2.15 The SWI-Prolog syntax

SWI-Prolog syntax is close to ISO-Prolog standard syntax, which is based on the Edinburgh Prolog syntax. A formal description can be found in the ISO standard document. For an informal introduction we refer to Prolog text books (see section 1) and online tutorials. In addition to the differences from the ISO standard documented here, SWI-Prolog offers several extensions, some of which also extend the syntax. See section 5 for more information.

2.15.1 ISO Syntax Support

This section lists various extensions w.r.t. the ISO Prolog syntax.

2.15.1.1 Processor Character Set

The processor character set specifies the class of each character used for parsing Prolog source text. Character classification is fixed to Unicode. See also section 2.18.

2.15.1.2 Nested comments

SWI-Prolog allows for nesting /* ... */ comments. Where the ISO standard accepts /* ... /* ... */ as a comment, SWI-Prolog will search for a terminating */. This is useful if some code with /* ... */ comment statements in it should be commented out. This modification also avoids unintended commenting in the example below, where the closing */ of the first comment has been forgotten.^{28Recent copies of GCC give a style warning if /* is encountered in a comment, which suggests that this problem has been recognised more widely.}

/* comment

code

/* second comment */

code

2.15.1.3 Character Escape Syntax

Within quoted atoms (using single quotes: ’<atom>’) special characters are represented using escape sequences. An escape sequence is led in by the backslash (\) character. The list of escape sequences is compatible with the ISO standard but contains some extensions, and the interpretation of numerically specified characters is slightly more flexible to improve compatibility. Undefined escape characters raise a syntax_error exception.^{29Up to SWI-Prolog 6.1.9, undefined escape characters were copied verbatim, i.e., removing the backslash.}

\a

Alert character. Normally the ASCII character 7 (beep).

\b

Backspace character.

\c

No output. All input characters up to but not including the first non-layout character are skipped. This allows for the specification of pretty-looking long lines. Not supported by ISO. Example:

format('This is a long line that looks better if it was \c
       split across multiple physical lines in the input')

\<NEWLINE>

When in ISO mode (see the Prolog flag iso), only skip this sequence. In native mode, white space that follows the newline is skipped as well and a warning is printed, indicating that this construct is deprecated and advising to use \c. We advise using \c or putting the layout before the \, as shown below. Using \c is supported by various other Prolog implementations and will remain supported by SWI-Prolog. The style shown below is the most compatible solution.^{30Future versions will interpret \<return> according to ISO.}

format('This is a long line that looks better if it was \
split across multiple physical lines in the input')

instead of

format('This is a long line that looks better if it was\
 split across multiple physical lines in the input')

Note that SWI-Prolog also allows unescaped newlines to appear in quoted material. This is not allowed by the ISO standard, but used to be common practice before.

\e

Escape character (ASCII 27). Not ISO, but widely supported.

\f

Form-feed character.

\n

Next-line character.

\r

Carriage-return only (i.e., go back to the start of the line).

\s

Space character. Intended to allow writing 0'\s to get the character code of the space character. Not ISO.

\t

Horizontal tab character.

\v

Vertical tab character (ASCII 11).

\xXX..\

Hexadecimal specification of a character. The closing \ is obligatory according to the ISO standard, but optional in SWI-Prolog to enhance compatibility with the older Edinburgh standard. The code \xa\3 emits the character 10 (hexadecimal‘a’) followed by‘3’. Characters specified this way are interpreted as Unicode characters. See also \u.

\uXXXX

Unicode character specification where the character is specified using exactly 4 hexadecimal digits. This is an extension to the ISO standard, fixing two problems. First, where \x defines a numeric character code, it doesn't specify the character set in which the character should be interpreted. Second, it is not needed to use the idiosyncratic closing \ ISO Prolog syntax.

\UXXXXXXXX

Same as \uXXXX, but using 8 digits to cover the whole Unicode set.

\40

Octal character specification. The rules and remarks for hexadecimal specifications apply to octal specifications as well.

\\

Escapes the backslash itself. Thus, '\\' is an atom consisting of a single \.

\’

Single quote. Note that '\'' and '''' both describe the atom with a single ’, i.e., '\'' == '''' is true.

\"

Double quote.

\‘

Back quote.

Character escaping is only available if current_prolog_flag(character_escapes, true) is active (default). See current_prolog_flag/2. Character escapes conflict with writef/2 in two ways: \40 is interpreted as decimal 40 by writef/2, but as octal 40 (decimal 32) by read. Also, the writef/2 sequence \l is illegal. It is advised to use the more widely supported format/[2,3] predicate instead. If you insist upon using writef/2, either switch character_escapes to false, or use double \\, as in writef('\\l').

2.15.1.4 Syntax for non-decimal numbers

SWI-Prolog implements both Edinburgh and ISO representations for non-decimal numbers. According to Edinburgh syntax, such numbers are written as <radix>’<number>, where <radix> is a number between 2 and 36. ISO defines binary, octal and hexadecimal numbers using 0[bxo]<number>. For example: A is 0b100 \/ 0xf00 is a valid expression. Such numbers are always unsigned.

2.15.1.5 Using digit groups in large integers

SWI-Prolog supports splitting long integers into digit groups. Digit groups can be separated with the sequence <underscore>, <optional white space>. If the <radix> is 10 or lower, they may also be separated with exactly one space. The following all express the integer 1 million:

1_000_000
1 000 000
1_000_/*more*/000

Integers can be printed using this notation with format/2, using the ~I format specifier. For example:

?- format('~I', [1000000]).
1_000_000

The current syntax has been proposed by Ulrich Neumerkel on the SWI-Prolog mailinglist.

2.15.1.6 Rational number syntax

As of version 8.1.22, SWI-Prolog supports rational numbers as a primary citizen atomic data type if SWI-Prolog is compiled with the GMP library. This can be tested using the bounded Prolog flag. An atomic type also requires a syntax. Unfortunately there are few options for adding rational numbers without breaking the ISO standard.^{31ECLiPSe uses numerator_denominator. This syntax conflicts with SWI-Prolog digit groups (see section 2.15.1.5) and does not have a recognised link to rational numbers. The notation 1/3r and 1/3R have also been proposed. The 1/3r is compatible to Ruby, but is hard to parse due to the required look-ahead and not very natural. See also https://en.wikipedia.org/wiki/Rational_data_type.}

ECLiPSe and SWI-Prolog have agreed to define the canonical syntax for rational numbers to be e.g., 1r3. In addition, ECLiPSe accepts 1_3 and SWI-Prolog can be asked to accept 1/3 using the module sensitive Prolog flag rational_syntax, which has the values below. Note that write_canonical/1 always uses the compatible 1r3 syntax.

natural

This is the default mode where we ignore the ambiguity issue and follow the most natural <integer>/<nonneg> alternative. Here, <integer> follows the normal rules for Prolog decimal integers and <nonneg> does the same, but does not allows for a sign. Note that the parser translates a rational number to its canonical form which implies there are no common divisors in the resulting numerator and denominator. Examples of ration numbers are:

1/2	1/2
2/4	1/2
1 000 000/33 000	1000/33
-3/5	-3/5

We expect very few programs to have text parsed into a rational number while a term was expected. Note that for rationals appearing in an arithmetic expression the only difference is that evaluation moves from runtime to compiletime. The utility list_rationals/0 may be used on a loaded program to check whether the program contains rational numbers inside clauses and thus may be subject to compatibility issues. If a term is intended this can be written as /(1,2), (1)/2, 1 / 2 or some variation thereof.

compatibility

Read and write rational numbers as e.g., 1r3. In other words, this adheres to the same rules as natural above, but using the‘r’instead of‘/’. Note that this may conflict with traditional Prolog as‘r’can be defined as an infix operator. The same argument holds for 0x23 and similar syntax for numbers that are part of the ISO standard.

While the syntax is controlled by the flag rational_syntax, behavior on integer division and exponentiation is controlled by the flag prefer_rationals. See section section 4.27.2.2 for arithmetic on rational numbers.

2.15.1.7 NaN and Infinity floats and their syntax

SWI-Prolog supports reading and printing‘special’floating point values according to Proposal for Prolog Standard core update wrt floating point arithmetic by Joachim Schimpf and available in ECLiPSe Prolog. In particular,

• Infinity is printed as 1.0Inf or -1.0Inf. Any sequence matching the regular expression [+-]?\sd+[.]\sd+Inf is mapped to plus or minus infinity.

• NaN (Not a Number) is printed as 1.xxxNaN, where 1.xxx is the float after replacing the exponent by‘1’. Such numbers are read, resulting in the same NaN. The NaN constant can also be produced using the function nan/0, e.g.,

  ?- A is nan.
  A = 1.5NaN.

By default SWI-Prolog arithmetic (see section 4.27) follows the ISO standard with describes that floating point operations either produce a normal floating point number or raise an exception. section 4.27.2.4 describes the Prolog flags that can be used to support the IEEE special float values. The ability to create, read and write such values facilitates the exchange of data with languages that can represent the full range of IEEE doubles.

2.15.1.8 Force only underscore to introduce a variable

According to the ISO standard and most Prolog systems, identifiers that start with an uppercase letter or an underscore are variables. In the past, Prolog by BIM provided an alternative syntax, where only the underscore (_) introduces a variable. As of SWI-Prolog 7.3.27 SWI-Prolog supports this alternative syntax, controlled by the Prolog flag var_prefix. As the character_escapes flag, this flag is maintained per module, where the default is false, supporting standard syntax.

Having only the underscore introduce a variable is particularly useful if code contains identifiers for case sensitive external languages. Examples are the RDF library where code frequently specifies property and class names^{32Samer Abdallah suggested this feature based on experience with non-Prolog users using the RDF library.} and the R interface for specifying functions or variables that start with an uppercase character. Lexical databases where part of the terms start with an uppercase letter is another category were the readability of the code improves using this option.

2.15.1.9 Unicode Prolog source

The ISO standard specifies the Prolog syntax in ASCII characters. As SWI-Prolog supports Unicode in source files we must extend the syntax. This section describes the implication for the source files, while writing international source files is described in section 3.1.3.

The SWI-Prolog Unicode character classification follows the Unicode release reported by the read-only Prolog flag unicode_syntax_version. Note that char_type/2 and friends, intended for processing arbitrary text rather than Prolog source code, are based on the C library locale-based classification routines, and that the predicates in library(unicode) report the version of the bundled utf8proc data, which may differ from the syntax classifier's version (see unicode_version/1).

• Quoted atoms and strings
  Any character of any script can be used in quoted atoms and strings. The escape sequences \uXXXX and \UXXXXXXXX (see section 2.15.1.3) were introduced to specify Unicode code points in ASCII files.

• Atoms and Variables
  We handle them in one item as they are closely related. The Unicode standard defines a syntax for identifiers in computer languages.^{33http://www.unicode.org/reports/tr31/} SWI-Prolog uses the XID_Start and XID_Continue sets: identifiers start with an XID_Start code point followed by a sequence of XID_Continue code points. As a profile addition, the superscript digits (², ³, ¹, and ⁰--⁹, i.e. U+00B2, U+00B3, U+00B9, U+2070, U+2074..U+2079) and the subscript digits (₀--₉, U+2080..U+2089) are also accepted as XID_Continue, allowing variables such as X² and X₁. Such sequences are handled as a single token. The token is a variable iff it starts with an underscore (_) or with a code point in general category Lu (uppercase letter). Otherwise it is an atom. Note that titlecase letters (general category Lt, e.g. Dž) start an atom, not a variable; this differs from earlier releases that used the broader derived Uppercase property. Many languages do not have the notion of character case; in such languages variables must be written as _name.

• Numbers
  

  In source code (read_term/2), numeric literals use ASCII digits 0--9 only. Conversion via atom_number/2, number_codes/2, and number_chars/2 additionally accepts any Unicode Nd block for integers, rational numbers (see section 2.15.1.6) and floating point numbers; in a single number all digits must come from the same block, i.e., if the numerator of a rational uses Indian script the denominator must too. The sign, rational separator, floating point ., and floating point exponent are always ASCII.

• White space
  The layout characters are exactly the Unicode Pattern_White_Space set defined by UAX #31: U+0009..U+000D, U+0020, U+0085, U+200E, U+200F, U+2028, and U+2029. NBSP (U+00A0) is deliberately excluded from Pattern_White_Space; appearing outside quoted material it raises a stray-character syntax error. Programs that paste from word processors will occasionally encounter NBSP in the wrong place, and reporting it explicitly is preferable to silently treating it as a separator.

• Line termination
  Seven of the Pattern_White_Space code points end a line: U+000A (LF), U+000B (VT), U+000C (FF), U+000D (CR), U+0085 (NEL), U+2028 (LINE SEPARATOR), and U+2029 (PARAGRAPH SEPARATOR). They terminate % line comments, drive the source-position line counter, and act as the newline for backslash-newline continuation inside quoted strings (\<EOL> followed by zero or more blanks is consumed). The same set is exposed to user code through prolog_end_of_line in code_type/2 and char_type/2; the unprefixed end_of_line stays restricted to the four ISO/POSIX control codes (LF, VT, FF, CR), and the eleven-member Pattern_White_Space set itself is prolog_layout.

• Stray characters in source text
  At token-start position (where layout is allowed), a code point that is not in any of the recognised syntax classes --- layout (see above), decimal digit, identifier-start, identifier-continue, solo, bracket open or quote open --- raises syntax_error(illegal_character). This includes the C0 and C1 control range, unassigned and noncharacter code points, surrogates, the Zs / Zl / Zp separator classes that are not in Pattern_White_Space (NBSP, OGHAM SPACE MARK, NARROW NO-BREAK SPACE, IDEOGRAPHIC SPACE, ...), Cf format characters that are not in Pattern_White_Space and not in Other_ID_Continue (SOFT HYPHEN, ZERO WIDTH SPACE, ...), enclosing combining marks (Me), and other-number characters (No, e.g. the vulgar fractions and Roman-numeral form U+00BC..U+00BE) that are not in the explicit super- or subscript-digit profile.

  Non-spacing combining marks (Mn, Mc) are likewise rejected at token-start position --- they do not start an identifier --- but are in XID_Continue so they absorb into a preceding identifier (the sequence U+0061 followed by U+0300 COMBINING GRAVE reads as a single-token identifier two code points long).

• Inside quoted material
  Inside single-quoted atoms ('...'), double-quoted strings ("..."), back-quoted text (`...`), Unicode quote pairs (see above), % comments, and /* ... */ comments, any Unicode scalar value (U+0000 to U+10FFFF, excluding surrogates which UTF-8 cannot encode) is accepted verbatim. The escape sequences \uXXXX and \UXXXXXXXX (section 2.15.1.3) are available for portability and explicit clarity, not as a gate. The single exception is the bidirectional override / isolate range (U+202A..U+202E and U+2066..U+2069), which is rejected as a Trojan-source defense (see unicode_atoms). Quoted writing (e.g. writeq/1) of an atom or string that contains a control or zero-width code point causes the atom or string to be quoted and the offending code points to be written using an escape sequence; see section 2.15.1.3.

• Other characters
  The first 128 characters follow the ISO Prolog standard. In particular, the ASCII symbol characters glue into compound atoms, giving the familiar operator tokens ==, =.., :- and so on. Beyond ASCII, all Unicode symbol characters (general categories Sm, Sc, Sk, So) and the connector, dash, and other-punctuation classes (Pc, Pd, Po) are treated as solo: each forms an atom on its own and does not glue with adjacent symbols. This is a deliberate change from earlier releases, in which Unicode symbols glued into compound atoms in the same way as ASCII symbols; the change ensures that characters such as ≤, €, and · keep their per-character meaning. Operators built from Unicode symbols must be declared explicitly with op/3. Numeric characters of other type (general category No, e.g. fractions and circled digits) are not part of the identifier set; only the explicitly listed super- and subscript digits extend identifiers.

• Brackets (paired delimiters)
  The opening / closing punctuation classes Ps and Pe form bracket pairs: an opening character followed by a Prolog term and the matching closing character reads as a unary compound whose functor is the two delimiter characters joined. This is the same shape as {Term} becoming '{}'(Term), generalised to the full Unicode Ps/Pe set (64 pairs, sourced from Unicode BidiMirroring.txt filtered by general category). Operators inside brackets are honoured; nesting works as expected. Mismatched or stray closes raise syntax_error. The analogy with {} is complete: an empty pair, optionally containing only layout, reads as the two-character atom rather than a compound; that atom followed by ( is a functor; and on output '<open><close>'(X) is written as <open>X<close> and the bare atom unquoted, both subject to the brace_terms(true) write option.

• Quotes (paired literal-text delimiters)
  The initial / final quotation classes Pi and Pf form quote pairs: an opening character followed by literal text and the matching closing character reads as a unary compound whose functor is the two delimiter characters joined and whose argument is the contained text in the form selected by double_quotes (string by default; also atom, codes, or chars). The contained text is not parsed as a Prolog term; escape sequences (\n, \uXXXX, ...) are processed as in ASCII quoted strings. For example, with double_quotes set to string, the source text «hello, world» reads as a compound whose functor is the two-character atom «» and whose single argument is the string "hello, world". The quote pairs come from Pi/Pf entries of BidiMirroring.txt (8 pairs) plus the standard left/right curly quote pairs U+2018/U+2019 and U+201C/U+201D, which are absent from BidiMirroring.txt. Mismatched closes and unmatched opens raise syntax_error.

The features above let source text contain Unicode without escapes:

p(X⁰, X) :-                   % Superscript variable profile for
    q(X⁰, X¹),                % threaded variables.
    r(X¹, X).

?- atom_number('१२३', N).     % Devanagari Nd via atom_number/2
N = 123.

?- atom_codes(≤, Cs).         % Unicode symbol stays solo
Cs = [8804].

?- term_string(T, "⟨a, b⟩"),  % bracket pair (Ps/Pe)
   display(T).
⟨⟩(','(a,b))
T = ⟨a, b⟩.

?- term_string(T, "⟨ ⟩").     % empty pair is the atom
T = ⟨⟩.

?- term_string(T, "«hello").  % quote pair (Pi/Pf)
T = '«»'("hello").

2.15.1.10 Singleton variable checking

A singleton variable is a variable that appears only one time in a clause. It can always be replaced by _, the anonymous variable. In some cases, however, people prefer to give the variable a name. As mistyping a variable is a common mistake, Prolog systems generally give a warning (controlled by style_check/1) if a variable is used only once. The system can be informed that a variable is meant to appear once by starting it with an underscore, e.g., _Name. Please note that any variable, except plain _, shares with variables of the same name. The term t(_X, _X) is equivalent to t(X, X), which is different from t(_, _).

As Unicode requires variables to start with an underscore in many languages, this schema has been extended.^{34After a proposal by Richard O'Keefe.} First we define the two classes of named variables.

• Named singleton variables
  Named singletons start with a double underscore (__) or a single underscore followed by an uppercase letter, e.g., __var or _Var.

• Normal variables
  All other variables are‘normal’variables. Note this makes _var a normal variable.^{35Some Prolog dialects write variables this way.}

Any normal variable appearing exactly once in the clause and any named singleton variables appearing more than once are reported. Below are some examples with warnings in the right column. Singleton messages can be suppressed using the style_check/1 directive.

Finally, variables named _<digit> are never subject to style checking. These variable names are emitted by write/1 and friends. This exception can also be used to pass singletons to debug/3. Passing singletons to debug/3 is otherwise problematic as using a normal variable results in a singleton warning when optimization removes the debug/3 statement while using a named anonymous variable results in a multiton warning. For example:

p(X) :-
    q(X,_0Y),
    debug(demo, 'q/2 says ~p', [_0Y]).

test(_).
test(_a).	Singleton variables: [_a]
test(A).	Singleton variables: [A]
test(_12).
test(_A).
test(__a).
test(_, _).
test(_a, _a).
test(__a, __a).	Singleton-marked variables appearing more than once: [__a]
test(_A, _A).	Singleton-marked variables appearing more than once: [_A]
test(A, A).

Semantic singletons

Starting with version 6.5.1, SWI-Prolog has syntactic singletons and semantic singletons. The first are checked by read_clause/3 (and read_term/3 using the option singletons(warning)). The latter are generated by the compiler for variables that appear alone in a branch. For example, in the code below the variable X is not a syntactic singleton, but the variable X does not communicate any bindings and replacing X with _ does not change the semantics.

test :-
        (   test_1(X)
        ;   test_2(X)
        ).