Lexical Syntax

Scala source code consists of Unicode text.

The nine Bidirectional explicit formatting characters \u202a - \u202e and \u2066 - \u2069 (inclusive) are forbidden from appearing in source files. Note that they can be represented using unicode escapes in string and character literals.

The program text is tokenized as described in this chapter. See the last section for special support for XML literals, which are parsed in XML mode.

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), title case letters (Lt), other letters (Lo), modifier letters (Lm), letter numerals (Nl) and the two characters \u0024 ‘$’ and \u005F ‘_’.
3. Digits ‘0’ | … | ‘9’.
4. Parentheses ‘(’ | ‘)’ | ‘[’ | ‘]’ | ‘{’ | ‘}’.
5. Delimiter characters ‘`’ | ‘'’ | ‘"’ | ‘.’ | ‘;’ | ‘,’.
6. Operator characters. These consist of all printable ASCII characters (\u0020 - \u007E) that are in none of the sets above, mathematical symbols (Sm) and other symbols (So).

Identifiers

op       ::=  opchar {opchar}
varid    ::=  lower idrest
boundvarid ::=  varid
             | ‘`’ varid ‘`’
plainid  ::=  upper idrest
           |  varid
           |  op
id       ::=  plainid
           |  ‘`’ { charNoBackQuoteOrNewline | escapeSeq } ‘`’
idrest   ::=  {letter | digit} [‘_’ op]
escapeSeq     ::= UnicodeEscape | charEscapeSeq
UnicodeEscape ::= ‘\’ ‘u’ {‘u’} hexDigit hexDigit hexDigit hexDigit
hexDigit      ::= ‘0’ | … | ‘9’ | ‘A’ | … | ‘F’ | ‘a’ | … | ‘f’

There are three ways to form an identifier. First, an identifier can start with a letter, 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 backquotes (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, the longest match rule applies. For instance, the string

big_bob++=`def`

decomposes into the three identifiers big_bob, ++=, and def.

Although / is an opchar, the sequence of characters // or /*, which open a comment, must be enclosed in backquotes when used in an identifier.

def `://`(s: String): URI
def `*/*`(d: Double): Double

The rules for pattern matching further distinguish between variable identifiers, which start with a lower case letter or _, and constant identifiers, which do not.

For this purpose, lower case letters include not only a-z, but also all characters in Unicode category Ll (lowercase letter), as well as all letters that have contributory property Other_Lowercase, except characters in category Nl (letter numerals), which are never taken as lower case.

The following are examples of variable identifiers:

    x         maxIndex   p2p   empty_?
    `yield`   αρετη      _y    dot_product_*
    __system  _MAX_LEN_
    ªpple     ʰelper

Some examples of constant identifiers are

    +    Object  $reserved  Džul    ǂnûm
    ⅰ_ⅲ  Ⅰ_Ⅲ     ↁelerious  ǃqhàà  ʹthatsaletter

The ‘$’ character is reserved for compiler-synthesized identifiers. User programs should not define identifiers that contain ‘$’ 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 new line 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):

• between the condition of a conditional expression or while loop and the next following expression,
• between the enumerators of a for-comprehension and the next following expression, and
• after the initial type keyword in a type definition or declaration.

A single new line token is accepted

• in front of an opening brace ‘{’, if that brace is a legal continuation of the current statement or expression,
• after an infix operator, if the first token on the next line can start an expression,
• in front of a parameter clause, and
• after an annotation.

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
           |  interpolatedString
           |  symbolLiteral
           |  ‘null’

Integer Literals

integerLiteral  ::=  (decimalNumeral | hexNumeral | binaryNumeral)
                       [‘L’ | ‘l’]
decimalNumeral  ::=  digit {digit}
hexNumeral      ::=  ‘0’ (‘x’ | ‘X’) hexDigit {hexDigit}
binaryNumeral   ::=  ‘0’ (‘b’ | ‘B’) binaryDigit {binaryDigit}

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.

Integer literals are usually of type Int, or of type Long when followed by a L or l suffix. (Lowercase l is deprecated for reasons of legibility.)

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:

The digits of a numeric literal may be separated by arbitrarily many underscores for purposes of legibility.

0           21_000      0x7F        -42L        0xFFFF_FFFF

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  ::=  ‘'’ (charNoQuoteOrNewline | escapeSeq) ‘'’

A character literal is a single character enclosed in quotes. The character can be any Unicode character except the single quote delimiter or \u000A (LF) or \u000D (CR); or any Unicode character represented by an escape sequence.

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

String Literals

stringLiteral  ::=  ‘"’ {stringElement} ‘"’
stringElement  ::=  charNoDoubleQuoteOrNewline | escapeSeq

A string literal is a sequence of characters in double quotes. The characters can be any Unicode character except the double quote delimiter or \u000A (LF) or \u000D (CR); or any Unicode character represented by an escape sequence.

If the string literal contains a double quote character, it must be escaped using "\"".

The value of a string literal is an instance of class String.

"Hello, world!\n"
"\"Hello,\" replied the world."

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. Escape sequences are not processed, except for Unicode escapes (this is deprecated since 2.13.2).

  """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.StringOps.

Interpolated string

interpolatedString     ::= alphaid ‘"’ {[‘\’] interpolatedStringPart | ‘\\’ | ‘\"’} ‘"’
                         | alphaid ‘"""’ {[‘"’] [‘"’] char \ (‘"’ | ‘$’) | escape} {‘"’} ‘"""’
interpolatedStringPart ::= printableChar \ (‘"’ | ‘$’ | ‘\’) | escape
escape                 ::= ‘$$’
                         | ‘$"’
                         | ‘$’ alphaid
                         | ‘$’ BlockExpr
alphaid                ::= upper idrest
                         |  varid

An interpolated string consists of an identifier starting with a letter immediately followed by a string literal. There may be no whitespace characters or comments between the leading identifier and the opening quote " of the string. The string literal in an interpolated string can be standard (single quote) or multi-line (triple quote).

Inside an interpolated string none of the usual escape characters are interpreted no matter whether the string literal is normal (enclosed in single quotes) or multi-line (enclosed in triple quotes). Note that the sequence \" does not close a normal string literal (enclosed in single quotes).

There are three forms of dollar sign escape. The most general form encloses an expression in ${ and }, i.e. ${expr}. The expression enclosed in the braces that follow the leading $ character is of syntactical category BlockExpr. Hence, it can contain multiple statements, and newlines are significant. Single ‘$’-signs are not permitted in isolation in an interpolated string. A single ‘$’-sign can still be obtained by doubling the ‘$’ character: ‘$$’. A single ‘"’-sign can be obtained by the sequence ‘\$"’.

The simpler form consists of a ‘$’-sign followed by an identifier starting with a letter and followed only by letters, digits, and underscore characters, e.g., $id. The simpler form is expanded by putting braces around the identifier, e.g., $id is equivalent to ${id}. In the following, unless we explicitly state otherwise, we assume that this expansion has already been performed.

The expanded expression is type checked normally. Usually, StringContext will resolve to the default implementation in the scala package, but it could also be user-defined. Note that new interpolators can also be added through implicit conversion of the built-in scala.StringContext.

One could write an extension

implicit class StringInterpolation(s: StringContext) {
  def id(args: Any*) = ???
}

Escape Sequences

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

In addition, Unicode escape sequences of the form \uxxxx, where each x is a hex digit are recognized in character and string literals.

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 deprecated shorthand for the expression scala.Symbol("x").

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.

Trailing Commas in Multi-line Expressions

If a comma (,) is followed immediately, ignoring whitespace, by a newline and a closing parenthesis ()), bracket (]), or brace (}), then the comma is treated as a "trailing comma" and is ignored. For example:

foo(
  23,
  "bar",
  true,
)

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

• the XML expression or the XML pattern started by the initial ‘<’ has been successfully parsed, or if
• the parser encounters an embedded Scala expression or pattern and forces the Scanner back to normal mode, until the Scala expression or pattern is successfully parsed. In this case, since code and XML fragments can be nested, the parser has to maintain a stack that reflects the nesting of XML and Scala expressions adequately.

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>