Scala's List unpacking - scala

Question

So my problem is simple. I have come across this many times, and my brain is unable to find a solution.
How can I unpack a list into another list, for an indefinite amount of variables?
Here is what I mean.
val list1 = List(List(7, 4), List(7, 6))
val list2 = List(List(1), List(5), List(8))
val desired_list1 = List(List(1, 7, 4), List(5, 7, 4), List(8, 7, 4))
val desired_list2 = List(List(1, 7, 6), List(5, 7, 6), List(8, 7, 6))
//** The desired_list1 and 2 must be a List[List[Int]] it cannot be List[List[Any]]
//Here's my attempt, which oddly enough completely ignores all elements of list1(0) which are not the first(7).
val attempt = list2.map(i => i +: list1(0)).map(j => j.collect{ case k:Int => k; case l # a :: b => a}).map(m => m.map{ case i:Any => i.toString.toInt})
//The result is
attempt: List[List[Int]] = List(List(1, 7), List(5, 7), List(8, 7))
//while it should be:
val desired_list1 = List(List(1, 7, 4), List(5, 7, 4), List(8, 7, 4))
I need a way to unpack that is not manual, please do not tell me to do this:
val attempt = list2.map(k => k +: list1(0)).map{ case (k, List(x, y)) => (k, x, y)}
Basically, list1 could have any number of elements. e.g
val list1 = List(List(99, 83, 2, 3, 4), List(99, 83, 2, 5 7))
However, these numbers are never repeated, so I guess it could be a set as well. But I don't know much about sets or if it'd help in any way.

Answer 1

This seems to be what you want:
val lists: List[List[Int]] = List(List(7, 4), List(7, 6))
val prefixes: List[List[Int]] = List(List(1), List(5), List(8))
val res: List[List[Int]] = for{
prefix <- prefixes.flatten
rest <- lists
} yield prefix :: res
// res: List[List[Int]] = List(List(1, 7, 4), List(1, 7, 6), List(5, 7, 4), List(5, 7, 6), List(8, 7, 4), List(8, 7, 6))
Unless, you actually want a list that contains your desired_list1 and desired_list2 together, in which case you need:
val res3 = lists.map{ rest =>
prefixes.flatten.map{prefix =>
prefix :: rest
}
}
/// res3: List[List[List[Int]]] =
// List(
// List(
// List(1, 7, 4), List(5, 7, 4), List(8, 7, 4)
// ),
// List(
// List(1, 7, 6), List(5, 7, 6), List(8, 7, 6)
// )
// )

Answer 2

I am not sure exactly why you would want that but try this out:
val list1 = List(List(7, 4), List(7, 6))
val list2 = List(1, 5, 8)
list2.flatMap(element=> list1.map(innerList=> element:: innerList))
output: List[List[Int]] = List(List(1, 7, 4), List(1, 7, 6), List(5, 7, 4), List(5, 7, 6), List(8, 7, 4), List(8, 7, 6))