Method Invocation

Generally speaking, method invocation in Scala follows Java conventions. In other words, there should not be a space between the invocation target and the dot (.), nor a space between the dot and the method name, nor should there be any space between the method name and the argument-delimiters (parentheses). Each argument should be separated by a single space following the comma (,):

foo(42, bar)
target.foo(42, bar)
target.foo()

As of version 2.8, Scala now has support for named parameters. Named parameters in a method invocation should be treated as regular parameters (spaced accordingly following the comma) with a space on either side of the equals sign:

foo(x = 6, y = 7)

While this style does create visual ambiguity with named parameters and variable assignment, the alternative (no spacing around the equals sign) results in code which can be very difficult to read, particularly for non-trivial expressions for the actuals.

Arity-0

Scala allows the omission of parentheses on methods of arity-0 (no arguments):

reply()

// is the same as

reply

However, this syntax should only be used when the method in question has no side-effects (purely-functional). In other words, it would be acceptable to omit parentheses when calling queue.size, but not when calling println(). This convention mirrors the method declaration convention given above.

Observing this convention improves code readability and will make it much easier to understand at a glance the most basic operation of any given method. Resist the urge to omit parentheses simply to save two characters!

Arity-1 (Infix Notation)

Scala has a special punctuation-free syntax for invoking methods of arity-1 (one argument). This should generally be avoided, but with the following exceptions for operators and higher-order functions. In these cases it should only be used for purely-functional methods (methods with no side-effects).

// recommended
names.mkString(",")

// also sometimes seen; controversial
names mkString ","

// wrong - has side-effects
javaList add item

Symbolic Methods/Operators

Symbolic methods (operators) should always be invoked using infix notation with spaces separating the target, the operator, and the parameter:

// right!
"daniel" + " " + "spiewak"
a + b

// wrong!
"daniel"+" "+"spiewak"
a+b
a.+(b)

For the most part, this idiom follows Java and Haskell syntactic conventions. A gray area is short, operator-like methods like max, especially if commutative:

// fairly common
a max b

Symbolic methods which take more than one parameter are discouraged. When they exist, they may still be invoked using infix notation, delimited by spaces:

foo ** (bar, baz)

Such methods are fairly rare, however, and should normally be avoided during API design. For example, the use of the (now deprecated) /: and :\ methods should be avoided in preference to their better-known names, foldLeft and foldRight.

Higher-Order Functions

Invoking higher-order functions may use parens or braces, but in either case, use dot notation and omit any space after the method name:

names.map(_.toUpperCase)

These are not recommended:

// wrong! missing dot
names map (_.toUpperCase)
// wrong! extra space
names.map (_.toUpperCase)

Experience has shown that these styles make code harder to read, especially when multiple such method calls are chained.