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@section CHR CHR: Constraint Handling Rules
@ingroup SWILibrary
@{
This chapter is written by Tom Schrijvers, K.U. Leuven for the hProlog
system. Adjusted by Jan Wielemaker to fit the SWI-Prolog documentation
infrastructure and remove hProlog specific references.
The CHR system of SWI-Prolog is the K.U.Leuven CHR system. The runtime
environment is written by Christian Holzbaur and Tom Schrijvers while the
compiler is written by Tom Schrijvers. Both are integrated with SWI-Prolog
and licenced under compatible conditions with permission from the authors.
The main reference for SWI-Prolog's CHR system is:
+ T. Schrijvers, and B. Demoen, <em>The K.U.Leuven CHR System: Implementation and Application</em>, First Workshop on Constraint Handling Rules: Selected
Contributions (Fruwirth, T. and Meister, M., eds.), pp. 1--5, 2004.
@defgroup CHR_Introduction Introduction
@ingroup CHR
Constraint Handling Rules (CHR) is a committed-choice bottom-up language
embedded in Prolog. It is designed for writing constraint solvers and is
particularily useful for providing application-specific constraints.
It has been used in many kinds of applications, like scheduling,
model checking, abduction, type checking among many others.
CHR has previously been implemented in other Prolog systems (SICStus,
Eclipse, Yap), Haskell and Java. This CHR system is based on the
compilation scheme and runtime environment of CHR in SICStus.
In this documentation we restrict ourselves to giving a short overview
of CHR in general and mainly focus on elements specific to this
implementation. For a more thorough review of CHR we refer the reader to
[Freuhwirth:98]. More background on CHR can be found at the CHR web site.
@defgroup CHR_Syntax_and_Semantics Syntax and Semantics
@ingroup CHR
@{
We present informally the syntax and semantics of CHR.
@defgroup CHR_Syntax CHR Syntax
@ingroup CHR_Syntax_and_Semantics
@{
The syntax of CHR rules in hProlog is the following:
~~~~~
rules --> rule, rules.
rules --> [].
rule --> name, actual_rule, pragma, [atom(`.`)].
name --> atom, [atom(`@`)].
name --> [].
actual_rule --> simplification_rule.
actual_rule --> propagation_rule.
actual_rule --> simpagation_rule.
simplification_rule --> constraints, [atom(`<=>`)], guard, body.
propagation_rule --> constraints, [atom(`==>`)], guard, body.
simpagation_rule --> constraints, [atom(`\`)], constraints, [atom(`<=>`)],
guard, body.
constraints --> constraint, constraint_id.
constraints --> constraint, [atom(`,`)], constraints.
constraint --> compound_term.
constraint_id --> [].
constraint_id --> [atom(`#`)], variable.
guard --> [].
guard --> goal, [atom(`|`)].
body --> goal.
pragma --> [].
pragma --> [atom(`pragma`)], actual_pragmas.
actual_pragmas --> actual_pragma.
actual_pragmas --> actual_pragma, [atom(`,`)], actual_pragmas.
actual_pragma --> [atom(`passive(`)], variable, [atom(`)`)].
~~~~~
Additional syntax-related terminology:
+ *head:* the constraints in an `actual_rule` before
the arrow (either `<=>` or `==>`)
@}
@defgroup Semantics Semantics
@ingroup CHR_Syntax_and_Semantics
@{
In this subsection the operational semantics of CHR in Prolog are presented
informally. They do not differ essentially from other CHR systems.
When a constraint is called, it is considered an active constraint and
the system will try to apply the rules to it. Rules are tried and executed
sequentially in the order they are written.
A rule is conceptually tried for an active constraint in the following
way. The active constraint is matched with a constraint in the head of
the rule. If more constraints appear in the head they are looked for
among the suspended constraints, which are called passive constraints in
this context. If the necessary passive constraints can be found and all
match with the head of the rule and the guard of the rule succeeds, then
the rule is committed and the body of the rule executed. If not all the
necessary passive constraint can be found, the matching fails or the
guard fails, then the body is not executed and the process of trying and
executing simply continues with the following rules. If for a rule,
there are multiple constraints in the head, the active constraint will
try the rule sequentially multiple times, each time trying to match with
another constraint.
This process ends either when the active constraint disappears, i.e. it
is removed by some rule, or after the last rule has been processed. In
the latter case the active constraint becomes suspended.
A suspended constraint is eligible as a passive constraint for an active
constraint. The other way it may interact again with the rules, is when
a variable appearing in the constraint becomes bound to either a nonvariable
or another variable involved in one or more constraints. In that case the
constraint is triggered, i.e. it becomes an active constraint and all
the rules are tried.
@defgroup CHR_Rule_Types Rules
@ingroup CHR
@{
There are three different kinds of rules, each with their specific semantics:
+ simplification
The simplification rule removes the constraints in its head and calls its body.
+ propagation
The propagation rule calls its body exactly once for the constraints in
its head.
+ simpagation
The simpagation rule removes the constraints in its head after the
`\` and then calls its body. It is an optimization of
simplification rules of the form: \[constraints_1, constraints_2 <=>
constraints_1, body \] Namely, in the simpagation form:
~~~~~
constraints1 \ constraints2 <=> body
~~~~~
_constraints1_
constraints are not called in the body.
@defgroup CHR_Rule_Names Rule Names
@ingroup CHR_Rule_Types
@{
Naming a rule is optional and has no semantical meaning. It only functions
as documentation for the programmer.
@defgroup CHRPragmas Pragmas
@ingroup CHR
@{
The semantics of the pragmas are:
+ passive(Identifier)
The constraint in the head of a rule _Identifier_ can only act as a
passive constraint in that rule.
Additional pragmas may be released in the future.
@defgroup CHR_Options Options
@ingroup CHR
@{
It is possible to specify options that apply to all the CHR rules in the module.
Options are specified with the `option/2` declaration:
~~~~~
option(Option,Value).
~~~~~
Available options are:
+ check_guard_bindings
This option controls whether guards should be checked for illegal
variable bindings or not. Possible values for this option are
`on`, to enable the checks, and `off`, to disable the
checks.
+ optimize
This is an experimental option controlling the degree of optimization.
Possible values are `full`, to enable all available
optimizations, and `off` (default), to disable all optimizations.
The default is derived from the SWI-Prolog flag `optimise`, where
`true` is mapped to `full`. Therefore the commandline
option `-O` provides full CHR optimization.
If optimization is enabled, debugging should be disabled.
+ debug
This options enables or disables the possibility to debug the CHR code.
Possible values are `on` (default) and `off`. See
`debugging` for more details on debugging. The default is
derived from the prolog flag `generate_debug_info`, which
is `true` by default. See `-nodebug`.
If debugging is enabled, optimization should be disabled.
+ mode
This option specifies the mode for a particular constraint. The
value is a term with functor and arity equal to that of a constraint.
The arguments can be one of `-`, `+` or `?`.
The latter is the default. The meaning is the following:
+ -
The corresponding argument of every occurrence
of the constraint is always unbound.
+ +
The corresponding argument of every occurrence
of the constraint is always ground.
+ ?
The corresponding argument of every occurrence
of the constraint can have any instantiation, which may change
over time. This is the default value.
The declaration is used by the compiler for various optimizations.
Note that it is up to the user the ensure that the mode declaration
is correct with respect to the use of the constraint.
This option may occur once for each constraint.
+ type_declaration
This option specifies the argument types for a particular constraint. The
value is a term with functor and arity equal to that of a constraint.
The arguments can be a user-defined type or one of
the built-in types:
+ int
The corresponding argument of every occurrence
of the constraint is an integer number.
+ float
...{} a floating point number.
+ number
...{} a number.
+ natural
...{} a positive integer.
+ any
The corresponding argument of every occurrence
of the constraint can have any type. This is the default value.
Currently, type declarations are only used to improve certain
optimizations (guard simplification, occurrence subsumption, ...{}).
+ type_definition
This option defines a new user-defined type which can be used in
type declarations. The value is a term of the form
`type(` _name_`,` _list_`)`, where
_name_ is a term and _list_ is a list of alternatives.
Variables can be used to define generic types. Recursive definitions
are allowed. Examples are
~~~~~
type(bool,[true,false]).
type(complex_number,[float + float * i]).
type(binary_tree(T),[ leaf(T) | node(binary_tree(T),binary_tree(T)) ]).
type(list(T),[ [] | [T | list(T)]).
~~~~~
The mode, type_declaration and type_definition options are provided
for backward compatibility. The new syntax is described below.
@}
@defgroup CHR_in_YAP_Programs CHR in Prolog Programs
@ingroup CHR
@{
The CHR constraints defined in a particulary chr file are
associated with a module. The default module is `user`. One should
never load different chr files with the same CHR module name.
@defgroup Constraint_declaration Constraint declaration
@ingroup CHR_in_YAP_Programs
@{
Every constraint used in CHR rules has to be declared.
There are two ways to do this. The old style is as follows:
~~~~~
option(type_definition,type(list(T),[ [] , [T|list(T)] ]).
option(mode,foo(+,?)).
option(type_declaration,foo(list(int),float)).
:- constraints foo/2, bar/0.
~~~~~
The new style is as follows:
~~~~~
:- chr_type list(T) ---> [] ; [T|list(T)].
:- constraints foo(+list(int),?float), bar.
~~~~~
@defgroup Compilation Compilation
@ingroup CHR_in_YAP
The
SWI-Prolog CHR compiler exploits term_expansion/2 rules to translate
the constraint handling rules to plain Prolog. These rules are loaded
from the library chr. They are activated if the compiled file
has the chr extension or after finding a declaration of the
format below.
~~~~~
:- constraints ...
~~~~~
It is adviced to define CHR rules in a module file, where the module
declaration is immediately followed by including the chr
library as examplified below:
~~~~~
:- module(zebra, [ zebra/0 ]).
:- use_module(library(chr)).
:- constraints ...
~~~~~
Using this style CHR rules can be defined in ordinary Prolog
pl files and the operator definitions required by CHR do not
leak into modules where they might cause conflicts.
@defgroup CHR_Debugging CHR Debugging
@ingroup CHR_in_YAP
@{
The CHR debugging facilities are currently rather limited. Only tracing
is currently available. To use the CHR debugging facilities for a CHR
file it must be compiled for debugging. Generating debug info is
controlled by the CHR option debug, whose default is derived
from the SWI-Prolog flag `generate_debug_info`. Therefore debug
info is provided unless the `-nodebug` is used.
@defgroup Ports Ports
@ingroup CHR_Debugging
For CHR constraints the four standard ports are defined:
+ call
A new constraint is called and becomes active.
+ exit
An active constraint exits: it has either been inserted in the store after
trying all rules or has been removed from the constraint store.
+ fail
An active constraint fails.
+ redo
An active constraint starts looking for an alternative solution.
In addition to the above ports, CHR constraints have five additional
ports:
+ wake
A suspended constraint is woken and becomes active.
+ insert
An active constraint has tried all rules and is suspended in
the constraint store.
+ remove
An active or passive constraint is removed from the constraint
store, if it had been inserted.
+ try
An active constraints tries a rule with possibly
some passive constraints. The try port is entered
just before committing to the rule.
+ apply
An active constraints commits to a rule with possibly
some passive constraints. The apply port is entered
just after committing to the rule.
@defgroup Tracing Tracing
@ingroup CHR_Debugging
Tracing is enabled with the chr_trace/0 predicate
and disabled with the chr_notrace/0 predicate.
When enabled the tracer will step through the `call`,
`exit`, `fail`, `wake` and `apply` ports,
accepting debug commands, and simply write out the other ports.
The following debug commans are currently supported:
~~~~~
CHR debug options:
<cr> creep c creep
s skip
g ancestors
n nodebug
b break
a abort
f fail
? help h help
~~~~~
Their meaning is:
+ creep
Step to the next port.
+ skip
Skip to exit port of this call or wake port.
+ ancestors
Print list of ancestor call and wake ports.
+ nodebug
Disable the tracer.
+ break
Enter a recursive Prolog toplevel. See break/0.
+ abort
Exit to the toplevel. See abort/0.
+ fail
Insert failure in execution.
+ help
Print the above available debug options.
@defgroup CHR_Debugging_Predicates CHR Debugging Predicates
@ingroup CHR_Debugging
The chr module contains several predicates that allow
inspecting and printing the content of the constraint store.
+ chr_trace
Activate the CHR tracer. By default the CHR tracer is activated and
deactivated automatically by the Prolog predicates trace/0 and
notrace/0.
@}
@defgroup CHR_Examples Examples
@ingroup CHR
@{
Here are two example constraint solvers written in CHR.
+
The program below defines a solver with one constraint,
`leq/2`, which is a less-than-or-equal constraint.
~~~~~
:- module(leq,[cycle/3, leq/2]).
:- use_module(library(chr)).
:- constraints leq/2.
reflexivity @ leq(X,X) <=> true.
antisymmetry @ leq(X,Y), leq(Y,X) <=> X = Y.
idempotence @ leq(X,Y) \ leq(X,Y) <=> true.
transitivity @ leq(X,Y), leq(Y,Z) ==> leq(X,Z).
cycle(X,Y,Z):-
leq(X,Y),
leq(Y,Z),
leq(Z,X).
~~~~~
+
The program below implements a simple finite domain
constraint solver.
~~~~~
:- module(dom,[dom/2]).
:- use_module(library(chr)).
:- constraints dom/2.
dom(X,[]) <=> fail.
dom(X,[Y]) <=> X = Y.
dom(X,L1), dom(X,L2) <=> intersection(L1,L2,L3), dom(X,L3).
intersection([],_,[]).
intersection([H|T],L2,[H|L3]) :-
member(H,L2), !,
intersection(T,L2,L3).
intersection([_|T],L2,L3) :-
intersection(T,L2,L3).
~~~~~
@}
@defgroup CHR_Compatibility Compatibility with SICStus CHR
@ingroup CHR
@{
There are small differences between CHR in SWI-Prolog and newer
YAPs and SICStus and older versions of YAP. Besides differences in
available options and pragmas, the following differences should be
noted:
+ [The handler/1 declaration]
In SICStus every CHR module requires a `handler/1`
declaration declaring a unique handler name. This declaration is valid
syntax in SWI-Prolog, but will have no effect. A warning will be given
during compilation.
+ [The rules/1 declaration]
In SICStus, for every CHR module it is possible to only enable a subset
of the available rules through the `rules/1` declaration. The
declaration is valid syntax in SWI-Prolog, but has no effect. A
warning is given during compilation.
+ [Sourcefile naming]
SICStus uses a two-step compiler, where chr files are
first translated into pl files. For SWI-Prolog CHR
rules may be defined in a file with any extension.
@}
@}
@defgroup CHR_Guidelines Guidelines
@ingroup CHR
@{
In this section we cover several guidelines on how to use CHR to write
constraint solvers and how to do so efficiently.
+ [Set semantics]
The CHR system allows the presence of identical constraints, i.e.
multiple constraints with the same functor, arity and arguments. For
most constraint solvers, this is not desirable: it affects efficiency
and possibly termination. Hence appropriate simpagation rules should be
added of the form:
~~~~~
{constraint \ constraint <=> true}.
~~~~~
+ [Multi-headed rules]
Multi-headed rules are executed more efficiently when the constraints
share one or more variables.
+ [Mode and type declarations]
Provide mode and type declarations to get more efficient program execution.
Make sure to disable debug (`-nodebug`) and enable optimization
(`-O`).
@}
@}

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/** @defgroup CLPQR Constraint Logic Programming over Reals
@ingroup SWILibrary
@{
YAP now uses the CLP(R) package developed by <em>Leslie De Koninck</em>,
K.U. Leuven as part of a thesis with supervisor Bart Demoen and daily
advisor Tom Schrijvers, and distributed with SWI-Prolog.
This CLP(R) system is a port of the CLP(Q,R) system of Sicstus Prolog
and YAP by Christian Holzbaur: Holzbaur C.: OFAI clp(q,r) Manual,
Edition 1.3.3, Austrian Research Institute for Artificial
Intelligence, Vienna, TR-95-09, 1995,
<http://www.ai.univie.ac.at/cgi-bin/tr-online?number+95-09> This
port only contains the part concerning real arithmetics. This manual
is roughly based on the manual of the above mentioned *CLP(QR)*
implementation.
Please note that the clpr library is <em>not</em> an
`autoload` library and therefore this library must be loaded
explicitely before using it:
~~~~~
:- use_module(library(clpr)).
~~~~~
@defgroup CLPR_Solver_Predicates Solver Predicates
@ingroup CLPQR
@{
The following predicates are provided to work with constraints:
* @defgroup CLPR_Syntax Syntax of the predicate arguments
@ingroup YAPPackages
@{
The arguments of the predicates defined in the subsection above are
defined in the following table. Failing to meet the syntax rules will
result in an exception.
~~~~~
<Constraints> ---> <Constraint> \ single constraint \
| <Constraint> , <Constraints> \ conjunction \
| <Constraint> ; <Constraints> \ disjunction \
<Constraint> ---> <Expression> {<} <Expression> \ less than \
| <Expression> {>} <Expression> \ greater than \
| <Expression> {=<} <Expression> \ less or equal \
| {<=}(<Expression>, <Expression>) \ less or equal \
| <Expression> {>=} <Expression> \ greater or equal \
| <Expression> {=\=} <Expression> \ not equal \
| <Expression> =:= <Expression> \ equal \
| <Expression> = <Expression> \ equal \
<Expression> ---> <Variable> \ Prolog variable \
| <Number> \ Prolog number (float, integer) \
| +<Expression> \ unary plus \
| -<Expression> \ unary minus \
| <Expression> + <Expression> \ addition \
| <Expression> - <Expression> \ substraction \
| <Expression> * <Expression> \ multiplication \
| <Expression> / <Expression> \ division \
| abs(<Expression>) \ absolute value \
| sin(<Expression>) \ sine \
| cos(<Expression>) \ cosine \
| tan(<Expression>) \ tangent \
| exp(<Expression>) \ exponent \
| pow(<Expression>) \ exponent \
| <Expression> {^} <Expression> \ exponent \
| min(<Expression>, <Expression>) \ minimum \
| max(<Expression>, <Expression>) \ maximum \
~~~~~
@defgroup CLPR_Unification Use of unification
@ingroup CLPQR
@{
Instead of using the `{}/1` predicate, you can also use the standard
unification mechanism to store constraints. The following code samples
are equivalent:
+ Unification with a variable
~~~~~
{X =:= Y}
{X = Y}
X = Y
~~~~~
+ Unification with a number
~~~~~
{X =:= 5.0}
{X = 5.0}
X = 5.0
~~~~~
@defgroup CLPR_NonhYlinear_Constraints Non-Linear Constraints
@ingroup CLPQR
@{
In this version, non-linear constraints do not get solved until certain
conditions are satisfied. We call these conditions the isolation axioms.
They are given in the following table.
~~~~~
A = B * C when B or C is ground or // A = 5 * C or A = B * 4 \\
A and (B or C) are ground // 20 = 5 * C or 20 = B * 4 \\
A = B / C when C is ground or // A = B / 3
A and B are ground // 4 = 12 / C
X = min(Y,Z) when Y and Z are ground or // X = min(4,3)
X = max(Y,Z) Y and Z are ground // X = max(4,3)
X = abs(Y) Y is ground // X = abs(-7)
X = pow(Y,Z) when X and Y are ground or // 8 = 2 ^ Z
X = exp(Y,Z) X and Z are ground // 8 = Y ^ 3
X = Y ^ Z Y and Z are ground // X = 2 ^ 3
X = sin(Y) when X is ground or // 1 = sin(Y)
X = cos(Y) Y is ground // X = sin(1.5707)
X = tan(Y)
~~~~~

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@defgroup Syntax YAP Syntax
@ingroup YAPProgramming
@{
@page YAPSyntax 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 \a tokens from which Prolog \a terms are
built.
@section Formal_Syntax Syntax of Terms
Below, we describe the syntax of YAP terms from the different
classes of tokens defined above. The formalism used will be <em>BNF</em>,
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.
@section Tokens Prolog Tokens
Prolog tokens are grouped into the following categories:
@subsection Numbers Numbers
Numbers can be further subdivided into integer and floating-point numbers.
@subsubsection Integers
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.
@subsubsection Floats
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.
@subsection Strings Character Strings
Strings are described by the following rules:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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` in any character except the double quote
and escape characters.
Examples:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"" "a string" "a double-quote:"""
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The first string is an empty string, the last string shows the use of
double-quoting. The implementation of YAP represents strings as
lists of integers. Since YAP 4.3.0 there is no static limit on string
size.
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 disable by using:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:- yap_flag(character_escapes,false).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@subsection 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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.
@subsection Variables Variables
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 <em>different</em>
anonymous variables.
@subsection Punctuation_Tokens Punctuation Tokens
Punctuation tokens consist of one of the following characters:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
( ) , [ ] { } |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
These characters are used to group terms.
@subsection Layout Layout
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.
@section Encoding Wide Character Support
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.
<em>Universal Character Set</em> (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.
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.
@subsection Stream_Encoding Wide character encodings on streams
Although characters are uniquely coded using the UCS standard
internally, streams and files are byte (8-bit) oriented and there are a
variety of ways to represent the larger UCS codes in an 8-bit octet
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 <em>encoding</em> which
specifies the used encoding that influence `get_code/2` and
`put_code/2` as well as all the other text I/O predicates.
The default encoding for files is derived from the Prolog flag
`encoding`, which is initialised from the environment. If 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). The encoding can 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
<http://www.unicode.org/>.
YAP currently defines and supports the following encodings:
+ octet
Default encoding for <em>binary</em> streams. This causes
the stream to be read and written fully untranslated.
+ 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
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
Multi-byte encoding of full UCS, compatible to `ascii`.
See above.
+ unicode_be
Unicode Big Endian. Reads input in pairs of bytes, most
significant byte first. Can only represent 16-bit characters.
+ unicode_le
Unicode Little 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.
@subsection BOM BOM: Byte Order Mark
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 <em>Byte Order Marker</em> is a technique
for identifying Unicode text-files as well as the encoding they
use. Such files start with the Unicode character `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 `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 `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`.
@subsection Operators Summary of YAP Predefined 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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@}

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@defgroup Threads Threads
@ingroup YAPExtensions
YAP implements a SWI-Prolog compatible multithreading
library. Like in SWI-Prolog, Prolog threads have their own stacks and
only share the Prolog <em>heap</em>: predicates, records, flags and other
global non-backtrackable data. The package is based on the POSIX thread
standard (Butenhof:1997:PPT) used on most popular systems except
for MS-Windows.
@defgroup Creating_and_Destroying_Prolog_Threads Creating and Destroying Prolog Threads
@ingroup Threads
@pred thread_create(: _Goal_, - _Id_, + _Options_)
Create a new Prolog thread (and underlying C-thread) and start it
by executing _Goal_. If the thread is created successfully, the
thread-identifier of the created thread is unified to _Id_.
_Options_ is a list of options. Currently defined options are:
+ stack
Set the limit in K-Bytes to which the Prolog stacks of
this thread may grow. If omitted, the limit of the calling thread is
used. See also the commandline `-S` option.
+ trail
Set the limit in K-Bytes to which the trail stack of this thread may
grow. If omitted, the limit of the calling thread is used. See also the
commandline option `-T`.
+ alias
Associate an alias-name with the thread. This named may be used to
refer to the thread and remains valid until the thread is joined
(see thread_join/2).
+ at_exit
Define an exit hook for the thread. This hook is called when the thread
terminates, no matter its exit status.
+ detached
If `false` (default), the thread can be waited for using
thread_join/2. thread_join/2 must be called on this thread
to reclaim the all resources associated to the thread. If `true`,
the system will reclaim all associated resources automatically after the
thread finishes. Please note that thread identifiers are freed for reuse
after a detached thread finishes or a normal thread has been joined.
See also thread_join/2 and thread_detach/1.
The _Goal_ argument is <em>copied</em> to the new Prolog engine.
This implies further instantiation of this term in either thread does
not have consequences for the other thread: Prolog threads do not share
data from their stacks.
@defgroup Monitoring_Threads Monitoring Threads
@ingroup Threads
Normal multi-threaded applications should not need these the predicates
from this section because almost any usage of these predicates is
unsafe. For example checking the existence of a thread before signalling
it is of no use as it may vanish between the two calls. Catching
exceptions using catch/3 is the only safe way to deal with
thread-existence errors.
These predicates are provided for diagnosis and monitoring tasks.
@defgroup Thread_Communication Thread communication
@ingroup Threads
Prolog threads can exchange data using dynamic predicates, database
records, and other globally shared data. These provide no suitable means
to wait for data or a condition as they can only be checked in an
expensive polling loop. <em>Message queues</em> provide a means for
threads to wait for data or conditions without using the CPU.
Each thread has a message-queue attached to it that is identified
by the thread. Additional queues are created using
`message_queue_create/2`.
@pred thread_send_message(+ _Term_)
Places _Term_ in the message-queue of the thread running the goal.
Any term can be placed in a message queue, but note that the term is
copied to the receiving thread and variable-bindings are thus lost.
This call returns immediately.
@defgroup Signalling_Threads Signalling Threads
@ingroup Threadas
These predicates provide a mechanism to make another thread execute some
goal as an <em>interrupt</em>. Signalling threads is safe as these
interrupts are only checked at safe points in the virtual machine.
Nevertheless, signalling in multi-threaded environments should be
handled with care as the receiving thread may hold a <em>mutex</em>
(see with_mutex/2). Signalling probably only makes sense to start
debugging threads and to cancel no-longer-needed threads with throw/1,
where the receiving thread should be designed carefully do handle
exceptions at any point.
@defgroup Threads_and_Dynamic_Predicates Threads and Dynamic Predicates
@ingroup Threads
Besides queues threads can share and exchange data using dynamic
predicates. The multi-threaded version knows about two types of
dynamic predicates. By default, a predicate declared <em>dynamic</em>
(see dynamic/1) is shared by all threads. Each thread may
assert, retract and run the dynamic predicate. Synchronisation inside
Prolog guarantees the consistency of the predicate. Updates are
<em>logical</em>: visible clauses are not affected by assert/retract
after a query started on the predicate. In many cases primitive from
thread synchronisation should be used to ensure application invariants on
the predicate are maintained.
Besides shared predicates, dynamic predicates can be declared with the
thread_local/1 directive. Such predicates share their
attributes, but the clause-list is different in each thread.
@defgroup Thread_Synchronisation Thread Synchronisation
All
internal Prolog operations are thread-safe. This implies two Prolog
threads can operate on the same dynamic predicate without corrupting the
consistency of the predicate. This section deals with user-level
<em>mutexes</em> (called <em>monitors</em> in ADA or
<em>critical-sections</em> by Microsoft). A mutex is a
<em>MUT</em>ual <em>EX</em>clusive device, which implies at most one thread
can <em>hold</em> a mutex.
Mutexes are used to realise related updates to the Prolog database.
With `related', we refer to the situation where a `transaction' implies
two or more changes to the Prolog database. For example, we have a
predicate `address/2`, representing the address of a person and we want
to change the address by retracting the old and asserting the new
address. Between these two operations the database is invalid: this
person has either no address or two addresses, depending on the
assert/retract order.
Here is how to realise a correct update:
~~~~~
:- initialization
mutex_create(addressbook).
change_address(Id, Address) :-
mutex_lock(addressbook),
retractall(address(Id, _)),
asserta(address(Id, Address)),
mutex_unlock(addressbook).
~~~~~

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