Task[List[List[A]]] to Task[A] if list has elements

Question

I have a method which returns Task[List[List[A]]] and I need to transform to Task[A] if the list is greater than 0

def method():Task[List[List[A]]] = {}

val d:Task[List[A]] = method().map(_.flatten)

How to get Task[A] is a list of A if the inner method has more than 0 elements

I am able to convert to Task[List[A]] as you can see above

You mean you want to take the first element from the list or transform `Task[List[A]]` to `List[Task[A]]`? — Krzysztof Atłasik, Dec 31 '19 at 12:59
You know... for example... getting an `Int` from a `List[Int]` you will need some operation to combine all elements or select one element or something similar. — sarveshseri, Dec 31 '19 at 13:05
What should happen if all lists are empty? Also you said you wanted the first element, but the first of all lists or ten first of the first list? It would be good if you could put some example inputs with expected outputs. — Luis Miguel Mejía Suárez, Dec 31 '19 at 13:14

gandaliter · Accepted Answer · 2019-12-31T13:12:28.233

1

You're flattening the List[List[A]] into a List[A] in the intuitive way, all wrapped in a Task. If you provide the method to go from a List[A] to an A (edit: see below), then you can call it from a map on the task as follows.

def method():Task[List[List[A]]] = {}

def listToItem(list: List[A]): A = ???

def d: Task[A] = method().map(_.flatten).map(listToItem(_))

You say that you want listToItem to take the first element of the list. Unfortunately, such a function wouldn't know what to do if the list were empty. You could use list.head, which will throw an exception if the list is empty, or you could use list.headOption, which will return an Option[T] rather than T.

edited Dec 31 '19 at 13:12

answered Dec 31 '19 at 13:06

gandaliter

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did you realise you just called `listOfA.map(listToA(_))`. `method().map(_.flatten.map(listToItem(_)))` should be changed to `method().map(_.flatten).map(listToItem(_))` – sarveshseri Dec 31 '19 at 13:08
@SarveshKumarSingh sure, but to fit with OP's code, it starts from `Task[List[List[A]]`, and I think this is the clearest way to show what's changed. – gandaliter Dec 31 '19 at 13:10
method().map(_.flatten.map(a=>a)) why this is wrong – Sumeet Kumar Yadav Dec 31 '19 at 13:10
@gandaliter your code will not compile. Types are wrong. – sarveshseri Dec 31 '19 at 13:11
@SarveshKumarSingh Ah, sorry, misunderstood. Edited - thanks – gandaliter Dec 31 '19 at 13:11
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@Sumeet `map` just allows you to operate inside the monad (`List` in this case, but similarly for `Task` elsewhere). Calling `list.map(a => a)` gives you another `List`, the same as the original one. – gandaliter Dec 31 '19 at 13:16

Task[List[List[A]]] to Task[A] if list has elements

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