Underscore in List.filter

Question

Why this is not working:

List(true,false).filter(_).size

The error says:

:8: error: missing parameter type f         


        

Answer 1

The first error is because Scala doesn't know what to make of _. So try this...

List(true,false).filter(_:Boolean).size

After that, you get more info:

:8: error: type mismatch;
found   : Boolean
required: (Boolean) => Boolean
 List(true,false).filter(_:Boolean).size

It's just evaluating the _ just as the value and not as function. Per the ScalaDoc

filter (pred: (A) ⇒ Boolean): GenTraversable[A]