Question

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Functions vs methods in Scala
What is the difference between def foo = {} and def foo() = {} in Scala?

In scala we can define

def foo():Unit = println ("hello")

or

def foo:Unit = println ("hello")

I know they are not the same but what is the difference, and which should be used when?

If this has been answered before please point me to that link.

Answer 1

A Scala 2.x method of 0-arity can be defined with or without parentheses (). This is used to signal the user that the method has some kind of side-effect (like printing out to std out or destroying data), as opposed to the one without, which can later be implemented as val.

See Programming in Scala:

Such parameterless methods are quite common in Scala. By contrast, methods defined with empty parentheses, such as def height(): Int, are called empty-paren methods. The recommended convention is to use a parameterless method whenever there are no parameters and the method accesses mutable state only by reading fields of the containing object (in particular, it does not change mutable state).

This convention supports the uniform access principle […]

To summarize, it is encouraged style in Scala to define methods that take no parameters and have no side effects as parameterless methods, i.e., leaving off the empty parentheses. On the other hand, you should never define a method that has side-effects without parentheses, because then invocations of that method would look like a field selection.

Terminology

There are some confusing terminology around 0-arity methods, so I’ll create a table here:

	Programming in Scala	scala/scala jargon
def foo: Int	parameterless methods	nullary method
def foo(): Int	empty-paren methods	nilary method

It might sound cool to say "nullary method", but often people say it wrong and the readers will also be confused, so I suggest sticking with parameterless vs empty-paren methods, unless you’re on a pull request where people are already using the jargons.

() is no longer optional in Scala 2.13 or 3.0

In The great () insert, Martin Odersky made change to Scala 3 to require () to call a method defined with (). This is documented in Scala 3 Migration Guide as:

Auto-application is the syntax of calling a nullary method without passing an empty argument list.

Note: Migration document gets the term wrong. It should read as:

Auto-application is the syntax of calling a empty-paren (or "nilary") method without passing an empty argument list.

Scala 2.13, followed Scala 3.x and deprecated the auto application of empty-paren methods in Eta-expand 0-arity method if expected type is Function0. A notable exception to this rule is Java-defined methods. We can continue to call Java methods such as toString without ().