Question

E.g. If I write this function:

def function1() : Unit = {            
 aVar = myList.map { ( s :Something ) => (s.id, s.name) } toList
 function2()
}

def function2() : Unit = {
   //bla
}

The compiler complains when calling function2 saying expecting Int but found Unit. I just can’t figure out where the required Int comes from.

Now, if I write function1 as follows it compiles and works exactly as I was expecting:

def function1() : Unit = {            
     aVar = myList.map { ( s :Something ) => (s.id, s.name) } toList
     var x = function2()
}

Why!!!!???? Just assigning function2 to a var changes the behavior?? In first place function2 is defined to return Unit…

Next it gets more tangled, any of the next two function1 definitions works perfectly:

def function1() : Unit = {            
         aVar = myList.map { ( s :Something ) => (s.id, s.name) }.toList
         function2()
    }

def function1() : Unit = {            
         aVar = (myList.map { ( s :Something ) => (s.id, s.name) } toList)
         function2()
    }

It looks like having an space instead a dot sometimes can be dangerous but I can’t figure out why.

Answer 1

Using space instead of . in scala should be used preferably with one parameter function in the following way:

myObject itsFunction param

Thus, when you type myMap toList, the compiler still waiting for a param, and take it from the next line of code, while, when you type myMap.toList it doesn’t wait for anything and it considers function2 as a independent instruction.
It’s quite the same when you assign the result of function2to a var. As soon as the compiler see the var declaration, it understands that the previous statement is closed and don’t expect a parameter anymore.

Here, you should wonder what parameter does the compiler expect, as toList doesn’t need any. Actually, toList returns a List and List has an applymethod… And that’s exactly what the compiler is waiting for: the parameter of List‘s apply method.

Maybe this REPL session will help you:

scala> Map(1 -> 2) toList 0
res0: (Int, Int) = (1,2)