Question

From Programming in Scala,we know that foreach is a high-order function which takes a procedure with return type Unit.So I think the following slice would work:

val abcde = List("a","b","c","d","e")
abcde.foreach(print _.toUpperCase)

however it tells me that:

1: error: ')' expected but '.' found.
  abcde foreach (println _.toUpperCase)
                          ^

But these two below both work well:

println("abcde".toUpperCase)
abcde.foreach(print _)

So what’s the difference?

Answer 1

These two are using _ in different ways:

abcde.foreach(print _.toUpperCase)
abcde.foreach(print _)

In the first case, you have an anonymous function where _ denotes a placeholder for a parameter.

In the second case, _ means you’d like a function value for the method print (an eta expansion).

So comparing the two is irrelevant.

More to the point would be this:

scala> print "abcde".toUpperCase
<console>:1: error: ';' expected but string literal found.
       print "abcde".toUpperCase
             ^

As you can see, this doesn’t work, so replacing "abcde" with _ wouldn’t work either.