Cannot prove that Unit <:< (T, U)

Question

When trying to remove all Unit - () from a list, I tried to call toMap.

scala> List((), ()).filter(_ != ()).toMap


        

Answer 1

It means that the type of an element in the list can't be viewed as a tuple which is required to build a Map. A Map in a sense is a collection of tuples (and more).

Illustration:

scala> List(1).toMap
:8: error: Cannot prove that Int <:< (T, U).
          List(1).toMap
                  ^
scala> List(1 -> 2).toMap
res1: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2)

I can build a map from a list of tuples, but not from a list of single cardinality elements.

Maybe you mean to say .map instead of .toMap? ;)