path: root/spec/01-lexical-syntax.md

---
title: Lexical Syntax
layout: default
chapter: 1
---

# 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.

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

<!--
TODO SI-4583: UnicodeEscape used to allow additional backslashes,
and there is something in the code `evenSlashPrefix` that alludes to it,
but I can't make it work nor can I imagine how this would make sense,
so I removed it for now.
-->

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

1. Whitespace characters. `\u0020 | \u0009 | \u000D | \u000A`.
1. 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.
1. Digits `‘0’ | … | ‘9’`.
1. Parentheses `‘(’ | ‘)’ | ‘[’ | ‘]’ | ‘{’ | ‘}’ `.
1. Delimiter characters ``‘`’ | ‘'’ | ‘"’ | ‘.’ | ‘;’ | ‘,’ ``.
1. 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

```ebnf
op       ::=  opchar {opchar}
varid    ::=  lower idrest
boundvarid ::=  varid
             | ‘`’ varid ‘`’
plainid  ::=  upper idrest
           |  varid
           |  op
id       ::=  plainid
           |  ‘`’ { charNoBackQuoteOrNewline | UnicodeEscape | charEscapeSeq } ‘`’
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

```scala
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.

```scala
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:
> ```scala
>     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

```ebnf
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.
1. The token immediately following the newline can begin a statement.
1. 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:

```scala
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:

```scala
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
1. 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
1. the interval between matching `[` and `]` bracket tokens, except for nested
   regions where newlines are enabled.
1. The interval between a `case` token and its matching
   `=>` token, except for nested regions where newlines are
   enabled.
1. Any regions analyzed in [XML mode](#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](13-syntax-summary.html))
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):

- between the condition of a
  [conditional expression](06-expressions.html#conditional-expressions)
  or [while loop](06-expressions.html#while-loop-expressions) and the next
  following expression,
- between the enumerators of a
  [for-comprehension](06-expressions.html#for-comprehensions-and-for-loops)
  and the next following expression, and
- after the initial `type` keyword in a
  [type definition or declaration](04-basic-declarations-and-definitions.html#type-declarations-and-type-aliases).

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](06-expressions.html#prefix,-infix,-and-postfix-operations),
  if the first token on the next line can start an expression,
- in front of a [parameter clause](04-basic-declarations-and-definitions.html#function-declarations-and-definitions), and
- after an [annotation](11-annotations.html#user-defined-annotations).

> The newline tokens between the two lines are not
> treated as statement separators.
>
> ```scala
> if (x > 0)
>   x = x - 1
>
> while (x > 0)
>   x = x / 2
>
> for (x <- 1 to 10)
>   println(x)
>
> type
>   IntList = List[Int]
> ```

<!-- -->

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

<!-- -->

> ```scala
>   x < 0 ||
>   x > 10
> ```
>
> With an additional newline character, the same code is interpreted as
> two expressions:
>
> ```scala
>   x < 0 ||
>
>   x > 10
> ```

<!-- -->

> ```scala
> 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:
>
> ```scala
> def func(x: Int)
>
>         (y: Int) = x + y
> ```

<!-- -->

> ```scala
> @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).
>
> ```scala
> @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.

<!-- TODO
  say that we take values from Java, give examples of some lits in
  particular float and double.
-->

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

### Integer Literals

```ebnf
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_](06-expressions.html#expression-typing) 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 $2\^{16}-1$       |

> ```scala
> 0          21          0xFFFFFFFF       -42L
> ```

### Floating Point Literals

```ebnf
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.

> ```scala
> 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

```ebnf
booleanLiteral  ::=  ‘true’ | ‘false’
```

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

### Character Literals

```ebnf
characterLiteral  ::=  ‘'’ (charNoQuoteOrNewline | UnicodeEscape | charEscapeSeq) ‘'’
```

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 either a
[Unicode escape](01-lexical-syntax.html) or by an [escape sequence](#escape-sequences).

> ```scala
> 'a'    '\u0041'    '\n'    '\t'
> ```

Note that although Unicode conversion is done early during parsing,
so that Unicode characters are generally equivalent to their escaped
expansion in the source text, literal parsing accepts arbitrary
Unicode escapes, including the character literal `'\u000A'`,
which can also be written using the escape sequence `'\n'`.

### String Literals

```ebnf
stringLiteral  ::=  ‘"’ {stringElement} ‘"’
stringElement  ::=  charNoDoubleQuoteOrNewline | UnicodeEscape | charEscapeSeq
```

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 either a
[Unicode escape](01-lexical-syntax.html) or by an [escape sequence](#escape-sequences).

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`.

> ```scala
> "Hello, world!\n"
> "\"Hello,\" replied the world."
> ```

#### Multi-Line String Literals

```ebnf
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](#escape-sequences) are interpreted.

> ```scala
>   """the present string
>      spans three
>      lines."""
> ```
>
> This would produce the string:
>
> ```scala
> 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
>
> ```scala
>  """the present string
>    |spans three
>    |lines.""".stripMargin
> ```
>
> evaluates to
>
> ```scala
> the present string
> spans three
> lines.
> ```
>
> Method `stripMargin` is defined in class
> [scala.collection.immutable.StringLike](http://www.scala-lang.org/api/current/#scala.collection.immutable.StringLike).
> Because there is a predefined
> [implicit conversion](06-expressions.html#implicit-conversions) 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

```ebnf
symbolLiteral  ::=  ‘'’ plainid
```

A symbol literal `'x` is a shorthand for the expression
`scala.Symbol("x")`. `Symbol` is a [case class](05-classes-and-objects.html#case-classes),
which is defined as follows.

```scala
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 `Symbol`s, 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.

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