Question

I can the require method in Scala’s Predef class with a String as second argument, e.g.

require ("foo" == "bar", "foobar")

First a thought the require method is overloaded for different parameters as second argument. But it is not. The Signature of the require method (Scala 2.9.1) is:

require(requirement: Boolean, message: ⇒ Any): Unit

Why is the above method call possible ?

Answer 1

I don’t fully understand the question, but here is a bit of explanation. require has one overloaded version in Predef:

def require(requirement: Boolean) //...
def require(requirement: Boolean, message: => Any) //...

The second one is a bit confusing due to message: => Any type. It would probably be easier if it was simply:

def require(requirement: Boolean, message: Any) //...

The second parameter is of course a message that is suppose to be appended to error message if assertions is not met. You could imagine message should be of String type but with Any you can simply write:

require(x == 4, x)

Which will add actual value of x (of type Int) into an error message if it is not equal to 4. That’s why Any was chosen – to allow arbitrary value.

But what about : => part? This is called call by name and basically means: evaluate this parameter when it is accessed. Imagine the following snippet:

require(list.isEmpty, list.size)

In this case you want to be sure the list is empty – and if it is not, add the actual list size to the error message. However with normal call convention the list.size part must be evaluated before the method is called – which might be wasteful. With call by name convention the list.size is only evaluated the first time it is used – when the error message is constructor (if required).