Scala Mock Syntax (class _).expects meaning?

Question

New to Scala, have searched far and wide for clarification on some ScalaMock syntax. As per this guide, I keep seeing the following general testing pattern:

(myClass.myMethod _).expects()

What exactly is happening here? What function does the class/method/space/underscore serve? How does the compiler treat this?

Answer 1

The appended _ forces the conversion of a method into a function.

To understand why this is necessary, let's try to re-build a tiny piece of Scalamock, namely the expects method. The expects method seems to be invoked on methods of mocked objects. But methods / functions do not have an expects method to begin with. Therefore, we have to use the "pimp my library"-pattern to attach the method expects to functions. We could do something like this:

implicit class ExpectsOp[A, B](f: A => B) {
  def expects(a: A): Unit = println("it compiles, ship it...")
}

Now let's define a class Bar with method baz:

class Bar {
  def baz(i: Int): Int = i * i
}

and also an instance of Bar:

val bar = new Bar

Let's see what happens if you try to invoke expects on bar.baz:

(bar.baz).expects(42)

error: missing argument list for method baz in class Bar Unapplied methods are only converted to functions when a function type is expected. You can make this conversion explicit by writing baz _ or baz(_) instead of baz.

So, it doesn't work without explicit conversion into a function, and we have to enforce this conversion by appending an _:

(bar.baz _).expects(42) // prints: "it compiles, ship it..."