style guide

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.

Religiously observing this convention will dramatically improve 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!

Infix notation

Scala has a special punctuation-free syntax for invoking methods that take one argument. Many Scala programmers use this notation for symbolic-named methods:

// recommended
a + b

// legal, but less readable
a+b

// legal, but definitely strange
a.+(b)

but avoid it for almost all alphabetic-named methods:

// recommended
names.mkString(",")

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

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 (they do exist!) 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 /: and :\ methods should be avoided in preference to their better-known names, foldLeft and foldRight.

Postfix Notation

Scala allows methods that take no arguments to be invoked using postfix notation:

// recommended
names.toList

// discourage
names toList

This style is unsafe, and should not be used. Since semicolons are optional, the compiler will attempt to treat it as an infix method if it can, potentially taking a term from the next line.

names toList
val answer = 42        // will not compile!

This may result in unexpected compile errors at best, and happily compiled faulty code at worst. Although the syntax is used by some DSLs, it should be considered deprecated, and avoided.

Since Scala 2.10, using postfix operator notation will result in a compiler warning.