Lexical Syntax

Scala programs are written using the Unicode Basic Multilingual Plane (BMP) character set; Unicode supplementary characters are not presently supported. This chapter defines the two modes of Scala's lexical syntax, the Scala mode and the XML mode. If not otherwise mentioned, the following descriptions of Scala tokens refer to Scala mode, and literal characters ‘c’ refer to the ASCII fragment \u0000 – \u007F.

In Scala mode, Unicode escapes are replaced by the corresponding Unicode character with the given hexadecimal code.

UnicodeEscape ::= ‘\’ ‘u’ {‘u’} hexDigit hexDigit hexDigit hexDigit
hexDigit      ::= ‘0’ | … | ‘9’ | ‘A’ | … | ‘F’ | ‘a’ | … | ‘f’

To construct tokens, characters are distinguished according to the following classes (Unicode general category given in parentheses):

  1. Whitespace characters. \u0020 | \u0009 | \u000D | \u000A.
  2. Letters, which include lower case letters (Ll), upper case letters (Lu), titlecase letters (Lt), other letters (Lo), letter numerals (Nl) and the two characters \u0024 ‘$’ and \u005F ‘_’, which both count as upper case letters.
  3. Digits ‘0’ | … | ‘9’.
  4. Parentheses ‘(’ | ‘)’ | ‘[’ | ‘]’ | ‘{’ | ‘}’.
  5. Delimiter characters ‘`’ | ‘'’ | ‘"’ | ‘.’ | ‘;’ | ‘,’.
  6. Operator characters. These consist of all printable ASCII characters \u0020 - \u007F which are in none of the sets above, mathematical symbols (Sm) and other symbols (So).

Identifiers

op       ::=  opchar {opchar}
varid    ::=  lower idrest
plainid  ::=  upper idrest
           |  varid
           |  op
id       ::=  plainid
           |  ‘`’ stringLiteral ‘`’
idrest   ::=  {letter | digit} [‘_’ op]

There are three ways to form an identifier. First, an identifier can start with a letter which can be followed by an arbitrary sequence of letters and digits. This may be followed by underscore ‘_‘ characters and another string composed of either letters and digits or of operator characters. Second, an identifier can start with an operator character followed by an arbitrary sequence of operator characters. The preceding two forms are called plain identifiers. Finally, an identifier may also be formed by an arbitrary string between back-quotes (host systems may impose some restrictions on which strings are legal for identifiers). The identifier then is composed of all characters excluding the backquotes themselves.

As usual, a longest match rule applies. For instance, the string

big_bob++=`def`

decomposes into the three identifiers big_bob, ++=, and def. The rules for pattern matching further distinguish between variable identifiers, which start with a lower case letter, and constant identifiers, which do not.

The ‘\$’ character is reserved for compiler-synthesized identifiers. User programs should not define identifiers which contain ‘\$’ characters.

The following names are reserved words instead of being members of the syntactic class id of lexical identifiers.

abstract    case        catch       class       def
do          else        extends     false       final
finally     for         forSome     if          implicit
import      lazy        macro       match       new
null        object      override    package     private
protected   return      sealed      super       this
throw       trait       try         true        type
val         var         while       with        yield
_    :    =    =>    <-    <:    <%     >:    #    @

The Unicode operators \u21D2 ‘$\Rightarrow$’ and \u2190 ‘$\leftarrow$’, which have the ASCII equivalents => and <-, are also reserved.

Here are examples of identifiers:

    x         Object        maxIndex   p2p      empty_?
    +         `yield`       αρετη     _y       dot_product_*
    __system  _MAX_LEN_

When one needs to access Java identifiers that are reserved words in Scala, use backquote-enclosed strings. For instance, the statement Thread.yield() is illegal, since yield is a reserved word in Scala. However, here's a work-around: Thread.`yield`()

Newline Characters

semi ::= ‘;’ |  nl {nl}

Scala is a line-oriented language where statements may be terminated by semi-colons or newlines. A newline in a Scala source text is treated as the special token “nl” if the three following criteria are satisfied:

  1. The token immediately preceding the newline can terminate a statement.
  2. The token immediately following the newline can begin a statement.
  3. The token appears in a region where newlines are enabled.

The tokens that can terminate a statement are: literals, identifiers and the following delimiters and reserved words:

this    null    true    false    return    type    <xml-start>
_       )       ]       }

The tokens that can begin a statement are all Scala tokens except the following delimiters and reserved words:

catch    else    extends    finally    forSome    match
with    yield    ,    .    ;    :    =    =>    <-    <:    <%
>:    #    [    )    ]    }

A case token can begin a statement only if followed by a class or object token.

Newlines are enabled in:

  1. all of a Scala source file, except for nested regions where newlines are disabled, and
  2. the interval between matching { and } brace tokens, except for nested regions where newlines are disabled.

Newlines are disabled in:

  1. the interval between matching ( and ) parenthesis tokens, except for nested regions where newlines are enabled, and
  2. the interval between matching [ and ] bracket tokens, except for nested regions where newlines are enabled.
  3. The interval between a case token and its matching => token, except for nested regions where newlines are enabled.
  4. Any regions analyzed in XML mode.

Note that the brace characters of {...} escapes in XML and string literals are not tokens, and therefore do not enclose a region where newlines are enabled.

Normally, only a single nl token is inserted between two consecutive non-newline tokens which are on different lines, even if there are multiple lines between the two tokens. However, if two tokens are separated by at least one completely blank line (i.e a line which contains no printable characters), then two nl tokens are inserted.

The Scala grammar (given in full here) contains productions where optional nl tokens, but not semicolons, are accepted. This has the effect that a newline in one of these positions does not terminate an expression or statement. These positions can be summarized as follows:

Multiple newline tokens are accepted in the following places (note that a semicolon in place of the newline would be illegal in every one of these cases):

A single new line token is accepted

The newline tokens between the two lines are not treated as statement separators.

if (x > 0)
  x = x - 1

while (x > 0)
  x = x / 2

for (x <- 1 to 10)
  println(x)

type
  IntList = List[Int]

new Iterator[Int]
{
  private var x = 0
  def hasNext = true
  def next = { x += 1; x }
}

With an additional newline character, the same code is interpreted as an object creation followed by a local block:

new Iterator[Int]

{
  private var x = 0
  def hasNext = true
  def next = { x += 1; x }
}

  x < 0 ||
  x > 10

With an additional newline character, the same code is interpreted as two expressions:

  x < 0 ||

  x > 10

def func(x: Int)
        (y: Int) = x + y

With an additional newline character, the same code is interpreted as an abstract function definition and a syntactically illegal statement:

def func(x: Int)

        (y: Int) = x + y

@serializable
protected class Data { ... }

With an additional newline character, the same code is interpreted as an attribute and a separate statement (which is syntactically illegal).

@serializable

protected class Data { ... }

Literals

There are literals for integer numbers, floating point numbers, characters, booleans, symbols, strings. The syntax of these literals is in each case as in Java.

Literal  ::=  [‘-’] integerLiteral
           |  [‘-’] floatingPointLiteral
           |  booleanLiteral
           |  characterLiteral
           |  stringLiteral
           |  symbolLiteral
           |  ‘null’

Integer Literals

integerLiteral  ::=  (decimalNumeral | hexNumeral)
                       [‘L’ | ‘l’]
decimalNumeral  ::=  ‘0’ | nonZeroDigit {digit}
hexNumeral      ::=  ‘0’ (‘x’ | ‘X’) hexDigit {hexDigit}
digit           ::=  ‘0’ | nonZeroDigit
nonZeroDigit    ::=  ‘1’ | … | ‘9’

Integer literals are usually of type Int, or of type Long when followed by a L or l suffix. Values of type Int are all integer numbers between $-2^{31}$ and $2^{31}-1$, inclusive. Values of type Long are all integer numbers between $-2^{63}$ and $2^{63}-1$, inclusive. A compile-time error occurs if an integer literal denotes a number outside these ranges.

However, if the expected type pt of a literal in an expression is either Byte, Short, or Char and the integer number fits in the numeric range defined by the type, then the number is converted to type pt and the literal's type is pt. The numeric ranges given by these types are:
Byte $-2^7$ to $2^7-1$
Short $-2^{15}$ to $2^{15}-1$
Char $0$ to $2^{16}-1$ 
0          21          0xFFFFFFFF       -42L

Floating Point Literals

floatingPointLiteral  ::=  digit {digit} ‘.’ digit {digit} [exponentPart] [floatType]
                        |  ‘.’ digit {digit} [exponentPart] [floatType]
                        |  digit {digit} exponentPart [floatType]
                        |  digit {digit} [exponentPart] floatType
exponentPart          ::=  (‘E’ | ‘e’) [‘+’ | ‘-’] digit {digit}
floatType             ::=  ‘F’ | ‘f’ | ‘D’ | ‘d’

Floating point literals are of type Float when followed by a floating point type suffix F or f, and are of type Double otherwise. The type Float consists of all IEEE 754 32-bit single-precision binary floating point values, whereas the type Double consists of all IEEE 754 64-bit double-precision binary floating point values.

If a floating point literal in a program is followed by a token starting with a letter, there must be at least one intervening whitespace character between the two tokens.

0.0        1e30f      3.14159f      1.0e-100      .1

The phrase 1.toString parses as three different tokens: the integer literal 1, a ., and the identifier toString.

1. is not a valid floating point literal because the mandatory digit after the . is missing.

Boolean Literals

booleanLiteral  ::=  ‘true’ | ‘false’

The boolean literals true and false are members of type Boolean.

Character Literals

characterLiteral  ::=  ‘'’ (printableChar | charEscapeSeq) ‘'’

A character literal is a single character enclosed in quotes. The character is either a printable unicode character or is described by an escape sequence.

'a'    '\u0041'    '\n'    '\t'

Note that '\u000A' is not a valid character literal because Unicode conversion is done before literal parsing and the Unicode character \u000A (line feed) is not a printable character. One can use instead the escape sequence '\n' or the octal escape '\12' (see here).

String Literals

stringLiteral  ::=  ‘"’ {stringElement} ‘"’
stringElement  ::=  printableCharNoDoubleQuote  |  charEscapeSeq

A string literal is a sequence of characters in double quotes. The characters are either printable unicode character or are described by escape sequences. If the string literal contains a double quote character, it must be escaped, i.e. "\"". The value of a string literal is an instance of class String.

"Hello,\nWorld!"
"This string contains a \" character."

Multi-Line String Literals

stringLiteral   ::=  ‘"""’ multiLineChars ‘"""’
multiLineChars  ::=  {[‘"’] [‘"’] charNoDoubleQuote} {‘"’}

A multi-line string literal is a sequence of characters enclosed in triple quotes """ ... """. The sequence of characters is arbitrary, except that it may contain three or more consecutive quote characters only at the very end. Characters must not necessarily be printable; newlines or other control characters are also permitted. Unicode escapes work as everywhere else, but none of the escape sequences here are interpreted.

  """the present string
     spans three
     lines."""

This would produce the string:

the present string
     spans three
     lines.

The Scala library contains a utility method stripMargin which can be used to strip leading whitespace from multi-line strings. The expression

 """the present string
   |spans three
   |lines.""".stripMargin

evaluates to

the present string
spans three
lines.

Method stripMargin is defined in class scala.collection.immutable.StringLike. Because there is a predefined implicit conversion from String to StringLike, the method is applicable to all strings.

Escape Sequences

The following escape sequences are recognized in character and string literals.

charEscapeSeq unicode name char
‘\‘ ‘b‘ \u0008 backspace BS
‘\‘ ‘t‘ \u0009 horizontal tab HT
‘\‘ ‘n‘ \u000a linefeed LF
‘\‘ ‘f‘ \u000c form feed FF
‘\‘ ‘r‘ \u000d carriage return CR
‘\‘ ‘"‘ \u0022 double quote "
‘\‘ ‘'‘ \u0027 single quote '
‘\‘ ‘\‘ \u005c backslash \

A character with Unicode between 0 and 255 may also be represented by an octal escape, i.e. a backslash '\' followed by a sequence of up to three octal characters.

It is a compile time error if a backslash character in a character or string literal does not start a valid escape sequence.

Symbol literals

symbolLiteral  ::=  ‘'’ plainid

A symbol literal 'x is a shorthand for the expression scala.Symbol("x"). Symbol is a case class, which is defined as follows.

package scala
final case class Symbol private (name: String) {
  override def toString: String = "'" + name
}

The apply method of Symbol's companion object caches weak references to Symbols, thus ensuring that identical symbol literals are equivalent with respect to reference equality.

Whitespace and Comments

Tokens may be separated by whitespace characters and/or comments. Comments come in two forms:

A single-line comment is a sequence of characters which starts with // and extends to the end of the line.

A multi-line comment is a sequence of characters between /* and */. Multi-line comments may be nested, but are required to be properly nested. Therefore, a comment like /* /* */ will be rejected as having an unterminated comment.

XML mode

In order to allow literal inclusion of XML fragments, lexical analysis switches from Scala mode to XML mode when encountering an opening angle bracket ‘<’ in the following circumstance: The ‘<’ must be preceded either by whitespace, an opening parenthesis or an opening brace and immediately followed by a character starting an XML name.

 ( whitespace | ‘(’ | ‘{’ ) ‘<’ (XNameStart | ‘!’ | ‘?’)

  XNameStart ::= ‘_’ | BaseChar | Ideographic // as in W3C XML, but without ‘:’

The scanner switches from XML mode to Scala mode if either

Note that no Scala tokens are constructed in XML mode, and that comments are interpreted as text.

The following value definition uses an XML literal with two embedded Scala expressions:

val b = <book>
          <title>The Scala Language Specification</title>
          <version>{scalaBook.version}</version>
          <authors>{scalaBook.authors.mkList("", ", ", "")}</authors>
        </book>