Why I can write something like this without compilation errors:
wordCount foreach(x => println("Word: " + x._1 + ", count: " + x._2)) // wordCount - is Map
i.e. I declared the x variable.
But I can't use magic _ symbol in this case:
wordCount foreach(println("Word: " + _._1 + ", count: " + _._2)) // wordCount - is
You should check this answer about placeholder syntax.
Two underscores mean two consecutive variables, so using println(_ + _) is a placeholder equivalent of (x, y) => println(x + y)
In first example, you just have a regular Tuple, which has accessors for first (._1) and second (._2) element.
it means that you can't use placeholder syntax when you want to reference only one variable multiple times.
Every underscore is positional. So your code is desugared to
wordCount foreach((x, y) => println("Word: " + x._1 + ", count: " + y._2))
Thanks to this, List(...).reduce(_ + _) is possible.
Moreover, since expansion is made relative to the closest paren it actually will look like:
wordCount foreach(println((x, y) => "Word: " + x._1 + ", count: " + y._2))
