In scala, use function name as return value?

Question

I am confused when I first come across the following piece of code.

In the class Element, there are three function definitions.

Why height and width can use contents directly as a variable of Array[String]?

Is it because every function in Scala is an object or some other rule?

I come from C++ world, so the definition really puzzles me.

abstract class Element {
  def contents: Array[String]
  def height: Int = contents.length
  def width: Int = if (height == 0) 0 else contents(0).length
}

I didn't notice this on the first reading of your question, but it's worth pointing out that there are actually no functions in the Element class as written. There are three methods. A (nearly) equivalent function for the contents method would be declared as val contents: () => Array[String]. See Difference between method and function in Scala and the blog posts linked from that question to understand why this distinction matters. — Aaron Novstrup
– Aaron Novstrup, Commented Sep 17, 2012 at 7:34

Aaron Novstrup · Accepted Answer · 2012-09-17 11:37:45Z

6

This is just syntax. The expression contents.length, for example, is compiled into a call of the contents method followed by a call of the length method on the result.

In general, parentheses can be omitted on zero-argument method calls. In fact, parentheses must be omitted when calling methods, such as your contents method, that have no parameter list in their definitions.

Note that this feature is part of Scala's commitment to the uniform access principle. The contents method could be replaced with a contents property without breaking callers.

edited Sep 17, 2012 at 11:37

answered Sep 17, 2012 at 4:39

Aaron Novstrup

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6 Comments

jerry Over a year ago

Thanks for the answer. But I think that "parentheses must be omitted on methods such as the contents method with no parameter list." is not entirely right. What about println()? I read this in Programming in Scala that it is better to not drop () in println() because of "side effect".

Aaron Novstrup Over a year ago

I'll clarify my answer -- I'm referring to methods that have no parameter list, not to methods (like println) that accept a variable number (including 0) of arguments.

Aaron Novstrup Over a year ago

I also added a note about the "uniform access principle", which is important to understanding much of Scala's design/conventions

Aaron Novstrup Over a year ago

@PéterTörök There's not anything to take sides about. The fact is that you can not legally use parentheses when calling a method that is defined with no parameter list (try it!). It's certainly true that such methods should be reserved for pure methods, but the language does not enforce that convention.

Péter Török Over a year ago

You are right, I did not read carefully enough, sorry :-( You were talking about function calls while I was talking about function definitions. I remove my comment above to avoid further confusion.

|

themel · Accepted Answer · 2012-09-17 06:47:47Z

2

Nothing very special is going on. Compared to C++, you're hitting the follownig differences:

invocation of methods without arguments in Scala doesn't need parentheses - length in this case is the same as length()
method definitions don't need to be wrapped in {} if they contain only one expression
functions return the value of the last expression in their body

A more C/Java like way to write this would be

def height() : Int = { return contents.length(); }

which would do the same, but be much more verbose - a nice demonstration of why Scala is an awesome language for reading code.

edited Sep 17, 2012 at 6:47

answered Sep 17, 2012 at 4:38

themel

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2 Comments

Alexey Romanov Over a year ago

Rather, def height() : Int.

themel Over a year ago

True. And I forgot a semicolon :)

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