0__ 0__ - 1 month ago 7
Scala Question

What do all of Scala's symbolic operators mean?

Scala syntax has a lot of symbols. Since these kinds of names are difficult to find using search engines, a comprehensive list of them would be helpful.

What are all of the symbols in Scala, and what does each of them do?

In particular, I'd like to know about

->
,
||=
,
++=
,
<=
,
_._
,
::
, and
:+=
.

Answer

I divide the operators, for the purpose of teaching, into four categories:

  • Keywords/reserved symbols
  • Automatically imported methods
  • Common methods
  • Syntactic sugars/composition

It is fortunate, then, that most categories are represented in the question:

->    // Automatically imported method
||=   // Syntactic sugar
++=   // Syntactic sugar/composition or common method
<=    // Common method
_._   // Typo, though it's probably based on Keyword/composition
::    // Common method
:+=   // Common method

The exact meaning of most of these methods depend on the class that is defining them. For example, <= on Int means "less than or equal to". The first one, ->, I'll give as example below. :: is probably the method defined on List (though it could be the object of the same name), and :+= is probably the method defined on various Buffer classes.

So, let's see them.

Keywords/reserved symbols

There are some symbols in Scala that are special. Two of them are considered proper keywords, while others are just "reserved". They are:

// Keywords
<-  // Used on for-comprehensions, to separate pattern from generator
=>  // Used for function types, function literals and import renaming

// Reserved
( )        // Delimit expressions and parameters
[ ]        // Delimit type parameters
{ }        // Delimit blocks
.          // Method call and path separator
// /* */   // Comments
#          // Used in type notations
:          // Type ascription or context bounds
<: >: <%   // Upper, lower and view bounds
<? <!      // Start token for various XML elements
" """      // Strings
'          // Indicate symbols and characters
@          // Annotations and variable binding on pattern matching
`          // Denote constant or enable arbitrary identifiers
,          // Parameter separator
;          // Statement separator
_*         // vararg expansion
_          // Many different meanings

These are all part of the language, and, as such, can be found in any text that properly describe the language, such as Scala Specification(PDF) itself.

The last one, the underscore, deserve a special description, because it is so widely used, and has so many different meanings. Here's a sample:

import scala._    // Wild card -- all of Scala is imported
import scala.{ Predef => _, _ } // Exception, everything except Predef
def f[M[_]]       // Higher kinded type parameter
def f(m: M[_])    // Existential type
_ + _             // Anonymous function placeholder parameter
m _               // Eta expansion of method into method value
m(_)              // Partial function application
_ => 5            // Discarded parameter
case _ =>         // Wild card pattern -- matches anything
f(xs: _*)         // Sequence xs is passed as multiple parameters to f(ys: T*)
case Seq(xs @ _*) // Identifier xs is bound to the whole matched sequence

I probably forgot some other meaning, though.

Automatically imported methods

So, if you did not find the symbol you are looking for in the list above, then it must be a method, or part of one. But, often, you'll see some symbol and the documentation for the class will not have that method. When this happens, either you are looking at a composition of one or more methods with something else, or the method has been imported into scope, or is available through an imported implicit conversion.

These can still be found on ScalaDoc: you just have to know where to look for them. Or, failing that, look at the index (presently broken on 2.9.1, but available on nightly).

Every Scala code has three automatic imports:

// Not necessarily in this order
import _root_.java.lang._      // _root_ denotes an absolute path
import _root_.scala._
import _root_.scala.Predef._

The first two only make classes and singleton objects available. The third one contains all implicit conversions and imported methods, since Predef is an object itself.

Looking inside Predef quickly show some symbols:

class <:<
class =:=
object <%<
object =:=

Any other symbol will be made available through an implicit conversion. Just look at the methods tagged with implicit that receive, as parameter, an object of type that is receiving the method. For example:

"a" -> 1  // Look for an implicit from String, AnyRef, Any or type parameter

In the above case, -> is defined in the class ArrowAssoc through the method any2ArrowAssoc that takes an object of type A, where A is an unbounded type parameter to the same method.

Common methods

So, many symbols are simply methods on a class. For instance, if you do

List(1, 2) ++ List(3, 4)

You'll find the method ++ right on the ScalaDoc for List. However, there's one convention that you must be aware when searching for methods. Methods ending in colon (:) bind to the right instead of the left. In other words, while the above method call is equivalent to:

List(1, 2).++(List(3, 4))

If I had, instead 1 :: List(2, 3), that would be equivalent to:

List(2, 3).::(1)

So you need to look at the type found on the right when looking for methods ending in colon. Consider, for instance:

1 +: List(2, 3) :+ 4

The first method (+:) binds to the right, and is found on List. The second method (:+) is just a normal method, and binds to the left -- again, on List.

Syntactic sugars/composition

So, here's a few syntactic sugars that may hide a method:

class Example(arr: Array[Int] = Array.fill(5)(0)) {
  def apply(n: Int) = arr(n)
  def update(n: Int, v: Int) = arr(n) = v
  def a = arr(0); def a_=(v: Int) = arr(0) = v
  def b = arr(1); def b_=(v: Int) = arr(1) = v
  def c = arr(2); def c_=(v: Int) = arr(2) = v
  def d = arr(3); def d_=(v: Int) = arr(3) = v
  def e = arr(4); def e_=(v: Int) = arr(4) = v
  def +(v: Int) = new Example(arr map (_ + v))
  def unapply(n: Int) = if (arr.indices contains n) Some(arr(n)) else None
}

val Ex = new Example // or var for the last example
println(Ex(0))  // calls apply(0)
Ex(0) = 2       // calls update(0, 2)
Ex.b = 3        // calls b_=(3)
// This requires Ex to be a "val"
val Ex(c) = 2   // calls unapply(2) and assigns result to c
// This requires Ex to be a "var"
Ex += 1         // substituted for Ex = Ex + 1

The last one is interesting, because any symbolic method can be combined to form an assignment-like method that way.

And, of course, there's various combinations that can appear in code:

(_+_) // An expression, or parameter, that is an anonymous function with
      // two parameters, used exactly where the underscores appear, and
      // which calls the "+" method on the first parameter passing the
      // second parameter as argument.