yap-6.3/docs/md/syntax.md

@defgroup YAPSyntax YAP Syntax

@{ @ingroup YAPProgrammming

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.

@defgroup Formal_Syntax Syntax of Terms @{ @ingroup YAPSyntax

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)

Notes:

• \a op(N,T) denotes an atom which has been previously declared with type \a T and base precedence \a N.

• Since ',' is itself a pre-declared operator with type \a xfy and precedence 1000, is \a subterm starts with a '(', \a op must be followed by a space to avoid ambiguity with the case of a functor followed by arguments, e.g.:

+ (a,b)        [the same as '+'(','(a,b)) of arity one]

  versus

+(a,b)         [the same as '+'(a,b) of arity two]

In the first rule for term(0) no blank space should exist between \a 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.

@} @defgroup Tokens Prolog Tokens

@{ @ingroup YAPSyntax

Prolog tokens are grouped into the following categories:

@defgroup Numbers Numbers @{ @ingroup Tokens

Numbers can be further subdivided into integer and floating-point numbers.

@defgroup Integers Integers @{ @ingroup Numbers

Integer numbers are described by the following regular expression:

<integer> := {<digit>+<single-quote>|0{xXo}}<alpha_numeric_char>+

where {...} stands for optionality, \a + optional repetition (one or more times), \a \<digit\> denotes one of the characters 0 ... 9, \a | denotes or, and \a \<single-quote\> denotes the character "'". The digits before the \a \<single-quote\> character, when present, form the number basis, that can go from 0, 1 and up to 36. Letters from A to Z are used when the basis is larger than 10.

Note that if no basis is specified then base 10 is assumed. Note also that the last digit of an integer token can not be immediately followed by one of the characters 'e', 'E', or '.'.

Following the ISO standard, YAP also accepts directives of the form 0x to represent numbers in hexadecimal base and of the form 0o to represent numbers in octal base. For usefulness, YAP also accepts directives of the form 0X to represent numbers in hexadecimal base.

Example: the following tokens all denote the same integer

10  2'1010  3'101  8'12  16'a  36'a  0xa  0o12

Numbers of the form 0'a are used to represent character constants. So, the following tokens denote the same integer:

0'd  100

YAP (version 6.3.4) supports integers that can fit the word size of the machine. This is 32 bits in most current machines, but 64 in some others, such as the Alpha running Linux or Digital Unix. The scanner will read larger or smaller integers erroneously. @}

@defgroup Floats Floats @} @ingroup Numbers

Floating-point numbers are described by:

   <float> := <digit>+{<dot><digit>+}
               <exponent-marker>{<sign>}<digit>+
            |<digit>+<dot><digit>+
               {<exponent-marker>{<sign>}<digit>+}

where \a \<dot\> denotes the decimal-point character '.', \a \<exponent-marker\> denotes one of 'e' or 'E', and \a \<sign\> denotes one of '+' or '-'.

Examples:

10.0   10e3   10e-3   3.1415e+3

Floating-point numbers are represented as a double in the target machine. This is usually a 64-bit number.

@} @} @defgroup Strings Character Strings @{

Strings are described by the following rules:

  string --> " 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.

Examples:

""   "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 \\, ", ', and ''. 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:

"\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).

@}

@addtogroup Atoms Atoms

@}

@ingroup Tokens

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

where:

   <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.

Examples:

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.

@}

@addtogroup Variables Variables @{ @ingroup Tokens

Variables are described by:

   <variable-starter><variable-character>+

where

  <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. Note that different occurrences of _ on the same term represent different anonymous variables.

@} @addtogroup Punctuation_Tokens Punctuation Tokens @{ @ingroup Tokens Punctuation tokens consist of one of the following characters:

( ) , [ ] { } |

These characters are used to group terms.

@} @defgroup LayoutComents Character Layout @{ @ingroup Tokens 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 \a % is taken to be a comment and ignored (including \a %). 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.

@} @} @addtogroup WideChars Encoding Wide Character Support @{ @ingroup YAPSyntax

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.

For this reason YAP, following SWI-Prolog, has two representations for atoms. If the text fits in ISO Latin-1, it is represented as an array of 8-bit characters. Otherwise the text is represented as an array of wide chars, which may take 16 or 32 bits. 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.

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.

@addtogroup Stream_Encoding Wide character encodings on streams @{ @ingroup WideChars

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

• 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 to ascii` . 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.

@}

@addtogroup BOM BOM: Byte Order Mark @{ @ingroup WideChars

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.

@} @} @addgroup Operators Summary of YAP Predefined Operators @{ @ingroup YapSyntax

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

means

a + (b * c)

as + and * have the following types and precedences:

:-op(500,yfx,'+').
:-op(400,yfx,'*').

Now defining

:-op(700,xfy,'++').
:-op(700,xfx,'=:=').
a ++ b =:= c

means

a ++ (b =:= c)

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

:-op(1200,fx,['?-', ':-']).
:-op(1200,xfx,[':-','-->']).
:-op(1150,fx,[block,dynamic,mode,public,multifile,meta_predicate,
              sequential,table,initialization]).
:-op(1100,xfy,[';','|']).
:-op(1050,xfy,->).
:-op(1000,xfy,',').
:-op(999,xfy,'.').
:-op(900,fy,['\+', not]).
:-op(900,fx,[nospy, spy]).
:-op(700,xfx,[@>=,@=<,@<,@>,<,=,>,=:=,=\=,\==,>=,=<,==,\=,=..,is]).
:-op(500,yfx,['\/','/\','+','-']).
:-op(500,fx,['+','-']).
:-op(400,yfx,['<<','>>','//','*','/']).
:-op(300,xfx,mod).
:-op(200,xfy,['^','**']).
:-op(50,xfx,same).

@} @}