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):
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 ‘$’ and \u005F ‘_’.
Operator characters. These consist of all printable ASCII characters
(\u0020 - \u007E) that are in none of the sets above, mathematical
symbols (Sm) and other symbols (So).
There are three ways to form an identifier. First, an identifier can
start with a letter, followed by an arbitrary sequence of
letters and digits. This may be followed by underscore ‘_‘
characters and another string composed of either letters and digits or
of operator characters. Second, an identifier can start with an operator
character followed by an arbitrary sequence of operator characters.
The preceding two forms are called plain identifiers. Finally,
an identifier may also be formed by an arbitrary string between
backquotes (host systems may impose some restrictions on which
strings are legal for identifiers). The identifier then is composed
of all characters excluding the backquotes themselves.
As usual, the longest match rule applies. For instance, the string
big_bob++=`def`
decomposes into the three identifiers big_bob, ++=, and
def.
Although / is an opchar, the sequence of characters // or /*,
which open a comment, must be enclosed in backquotes when used in an identifier.
def `://`(s: String): URI
def `*/*`(d: Double): Double
The rules for pattern matching further distinguish between
variable identifiers, which start with a lower case letter
or _, and constant identifiers, which do not.
For this purpose, lower case letters include not only a-z,
but also all characters in Unicode category Ll (lowercase letter),
as well as all letters that have contributory property
Other_Lowercase, except characters in category Nl (letter numerals),
which are never taken as lower case.
The following are examples of variable identifiers:
The ‘$’ 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.
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:
The token immediately preceding the newline can terminate a statement.
The token immediately following the newline can begin a statement.
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:
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):
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.
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:
Byte
´-2^7´ to ´2^7-1´
Short
´-2^{15}´ to ´2^{15}-1´
Char
´0´ to ´2^{16}-1´
The digits of a numeric literal may be separated by
arbitrarily many underscores for purposes of legibility.
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.
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 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.
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
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.
The following character 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
\
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>