YAP 7.1.0
YAP Syntax

We will describe the syntax of YAP at two levels We first will describe the syntax for Prolog terms In a second level we describe the tokens from which Prolog terms are built

[Syntax of Terms ](Formal_Syntax)

Below, we describe the syntax of YAP terms from the different classes of tokens defined above The formalism used will be BNF, extended where necessary with attributes denoting integer precedence or operator type

term ----> subterm(1200) end_of_term_marker
subterm(N) ----> term(M) [M <= N]
term(N) ----> op(N, fx) subterm(N-1)
| op(N, fy) subterm(N)
| subterm(N-1) op(N, xfx) subterm(N-1)
| subterm(N-1) op(N, xfy) subterm(N)
| subterm(N) op(N, yfx) subterm(N-1)
| subterm(N-1) op(N, xf)
| subterm(N) op(N, yf)
term(0) ----> atom '(' arguments ')'
| '(' subterm(1200) ')'
| '{' subterm(1200) '}'
| list
| string
| number
| atom
| variable
arguments ----> subterm(999)
| subterm(999) ',' arguments
list ----> '[]'
| '[' list_expr ']'
list_expr ----> subterm(999)
| subterm(999) list_tail
list_tail ----> ',' list_expr
| ',..' subterm(999)
| '|' subterm(999)


  • op(N,T) denotes an atom which has been previously declared with type T and base precedence N
  • Since , is itself a pre-declared operator with type xfy and precedence 1000, if a subterm starts with a '(', op must be followed by a space to avoid ambiguity with the case of a functor followed by arguments, e.g:
+ (a,b)

is the same as +(,(a,b)) of arity one; in contrast


is the same as +(a,b) of arity two

  • In the first rule for term(0) no blank space should exist between atom and (
  • Each term to be read by the YAP parser must end with a single dot, followed by a blank (in the sense mentioned in the previous paragraph) When a name consisting of a single dot could be taken for the end of term marker, the ambiguity should be avoided by surrounding the dot with single quotes

[Prolog Tokens ](Tokens)

" string_quoted_characters " string --> ` string_quoted_characters `

string_quoted_characters --> '"' '"' string_quoted_characters string_quoted_characters --> '\' escape_sequence string_quoted_characters string_quoted_characters --> string_character string_quoted_characters

escape_sequence --> 'a' | 'b' | 'r' | 'f' | 't' | 'n' | 'v' escape_sequence --> '\' | '"' | ''' | '`' escape_sequence --> at_most_3_octal_digit_seq_char '\' escape_sequence --> 'x' at_most_2_hexa_digit_seq_char '\'

where string_character is any character except the double quote (back quote) and escape characters

YAP supports four different textual elements:

  • Atoms, mentioned above, are textual representations of symbols, that are interned in the data-base They are stored either in ISO-LATIN-1 (first 256 code points), or as UTF-32
  • Strings are atomic representations of text The back-quote character is used to identify these objects in the program Strings exist as stack objects, in the same way as other Prolog terms As Prolog unification cannot be used to manipulate strings, YAP includes built-ins such as string_arg/3 , sub_string/5 , or string_concat to manipulate them efficiently Strings are stored as opaque objects containing a
  • Lists of codes represent text as a list of numbers, where each number is a character code A string of N bytes requires N pairs, that is 2N cells, leading to a total of 16 bytes per character on 64 byte machines Thus, they are a very expensive, but very flexible representation, as one can use unification to construct and access string elements
  • Lists of atoms represent text as a list of atoms, where each number has a single character code A string of N bytes also requires 2N pairs They have similar properties to lists of codes

The flags double_quotes and backquoted_string change the interpretation of text strings, they can take the values atom, string, codes, and chars


"" "a string" "a double-quote:"""

The first string is an empty string, the last string shows the use of double-quoting

Escape sequences can be used to include the non-printable characters a (alert), b (backspace), r (carriage return), f (form feed), t (horizontal tabulation), n (new line), and v (vertical tabulation) Escape sequences also be include the meta-characters \\, "</tt>, &lsquo;&rsquo;<tt>, and </tt>''` Last, one can use escape sequences to include the characters either as an octal or hexadecimal number The next examples demonstrates the use of escape sequences in YAP: @code "\x0c" "\01\" "\f" "\\"

The first three examples return a list including only character 12 (form feed) The last example escapes the escape character

Escape sequences were not available in C-Prolog and in original versions of YAP up to 4.2.0 Escape sequences can be disabled by using:

:- yap_flag(character_escapes,false).

[Atoms ](Atoms)

Atoms are defined by one of the following rules:

atom --> solo-character
atom --> lower-case-letter name-character*
atom --> symbol-character+
atom --> single-quote single-quote
atom --> ''' atom_quoted_characters '''
atom_quoted_characters --> ''' ''' atom_quoted_characters
atom_quoted_characters --> '\' atom_sequence string_quoted_characters
atom_quoted_characters --> character string_quoted_characters


<solo-character> denotes one of: ! ;
<symbol-character> denotes one of: # & * + - . / : <
= > ? @ \ ^ ~ `
<lower-case-letter> denotes one of: a...z
<name-character> denotes one of: _ a...z A...Z 0....9
<single-quote> denotes: '

and string_character denotes any character except the double quote and escape characters Note that escape sequences in strings and atoms follow the same rules


a a12x '$a' ! => '1 2'

Version 4.2.0 of YAP removed the previous limit of 256 characters on an atom Size of an atom is now only limited by the space available in the system

[Variables ](Variables)

Variables are described by:



<variable-starter> denotes one of: _ A...Z
<variable-character> denotes one of: _ a...z A...Z

If a variable is referred only once in a term, it needs not to be named and one can use the character _ to represent the variable These variables are known as anonymous variables Please note that different occurrences of _ on the same term represent different anonymous variables

YAP accepts both the character _, or _ followed by the usual alphanumeric characters In the former case, each _ corresponds to a different anonymous variable In the latter case, a name can be used to represent a singleton variable This allows preserving the name of arguments across clauses, as in the following example:

member(El, [El|_Tail]).
member(El, [_El|Tail]) :-
member(El, Tail).

In the example El and Tail refer to the head and tail of the clause clause Using the underscore, we can preserve the name while declaring the variables are singletons

[Punctuation Tokens ](Punctuation_Tokens)

Punctuation tokens consist of one of the following characters:

( ) , [ ] { } |

These characters are used to group terms

[Character Layout ](LayoutComents)

Any characters with ASCII code less than or equal to 32 appearing before a token are ignored

All the text appearing in a line after the character % is taken to be a comment and ignored (including %) Comments can also be inserted by using the sequence /\* to start the comment and \* followed by / to finish it In the presence of any sequence of comments or layout characters, the YAP parser behaves as if it had found a single blank character The end of a file also counts as a blank character for this purpose

[Encoding Wide Character Support ](WideChars)

YAP now implements a SWI-Prolog compatible interface to wide characters and the Universal Character Set (UCS) The following text was adapted from the SWI-Prolog manual

YAP now supports wide characters, characters with character codes above 255 that cannot be represented in a single byte Universal Character Set (UCS) is the ISO/IEC 10646 standard that specifies a unique 31-bits unsigned integer for any character in any language It is a superset of 16-bit Unicode, which in turn is a superset of ISO 8859-1 (ISO Latin-1), a superset of US-ASCII UCS can handle strings holding characters from multiple languages and character classification (uppercase, lowercase, digit, etc.) and operations such as case-conversion are unambiguously defined

YAP uses UTF-8 internally In this encoding, the representation of code points can have variable length ASCII codes still take a single byte, but western european latin text will have accented characeters that require two bytes in UTF-8 This representational issue is completely transparent to the Prolog user Users of the foreign language interface sometimes need to be aware of these issues though Notice that this will likely change in the future, we probably will use an UTF-8 based representation

YAP assumes that strings are null terminated UTF-8 has no support for encoding null characters within a sequence of codes, but YAP does prove some limited support for an UTF extension that allows such encodings

Character coding comes into view when characters of strings need to be read from or written to file or when they have to be communicated to other software components using the foreign language interface In this section we only deal with I/O through streams, which includes file I/O as well as I/O through network sockets

[Wide character encodings on streams ](Stream_Encoding)

The UCS standard describes all possible characters (or code points, as they include ideograms, ligatures, and other symbols) The current version, Unicode 8.0, allows code points up to 0x10FFFF, and thus allows for 1,114,112 code points See Unicode Charts for the supported languages

Notice that most symbols are rarely used Encodings represent the Unicode characters in a way that is more suited for communication The most popular encoding, especially in the context of the web and in the Unix/Linux/BSD/Mac communities, is UTF-8 UTF-8 is compact and as it uses bytes, does not have different endianesses Bytes 0...127 represent simply the corresponding US-ASCII character, while bytes 128...255 are used for multi-byte encoding of characters placed higher in the UCS space

Especially on MS-Windows and Java the 16-bit Unicode standard, represented by pairs of bytes is also popular Originally, Microsoft supported a UCS-2 with 16 bits that could represent only up to 64k characters This was later extended to support the full Unicode, we will call the latter version UTF-16 The extension uses a hole in the first 64K code points Characters above 0xFFFF are divided into two 2-byte words, each one in that hole There are two versions of UTF-16: big and low endian By default, UTF-16 is big endian, in practice most often it is used on Intel hardware that is naturally little endian

UTF-32, often called UCS-4, provides a natural interface where a code point is coded as four octets Unfortunately, it is also more expensive, so it is not as widely used

Last, other encodings are also commonly used One such legacy encoding is ISO-LATIN-1, that supported latin based languages in western europe YAP currently uses either ISO-LATIN-1 or UTF-32 internally

Prolog supports the default encoding used by the Operating System, Namely, YAP checks the variables LANG, LC_ALL and LC_TYPE Say, if at boot YAP detects that the environment variable LANG ends in "UTF-8", this encoding is assumed Otherwise, the default is text and the translation is left to the wide-character functions of the C-library (note that the Prolog native UTF-8 mode is considerably faster than the generic mbrtowc() one)

Prolog allows the encoding to be specified explicitly in load_files/2 for loading Prolog source with an alternative encoding, open/4 when opening files or using set_stream/2 on any open stream (not yet implemented) For Prolog source files we also provide the encoding/1 directive that can be used to switch between encodings that are compatible to US-ASCII (ascii, iso_latin_1, utf8 and many locales)

For additional information and Unicode resources, please visit the unicode organization web page

YAP currently defines and supports the following encodings:

  • octet Default encoding for binary streams This causes the stream to be read and written fully untranslated
  • ascii or US_ASCII 7-bit encoding in 8-bit bytes Equivalent to iso_latin_1, but generates errors and warnings on encountering values above 127
  • iso_latin_1 or ISO-8859-1 8-bit encoding supporting many western languages This causes the stream to be read and written fully untranslated
  • text C-library default locale encoding for text files Files are read and written using the C-library functions mbrtowc() and wcrtomb() This may be the same as one of the other locales, notably it may be the same as iso_latin_1 for western languages and utf8 in a UTF-8 context
  • utf8, iso_utf8, or UTF-8 Multi-byte encoding of the full Unicode 8, compatible toascii` See above
  • unicode_be or UCS-2BE Unicode Big Endian Reads input in pairs of bytes, most significant byte first Can only represent 16-bit characters
  • unicode_le or UCS-2LE Unicode Little Endian Reads input in pairs of bytes, least significant byte first Can only represent 16-bit characters
  • utf16_le or UTF-16LE (experimental) UTF-16 Little Endian Reads input in pairs of bytes, least significant byte first Can represent the full Unicode
  • utf16_le or UTF-16BE (experimental) Unicode Big Endian Reads input in pairs of bytes, least significant byte first Can represent the full Unicode
  • utf32_le or UTF-32LE (experimental) UTF-16 Little Endian Reads input in pairs of bytes, least significant byte first Can represent the full Unicode
  • utf32_le or UTF-32BE (experimental) Unicode Big Endian Reads input in pairs of bytes, least significant byte first Can only represent 16-bit characters

Note that not all encodings can represent all characters This implies that writing text to a stream may cause errors because the stream cannot represent these characters The behaviour of a stream on these errors can be controlled using open/4 or set_stream/2 (not implemented) Initially the terminal stream write the characters using Prolog escape sequences while other streams generate an I/O exception

[BOM: Byte Order Mark ](BOM)

From Stream Encoding, you may have got the impression that text-files are complicated This section deals with a related topic, making live often easier for the user, but providing another worry to the programmer BOM or Byte Order Marker is a technique for identifying Unicode text-files as well as the encoding they use Please read the [W3C](https://www.w3.org/International/questions/qa-byte-order-mark.en.php] page for a detailed explanation of byte-order marks

BOMa are necessary on multi-byte encodings, such as UTF-16 and UTF-32 There is a BOM for UTF-8, but it is rarely used The BOM is handled by the open/4 predicate By default, text-files are probed for the BOM when opened for reading If a BOM is found, the encoding is set accordingly and the property bom(true) is available through stream_property/2 When opening a file for writing, writing a BOM can be requested using the option bom(true) with open/4 YAP will parse an UTF-8 file for a BOM only if explicitly required to do so Do notice that YAP will write a BOM by default on UTF-16 (including UCS-2) and UTF-32; otherwise the default is not to write a BOM BOMs are not avaliable for ASCII and ISO-LATIN-1

[Summary of YAP Predefined Operators ](Operators)

The Prolog syntax caters for operators of three main kinds:

  • prefix;
  • infix;
  • postfix

Each operator has precedence in the range 1 to 1200, and this precedence is used to disambiguate expressions where the structure of the term denoted is not made explicit using brackets The operator of higher precedence is the main functor

If there are two operators with the highest precedence, the ambiguity is solved analyzing the types of the operators The possible infix types are: xfx, xfy, and yfx

With an operator of type xfx both sub-expressions must have lower precedence than the operator itself, unless they are bracketed (which assigns to them zero precedence) With an operator type xfy only the
left-hand sub-expression must have lower precedence The opposite happens for yfx type

A prefix operator can be of type fx or fy A postfix operator can be of type xf or yf The meaning of the notation is analogous to the above

a + b * c


a + (b * c)

as + and * have the following types and precedences:


Now defining

a ++ b =:= c


a ++ (b =:= c)

The following is the list of the declarations of the predefined operators:

:-op(1200,fx,['?-', ':-']).
:-op(900,fy,['\+', not]).
:-op(900,fx,[nospy, spy]).