Notation

The following notation is used in the syntax specification in this chapter:

• a term_h is a term which is the head of the clause.
• a term_h(N) is a term_h of maximum precedence N.
• a term_g is a term which is a goal (body) of the clause.
• a term_g(N) is a term_g of maximum precedence N.
• a term_a is a term which is an argument of a compound term or a list.
• a term(N) can be any term (term_h, term_a or term_h) of maximum precedence N.
• fx(N) is a prefix operator of precedence N which is not right associative.
• fy(N) is a prefix operator of precedence N which is right associative.
• similar definitions apply for infix (xfx, xfy, yfx) and postfix (xf, yf) operators.

Character Classes

The following character classes exist:

Character Class	Notation Used	Default Members
upper_case	UC	all upper case letters
underline	UL	_
lower_case	LC	all lower case letters
digit	N	digits
blank_space	BS	space, tab and nonprintable ASCII characters
end_of_line	NL	line feed
atom_quote	AQ	'
string_quote	SQ	"
list_quote	LQ
radix	RA
ascii	AS
solo	SL	( ) ] }
special	SP	! , ; [ { |
line_comment	CM	%
escape	ES	\
first_comment	CM1	/
second_comment	CM2	*
symbol	SY	# + - . : < = > ? @ ^ ` ~ $ &

The character class of any character can be modified by a chtab-declaration.

Groups of characters

Group Type	Notation	Valid Characters
alphanumerical	ALP	UC UL LC N
delimiter	DE	) } ] , |
any character	ANY
non escape	NES	any character except escape
sign	SGN	+ -

Valid Tokens

Terms are defined in terms of tokens, and tokens are defined in terms of characters and character classes. Individual tokens can be read with the predicates read_token/2 and read_token/3. The description of the valid tokens follows.

Constants

1.

atoms

ATOM    = (LC ALP*)
        | (SY | CM1 | CM2 | ES)+
        | (AQ (NES | ES ANY+)* AQ)
        | |
        | ;
        | []
        | {}
        | !

2.

numbers

(a)

integers

INT = [SGN] BS* N+

(b)

based integers

INTBAS = N+ (AQ | RA) (N | LC | UC)+

The base must be an integer between 1 and 36 included, the value being valid for this base.

If the syntax option iso_base_prefix is active, the syntax for based integers is instead

INTBAS = 0 (b | o | x) (N | LC | UC)+

which allows binary, octal and hexadecimal numbers respectively.

(c)

ASCII codes

ASCII = 0 (AQ | RA) ANY | AS ANY

The value of the integer is the ASCII code of the last character.

(d)

rationals

RAT = [SGN] BS* N+ UL N+

(e)

floats

FLOAT = [SGN] BS* N+ . N+ [ (e | E) [SGN] N+ | Inf ]
     | [SGN] BS* N+        (e | E) [SGN] N+

checks are performed that the numbers are in a valid range.

(f)

bounded reals

BREAL = FLOAT UL UL FLOAT

where the first float must be less or equal to the second.

3.

strings

STRING = SQ (NES | ES ANY+ | SQ BS* SQ)* SQ

By default, consecutive strings are concatenated into a single string. This behaviour can be disabled by the syntax option doubled_quote_is_quote, which causes doubled quotes to be interpreted as a single occurrence of the quote within the string.

4.

lists of ASCII codes

LIST = LQ (NES | ES ANY+ | LQ BS* LQ)* LQ

By default, consecutive character lists are concatenated into a single character list. This behaviour can be disabled by the syntax option doubled_quote_is_quote, which causes doubled quotes to be interpreted as a single occurrence of the quote within the string.

Variables

VAR = (UC | UL) ALP*

End of clause

EOCL = . (BS | NL | <end of file>) | <end of file>

Escape Sequences within Strings and Atoms

Within atoms and strings, the escape sequences (ES ANY+) are interpreted: if the sequence matches one of the following valid escape sequences, the corresponding special character is inserted into the quoted item.

Escape Sequence	Result
ES a	ASCII alert (7)
ES b	ASCII backspace (8)
ES f	ASCII form feed (12)
ES n	ASCII newline (10)
ES r	ASCII carriage return (13)
ES t	ASCII tabulation (9)
ES v	ASCII vertical tab (11)
ES e	ASCII escape (27)
ES d	ASCII delete (127)
ES ES	the ES character itself
ES AQ	the AQ character itself
ES SQ	the SQ character itself
ES LQ	the LQ character itself
ES NL	ignored
ES c (BS|NL)*	ignored
ES three octal digits	character whose character code is the given octal value
ES x hex digits ES	character whose character code is the given hexadecimal value

Any other character following the ES constitutes a syntax error. If the syntax option iso_escapes is active, the octal escape sequence can be of any length and must be terminated with an ES character.