Question

List("This","is","Scala").foreach(a => print(a+" "))

compiles fine, but

List("This","is","Scala").foreach(print(_+" "))

fails complaining of missing parameter type. I couldn’t figure out why it fails.

EDIT: I meant print not println – not that it makes logical difference.

Answer 1

The problem is that this

List("This","is","Scala").foreach(print(_+" "))

is not equivalent to

List("This","is","Scala").foreach(a => print(a+" "))

but to

List("This","is","Scala").foreach(print(a => a+" "))

Now, let’s see the type signature of foreach:

def foreach [B] (f: (A) ⇒ B) : Unit

where A is the type parameter of the List itself. Since we have a List[String], the compiler knows one has to pass to foreach a Function[String, B].

In a => print(a+" ") the type of a is already known then: String.

In print(a => a+" ") there is a problem, as print is not a Function. However, the compiler hasn’t considered that yet — it’s still trying to compile a => a+" ". So let’s look at the type of Predef.print:

def print (x: Any) : Unit

So a => a+" " must be of type Any, which, of course, means it can be anything. It doesn’t help the compiler in asserting what the type of a is. Which doesn’t really matter, because you didn’t want to print a Function in first place.