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?
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 _orbaz(_)instead ofbaz.
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..."
