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docs/syntax.tex
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docs/syntax.tex
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@c -*- mode: texinfo; coding: utf-8; -*-
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@node Syntax, Loading Programs, Run, Top
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@chapter Syntax
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We will describe the syntax of YAP at two levels. We first will
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describe the syntax for Prolog terms. In a second level we describe
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the @i{tokens} from which Prolog @i{terms} are
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built.
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@menu
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* Formal Syntax:: Syntax of terms
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* Tokens:: Syntax of Prolog tokens
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* Encoding:: How characters are encoded and Wide Character Support
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@end menu
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@node Formal Syntax, Tokens, ,Syntax
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@section Syntax of Terms
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@cindex syntax
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Below, we describe the syntax of YAP terms from the different
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classes of tokens defined above. The formalism used will be @emph{BNF},
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extended where necessary with attributes denoting integer precedence or
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operator type.
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@example
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term ----> subterm(1200) end_of_term_marker
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subterm(N) ----> term(M) [M <= N]
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term(N) ----> op(N, fx) subterm(N-1)
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| op(N, fy) subterm(N)
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| subterm(N-1) op(N, xfx) subterm(N-1)
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| subterm(N-1) op(N, xfy) subterm(N)
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| subterm(N) op(N, yfx) subterm(N-1)
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| subterm(N-1) op(N, xf)
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| subterm(N) op(N, yf)
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term(0) ----> atom '(' arguments ')'
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| '(' subterm(1200) ')'
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| '@{' subterm(1200) '@}'
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| list
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| string
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| number
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| atom
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| variable
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arguments ----> subterm(999)
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| subterm(999) ',' arguments
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list ----> '[]'
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| '[' list_expr ']'
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list_expr ----> subterm(999)
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| subterm(999) list_tail
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list_tail ----> ',' list_expr
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| ',..' subterm(999)
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| '|' subterm(999)
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@end example
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@noindent
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Notes:
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@itemize @bullet
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@item
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@i{op(N,T)} denotes an atom which has been previously declared with type
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@i{T} and base precedence @i{N}.
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@item
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Since ',' is itself a pre-declared operator with type @i{xfy} and
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precedence 1000, is @i{subterm} starts with a '(', @i{op} must be
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followed by a space to avoid ambiguity with the case of a functor
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followed by arguments, e.g.:
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@example
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+ (a,b) [the same as '+'(','(a,b)) of arity one]
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@end example
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versus
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@example
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+(a,b) [the same as '+'(a,b) of arity two]
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@end example
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@item
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In the first rule for term(0) no blank space should exist between
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@i{atom} and '('.
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@item
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@cindex end of term
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Each term to be read by the YAP parser must end with a single
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dot, followed by a blank (in the sense mentioned in the previous
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paragraph). When a name consisting of a single dot could be taken for
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the end of term marker, the ambiguity should be avoided by surrounding the
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dot with single quotes.
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@end itemize
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@node Tokens, Encoding, Formal Syntax, Syntax
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@section Prolog Tokens
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@cindex token
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Prolog tokens are grouped into the following categories:
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@menu
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* Numbers:: Integer and Floating-Point Numbers
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* Strings:: Sequences of Characters
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* Atoms:: Atomic Constants
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* Variables:: Logical Variables
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* Punctuation Tokens:: Tokens that separate other tokens
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* Layout:: Comments and Other Layout Rules
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@end menu
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@node Numbers, Strings, ,Tokens
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@subsection Numbers
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@cindex number
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Numbers can be further subdivided into integer and floating-point numbers.
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@menu
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* Integers:: How Integers are read and represented
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* Floats:: Floating Point Numbers
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@end menu
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@node Integers, Floats, ,Numbers
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@subsubsection Integers
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@cindex integer
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Integer numbers
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are described by the following regular expression:
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@example
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<integer> := @{<digit>+<single-quote>|0@{xXo@}@}<alpha_numeric_char>+
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@end example
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@noindent
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where @{...@} stands for optionality, @i{+} optional repetition (one or
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more times), @i{<digit>} denotes one of the characters 0 ... 9, @i{|}
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denotes or, and @i{<single-quote>} denotes the character "'". The digits
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before the @i{<single-quote>} character, when present, form the number
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basis, that can go from 0, 1 and up to 36. Letters from @code{A} to
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@code{Z} are used when the basis is larger than 10.
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Note that if no basis is specified then base 10 is assumed. Note also
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that the last digit of an integer token can not be immediately followed
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by one of the characters 'e', 'E', or '.'.
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Following the ISO standard, YAP also accepts directives of the
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form @code{0x} to represent numbers in hexadecimal base and of the form
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@code{0o} to represent numbers in octal base. For usefulness,
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YAP also accepts directives of the form @code{0X} to represent
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numbers in hexadecimal base.
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Example:
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the following tokens all denote the same integer
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@example
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@code{10 2'1010 3'101 8'12 16'a 36'a 0xa 0o12}
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@end example
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Numbers of the form @code{0'a} are used to represent character
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constants. So, the following tokens denote the same integer:
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@example
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@code{0'd 100}
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@end example
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YAP (version @value{VERSION}) supports integers that can fit
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the word size of the machine. This is 32 bits in most current machines,
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but 64 in some others, such as the Alpha running Linux or Digital
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Unix. The scanner will read larger or smaller integers erroneously.
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@node Floats, , Integers,Numbers
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@subsubsection Floating-point Numbers
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@cindex floating-point number
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Floating-point numbers are described by:
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@example
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<float> := <digit>+@{<dot><digit>+@}
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<exponent-marker>@{<sign>@}<digit>+
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|<digit>+<dot><digit>+
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@{<exponent-marker>@{<sign>@}<digit>+@}
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@end example
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@noindent
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where @i{<dot>} denotes the decimal-point character '.',
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@i{<exponent-marker>} denotes one of 'e' or 'E', and @i{<sign>} denotes
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one of '+' or '-'.
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Examples:
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@example
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@code{10.0 10e3 10e-3 3.1415e+3}
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@end example
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Floating-point numbers are represented as a double in the target
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machine. This is usually a 64-bit number.
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@node Strings, Atoms, Numbers,Tokens
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@subsection Character Strings
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@cindex string
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Strings are described by the following rules:
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@example
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string --> '"' string_quoted_characters '"'
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string_quoted_characters --> '"' '"' string_quoted_characters
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string_quoted_characters --> '\'
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escape_sequence string_quoted_characters
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string_quoted_characters -->
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string_character string_quoted_characters
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escape_sequence --> 'a' | 'b' | 'r' | 'f' | 't' | 'n' | 'v'
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escape_sequence --> '\' | '"' | ''' | '`'
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escape_sequence --> at_most_3_octal_digit_seq_char '\'
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escape_sequence --> 'x' at_most_2_hexa_digit_seq_char '\'
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@end example
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where @code{string_character} in any character except the double quote
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and escape characters.
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Examples:
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@example
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@code{"" "a string" "a double-quote:""" }
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@end example
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The first string is an empty string, the last string shows the use of
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double-quoting. The implementation of YAP represents strings as
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lists of integers. Since YAP 4.3.0 there is no static limit on string
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size.
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Escape sequences can be used to include the non-printable characters
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@code{a} (alert), @code{b} (backspace), @code{r} (carriage return),
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@code{f} (form feed), @code{t} (horizontal tabulation), @code{n} (new
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line), and @code{v} (vertical tabulation). Escape sequences also be
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include the meta-characters @code{\}, @code{"}, @code{'}, and
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@code{`}. Last, one can use escape sequences to include the characters
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either as an octal or hexadecimal number.
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The next examples demonstrates the use of escape sequences in YAP:
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@example
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@code{"\x0c\" "\01\" "\f" "\\" }
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@end example
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The first three examples return a list including only character 12 (form
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feed). The last example escapes the escape character.
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Escape sequences were not available in C-Prolog and in original
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versions of YAP up to 4.2.0. Escape sequences can be disable by using:
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@example
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@code{:- yap_flag(character_escapes,false).}
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@end example
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@node Atoms, Variables, Strings, Tokens
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@subsection Atoms
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@cindex atom
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Atoms are defined by one of the following rules:
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@example
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atom --> solo-character
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atom --> lower-case-letter name-character*
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atom --> symbol-character+
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atom --> single-quote single-quote
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atom --> ''' atom_quoted_characters '''
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atom_quoted_characters --> ''' ''' atom_quoted_characters
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atom_quoted_characters --> '\' atom_sequence string_quoted_characters
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atom_quoted_characters --> character string_quoted_characters
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@end example
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where:
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@example
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<solo-character> denotes one of: ! ;
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<symbol-character> denotes one of: # & * + - . / : <
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= > ? @@ \ ^ ~ `
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<lower-case-letter> denotes one of: a...z
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<name-character> denotes one of: _ a...z A...Z 0....9
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<single-quote> denotes: '
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@end example
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and @code{string_character} denotes any character except the double quote
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and escape characters. Note that escape sequences in strings and atoms
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follow the same rules.
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Examples:
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@example
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@code{a a12x '$a' ! => '1 2'}
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@end example
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Version @code{4.2.0} of YAP removed the previous limit of 256
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characters on an atom. Size of an atom is now only limited by the space
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available in the system.
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@node Variables, Punctuation Tokens, Atoms, Tokens
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@subsection Variables
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@cindex variable
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Variables are described by:
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@example
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<variable-starter><variable-character>+
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@end example
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where
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@example
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<variable-starter> denotes one of: _ A...Z
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<variable-character> denotes one of: _ a...z A...Z
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@end example
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@cindex anonymous variable
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If a variable is referred only once in a term, it needs not to be named
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and one can use the character @code{_} to represent the variable. These
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variables are known as anonymous variables. Note that different
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occurrences of @code{_} on the same term represent @emph{different}
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anonymous variables.
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@node Punctuation Tokens, Layout, Variables, Tokens
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@subsection Punctuation Tokens
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@cindex punctuation token
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Punctuation tokens consist of one of the following characters:
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@example
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( ) , [ ] @{ @} |
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@end example
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These characters are used to group terms.
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@node Layout, ,Punctuation Tokens, Tokens
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@subsection Layout
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@cindex comment
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Any characters with ASCII code less than or equal to 32 appearing before
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a token are ignored.
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All the text appearing in a line after the character @i{%} is taken to
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be a comment and ignored (including @i{%}). Comments can also be
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inserted by using the sequence @code{/*} to start the comment and
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@code{*} followed by @code{/} to finish it. In the presence of any sequence of comments or
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layout characters, the YAP parser behaves as if it had found a
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single blank character. The end of a file also counts as a blank
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character for this purpose.
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@node Encoding, , Tokens, Syntax
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@section Wide Character Support
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|
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@cindex encodings
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|
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@menu
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* Stream Encoding:: How Prolog Streams can be coded
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* BOM:: The Byte Order Mark
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@end menu
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@cindex UTF-8
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@cindex Unicode
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|
@cindex UCS
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@cindex internationalization
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|
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YAP now implements a SWI-Prolog compatible interface to wide
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characters and the Universal Character Set (UCS). The following text
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|
was adapted from the SWI-Prolog manual.
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YAP now supports wide characters, characters with character
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|
codes above 255 that cannot be represented in a single byte.
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@emph{Universal Character Set} (UCS) is the ISO/IEC 10646 standard
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that specifies a unique 31-bits unsigned integer for any character in
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any language. It is a superset of 16-bit Unicode, which in turn is
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a superset of ISO 8859-1 (ISO Latin-1), a superset of US-ASCII. UCS
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can handle strings holding characters from multiple languages and
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character classification (uppercase, lowercase, digit, etc.) and
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|
operations such as case-conversion are unambiguously defined.
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|
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|
For this reason YAP, following SWI-Prolog, has two representations for
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|
atoms. If the text fits in ISO Latin-1, it is represented as an array
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|
of 8-bit characters. Otherwise the text is represented as an array of
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wide chars, which may take 16 or 32 bits. This representational issue
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|
is completely transparent to the Prolog user. Users of the foreign
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|
language interface sometimes need to be aware of these issues though.
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|
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Character coding comes into view when characters of strings need to be
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|
read from or written to file or when they have to be communicated to
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|
other software components using the foreign language interface. In this
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||||||
|
section we only deal with I/O through streams, which includes file I/O
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|
as well as I/O through network sockets.
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|
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|
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|
@node Stream Encoding, , BOM, Encoding
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@subsection Wide character encodings on streams
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|
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|
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|
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||||||
|
Although characters are uniquely coded using the UCS standard
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|
internally, streams and files are byte (8-bit) oriented and there are a
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|
variety of ways to represent the larger UCS codes in an 8-bit octet
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||||||
|
stream. The most popular one, especially in the context of the web, is
|
||||||
|
UTF-8. 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 the 16-bit Unicode standard, represented by pairs of bytes is
|
||||||
|
also popular.
|
||||||
|
|
||||||
|
Prolog I/O streams have a property called @emph{encoding} which
|
||||||
|
specifies the used encoding that influence @code{get_code/2} and
|
||||||
|
@code{put_code/2} as well as all the other text I/O predicates.
|
||||||
|
|
||||||
|
The default encoding for files is derived from the Prolog flag
|
||||||
|
@code{encoding}, which is initialised from the environment. If the
|
||||||
|
environment variable @env{LANG} ends in "UTF-8", this encoding is
|
||||||
|
assumed. Otherwise the default is @code{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
|
||||||
|
@code{mbrtowc()} one). The encoding can be specified explicitly in
|
||||||
|
@code{load_files/2} for loading Prolog source with an alternative
|
||||||
|
encoding, @code{open/4} when opening files or using @code{set_stream/2} on
|
||||||
|
any open stream (not yet implemented). For Prolog source files we also
|
||||||
|
provide the @code{encoding/1} directive that can be used to switch
|
||||||
|
between encodings that are compatible to US-ASCII (@code{ascii},
|
||||||
|
@code{iso_latin_1}, @code{utf8} and many locales).
|
||||||
|
@c See also
|
||||||
|
@c \secref{intsrcfile} for writing Prolog files with non-US-ASCII
|
||||||
|
@c characters and \secref{unicodesyntax} for syntax issues.
|
||||||
|
For
|
||||||
|
additional information and Unicode resources, please visit
|
||||||
|
@uref{http://www.unicode.org/}.
|
||||||
|
|
||||||
|
YAP currently defines and supports the following encodings:
|
||||||
|
|
||||||
|
@itemize @bullet
|
||||||
|
@item octet
|
||||||
|
Default encoding for @emph{binary} streams. This causes
|
||||||
|
the stream to be read and written fully untranslated.
|
||||||
|
|
||||||
|
@item ascii
|
||||||
|
7-bit encoding in 8-bit bytes. Equivalent to @code{iso_latin_1},
|
||||||
|
but generates errors and warnings on encountering values above
|
||||||
|
127.
|
||||||
|
|
||||||
|
@item iso_latin_1
|
||||||
|
8-bit encoding supporting many western languages. This causes
|
||||||
|
the stream to be read and written fully untranslated.
|
||||||
|
|
||||||
|
@item text
|
||||||
|
C-library default locale encoding for text files. Files are read and
|
||||||
|
written using the C-library functions @code{mbrtowc()} and
|
||||||
|
@code{wcrtomb()}. This may be the same as one of the other locales,
|
||||||
|
notably it may be the same as @code{iso_latin_1} for western
|
||||||
|
languages and @code{utf8} in a UTF-8 context.
|
||||||
|
|
||||||
|
@item utf8
|
||||||
|
Multi-byte encoding of full UCS, compatible to @code{ascii}.
|
||||||
|
See above.
|
||||||
|
|
||||||
|
@item unicode_be
|
||||||
|
Unicode Big Endian. Reads input in pairs of bytes, most
|
||||||
|
significant byte first. Can only represent 16-bit characters.
|
||||||
|
|
||||||
|
@item unicode_le
|
||||||
|
Unicode Little Endian. Reads input in pairs of bytes, least
|
||||||
|
significant byte first. Can only represent 16-bit characters.
|
||||||
|
@end itemize
|
||||||
|
|
||||||
|
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 @code{open/4} or @code{set_stream/2} (not
|
||||||
|
implemented). Initially the terminal stream write the characters using
|
||||||
|
Prolog escape sequences while other streams generate an I/O exception.
|
||||||
|
|
||||||
|
|
||||||
|
@node BOM, Stream Encoding, , Encoding
|
||||||
|
@subsection BOM: Byte Order Mark
|
||||||
|
|
||||||
|
@cindex BOM
|
||||||
|
@cindex Byte Order Mark
|
||||||
|
From @ref{Stream Encoding}, you may have got the impression 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.
|
||||||
|
@strong{BOM} or @emph{Byte Order Marker} is a technique for
|
||||||
|
identifying Unicode text-files as well as the encoding they use. Such
|
||||||
|
files start with the Unicode character @code{0xFEFF}, a non-breaking,
|
||||||
|
zero-width space character. This is a pretty unique sequence that is not
|
||||||
|
likely to be the start of a non-Unicode file and uniquely distinguishes
|
||||||
|
the various Unicode file formats. As it is a zero-width blank, it even
|
||||||
|
doesn't produce any output. This solves all problems, or ...
|
||||||
|
|
||||||
|
Some formats start of as US-ASCII and may contain some encoding mark to
|
||||||
|
switch to UTF-8, such as the @code{encoding="UTF-8"} in an XML header.
|
||||||
|
Such formats often explicitly forbid the the use of a UTF-8 BOM. In
|
||||||
|
other cases there is additional information telling the encoding making
|
||||||
|
the use of a BOM redundant or even illegal.
|
||||||
|
|
||||||
|
The BOM is handled by the @code{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 @code{bom(true)} is
|
||||||
|
available through @code{stream_property/2}. When opening a file for
|
||||||
|
writing, writing a BOM can be requested using the option
|
||||||
|
@code{bom(true)} with @code{open/4}.
|
||||||
|
|
||||||
Reference in New Issue
Block a user