Question

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What are all the uses of an underscore in Scala?

When I follow a particular tutorial, I happen to see that the following two usages gives the same result. I understand the first one, but I do not understand why the 2nd one also works. Can someone give me an explanation, and at the same time give a summary of _ usage?

def sum (a:Int, b:Int) = a + b

val sumAsFunction1 = sum(_:Int, _:Int)  
// I understand this, _ used as placeholder of parameters

val sumAsFunction2 = sum _
// why this usage has the same effect as sumAsFunction1?

Answer 1

This is one of the few places where eta-expansion mechanism needs some help. Consider the simpler example:

def foo() = {
  println("foo"); 
  42
}

val bar1 = foo
val bar2 = foo _

There is a fundamental difference between bar1 and bar2. The former one is interpreted as call foo and assign value to bar while the latter: Change method foo into a function and assign it to bar2. As a result bar1 is just a simple Int variable while bar2 is actually a function that will call original foo() method (and print "foo").