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.
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
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) 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)
:\ methods should be avoided in
preference to their better-known names,
Invoking higher-order functions may use parens or braces, but in either case, use dot notation and omit any space after the method name:
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.