the result of the anonymous function in another future, flatMap instead needs you to wrap the result in a future, in both cases the return type is Future[T]. - Ende Neu
I have to get the string value out of the future in order to pass it to another method that tack a string - MIkCode
next.map(otherFunctionThatTakesAString) - Ryan

2 Answers

0
votes

http://docs.scala-lang.org/overviews/core/futures.html

From oficial tutorials, regarding map:

One of the basic combinators is map, which, given a future and a mapping function for the value of the future, produces a new future that is completed with the mapped value once the original future is successfully completed.

Regarding flat map.

The flatMap method takes a function that maps the value to a new future g, and then returns a future which is completed once g is completed.

So in your case since you just want to convert response to some other value, you need a map function, which would map result of your future to String.

val next: Future[String] = (chunk.map(_.\("paging").\("next").as[String]))

Next value here is a either failure from chunk or result of mapping chunk js value to String.

If you would have some additional asynchronous calculations, like 

def calculations: (x: String): Future[Int] = ...

val next: Future[Int] = chunk.flatMap(js=> {
   val x = js.\("paging").\("next").as[String]
   calculations(x)
})

You would need to use flatMap, combining two dependent asynchronous futures in one. Next value here is a result of calculations function, applied to result returned from chunk.

0
votes

If you want to unwrap your Future you have to block. This is a very bad idea and should be avoided if possible!

def nextFunction(s:String):Unit = ???
val chunk: Future[JsValue] = BusinessLogic.Methods.getJsonValue(url)
val next: String = Await.result(chunk, 1.second)
nextFunction(next)

As mentioned above this is a bad idea! The better solution would be:

def nextFunction(s:String):Unit = ???
val chunk: Future[JsValue] = BusinessLogic.Methods.getJsonValue(url)
chunk.foreach(nextFunction)

This basically calls the nextFunction with the value as soon as it is available and we never have to block.