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

I see the error message is much improved! Let's see what the error message said you wrote, and your working version:

((x$1) => List(true, false).filter(x$1).size)
          List(true,false).filter(a => a).size

Or, adjusting spaces, parenthesis and variable names:

a => List(true, false).filter(a     ).size
     List(true, false).filter(a => a).size

Does it look the same now?

Briefly, when you pass underscore in place of a parameter, you are doing partial function application. You are probably more familiar with underscores being used as placeholders for parameters in anonymous functions, which is what happens when it appears in an expression, like _ + 1. These two uses are different, even if they both result in an anonymous function.