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@page run Running YAP
We next describe how to invoke YAP from the command-line, either interactively or as a script:
* @subpage Running_YAP_Interactively
* @subpage
@page Running_YAP_Interactively Running YAP Interactively
Most often you will want to use YAP in interactive mode. Assuming that
YAP is in the user's search path, the top-level can be invoked under
Unix with the following command:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
yap [-s n] [-h n] [-a n] [-c IP_HOST port ] [filename]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
All the arguments and flags are optional and have the following meaning:
+ -?
print a short error message.
+ -s _Size_
allocate _Size_ KBytes for local and global stacks. The user may
specify <tt>M</tt> bytes.
+ -h _Size_
allocate _Size_ KBytes for heap and auxiliary stacks
+ -t _Size_
allocate _Size_ KBytes for the trail stack
+ -L _Size_
SWI-compatible option to allocate _Size_ K bytes for local and global stacks, the local stack
cannot be expanded. To avoid confusion with the load option, _Size_
must immediately follow the letter `L`.
+ -G _Size_
SWI-compatible option to allocate _Size_ K bytes for local and global stacks; the global
stack cannot be expanded
+ -T _Size_
SWI-compatible option to allocate _Size_ K bytes for the trail stack; the trail cannot be expanded.
+ -l _YAP_FILE_
compile the Prolog file _YAP_FILE_ before entering the top-level.
+ -L _YAP_FILE_
compile the Prolog file _YAP_FILE_ and then halt. This option is
useful for implementing scripts.
+ -g _Goal_
run the goal _Goal_ before top-level. The goal is converted from
an atom to a Prolog term.
+ -z _Goal_
run the goal _Goal_ as top-level. The goal is converted from
an atom to a Prolog term.
+ -b _BOOT_FILE_
boot code is in Prolog file _BOOT_FILE_. The filename must define
the predicate `'$live'/0`.
3333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333connect standard streams to host <tt>IP_HOST</tt> at port <tt>port</tt>
+ filename
restore state saved in the given file
+ -f
do not consult initial files
+ -q
do not print informational messages
+ --
separator for arguments to Prolog code. These arguments are visible
through the unix/1 built-in predicate.
Note that YAP will output an error message on the following conditions:
+
a file name was given but the file does not exist or is not a saved
YAP state;
+
the necessary amount of memory could not be allocated;
+
the allocated memory is not enough to restore the state.
When restoring a saved state, YAP will allocate the
same amount of memory as that in use when the state was saved, unless a
different amount is specified by flags in the command line. By default,
YAP restores the file startup.yss from the current directory or from
the YAP library.
+
YAP usually boots from a saved state. The saved state will use the default
installation directory to search for the YAP binary unless you define
the environment variable YAPBINDIR.
+
YAP always tries to find saved states from the current directory
first. If it cannot it will use the environment variable YAPLIBDIR, if
defined, or search the default library directory.
YAP will try to find library files from the YAPSHAREDIR/library
directory.
@subpage Running_Prolog_Files Running Prolog Files
YAP can also be used to run Prolog files as scripts, at least in
Unix-like environments. A simple example is shown next (do not forget
that the shell comments are very important):
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#!/usr/local/bin/yap -L --
#
# Hello World script file using YAP
#
# put a dot because of syntax errors .
vvvvvvvvvvvvvvvvvvvvvvvvvvv :- write('Hello World'), nl.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The `#!` characters specify that the script should call the binary
file YAP. Notice that many systems will require the complete path to the
YAP binary. The `-L` flag indicates that YAP should consult the
current file when booting and then halt. The remaining arguments are
then passed to YAP. Note that YAP will skip the first lines if they
start with `#` (the comment sign for Unix's shell). YAP will
consult the file and execute any commands.
A slightly more sophisticated example is:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#!/usr/bin/yap -L --
#
# Hello Wor ld script file using YAP
# .
:- initialization(main).
main :- write('Hello World'), nl.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The `initialization` directive tells YAP to execute the goal main
after consulting the file. Source code is thus compiled and `main`
executed at the end. The `.` is useful while debugging the script
as a Prolog program: it guarantees that the syntax error will not
propagate to the Prolog code.
Notice that the `--` is required so that the shell passes the extra
arguments to YAP. As an example, consider the following script
`dump_args`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#!/usr/bin/yap -L --
#.
main( [] ).
main( [H|T] ) :-
write( H ), nl,
main( T ).
:- unix( argv(AllArgs) ), main( AllArgs ).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you this run this script with the arguments:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./dump_args -s 10000
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
the script will start an YAP process with stack size `10MB`, and
the list of arguments to the process will be empty.
Often one wants to run the script as any other program, and for this it
is convenient to ignore arguments to YAP. This is possible by using
`L --` as in the next version of `dump_args`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#!/usr/bin/yap -L --
main( [] ).
main( [H|T] ) :-
write( H ), nl,
main( T ).
:- unix( argv(AllArgs) ), main( AllArgs ).
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The `--` indicates the next arguments are not for YAP. Instead,
they must be sent directly to the argv built-in. Hence, running
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./dump_args test
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
will write `test` on the standard output.

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@ingroup extensions
YAP supports attributed variables, originally developed at OFAI by
Christian Holzbaur. Attributes are a means of declaring that an
arbitrary term is a property for a variable. These properties can be
updated during forward execution. Moreover, the unification algorithm is
aware of attributed variables and will call user defined handlers when
trying to unify these variables.
Attributed variables provide an elegant abstraction over which one can
extend Prolog systems. Their main application so far has been in
implementing constraint handlers, such as Holzbaur's CLPQR, Fruewirth
and Holzbaur's CHR, and CLP(BN).
Different Prolog systems implement attributed variables in different
ways. Originally, YAP used the interface designed by SICStus
Prolog. This interface is still
available through the <tt>atts</tt> library, and is used by CLPBN.
From YAP-6.0.3 onwards we recommend using the hProlog, SWI style
interface. We believe that this design is easier to understand and
work with. Most packages included in YAP that use attributed
variables, such as CHR, CLP(FD), and CLP(QR), rely on the SWI-Prolog
interface.
+ @ref SICS_attributes
+ @ref sicsatts
+ @ref New_Style_Attribute_Declarations
+ @ref AttributedVariables_Builtins
+ @ref corout
### SICStus Style attribute declarations. {#SICS_attributes}
The YAP library `atts` implements attribute variables in the style of
SICStus Prolog. Attributed variables work as follows:
+ Each attribute must be declared beforehand. Attributes are described
as a functor with name and arity and are local to a module. Each
Prolog module declares its own sets of attributes. Different modules
may have attributes with the same name and arity.
+ The built-in put_atts/2 adds or deletes attributes to a
variable. The variable may be unbound or may be an attributed
variable. In the latter case, YAP discards previous values for the
attributes.
+ The built-in get_atts/2 can be used to check the values of
an attribute associated with a variable.
+ The unification algorithm calls the user-defined predicate
verify_attributes/3 before trying to bind an attributed
variable. Unification will resume after this call.
+ The user-defined predicate
<tt>attribute_goal/2</tt> converts from an attribute to a goal.
+ The user-defined predicate
<tt>project_attributes/2</tt> is used from a set of variables into a set of
constraints or goals. One application of <tt>project_attributes/2</tt> is in
the top-level, where it is used to output the set of
floundered constraints at the end of a query.
Attributes are compound terms associated with a variable. Each attribute
has a <em>name</em> which is <em>private</em> to the module in which the
attribute was defined. Variables may have at most one attribute with a
name. Attribute names are defined through the following declaration:
~~~~~
:- attribute AttributeSpec, ..., AttributeSpec.
~~~~~
where each _AttributeSpec_ has the form ( _Name_/ _Arity_).
One single such declaration is allowed per module _Module_.
Although the YAP module system is predicate based, attributes are local
to modules. This is implemented by rewriting all calls to the
built-ins that manipulate attributes so that attribute names are
preprocessed depending on the module. The `user:goal_expansion/3`
mechanism is used for this purpose.
The attribute manipulation predicates always work as follows:
+ The first argument is the unbound variable associated with
attributes,
+ The second argument is a list of attributes. Each attribute will
be a Prolog term or a constant, prefixed with the <tt>+</tt> and <tt>-</tt> unary
operators. The prefix <tt>+</tt> may be dropped for convenience.
The following three procedures are available to the user. Notice that
these built-ins are rewritten by the system into internal built-ins, and
that the rewriting process <em>depends</em> on the module on which the
built-ins have been invoked.
The user-predicate predicate verify_attributes/3 is called when
attempting to unify an attributed variable which might have attributes
in some _Module_.
Attributes are usually presented as goals. The following routines are
used by built-in predicates such as call_residue/2 and by the
Prolog top-level to display attributes:
Constraint solvers must be able to project a set of constraints to a set
of variables. This is useful when displaying the solution to a goal, but
may also be used to manipulate computations. The user-defined
project_attributes/2 is responsible for implementing this
projection.
The following examples are taken from the SICStus Prolog
manual. The sketches the implementation of a simple finite domain
`solver`. Note that an industrial strength solver would have to
provide a wider range of functionality and that it quite likely would
utilize a more efficient representation for the domains proper. The
module exports a single predicate `domain( _-Var_, _?Domain_)` which
associates _Domain_ (a list of terms) with _Var_. A variable can be
queried for its domain by leaving _Domain_ unbound.
We do not present here a definition for project_attributes/2.
Projecting finite domain constraints happens to be difficult.
~~~~~
:- module(domain, [domain/2]).
:- use_module(library(atts)).
:- use_module(library(ordsets), [
ord_intersection/3,
ord_intersect/2,
list_to_ord_set/2
]).
:- attribute dom/1.
verify_attributes(Var, Other, Goals) :-
get_atts(Var, dom(Da)), !, % are we involved?
( var(Other) -> % must be attributed then
( get_atts(Other, dom(Db)) -> % has a domain?
ord_intersection(Da, Db, Dc),
Dc = [El|Els], % at least one element
( Els = [] -> % exactly one element
Goals = [Other=El] % implied binding
; Goals = [],
put_atts(Other, dom(Dc))% rescue intersection
)
; Goals = [],
put_atts(Other, dom(Da)) % rescue the domain
)
; Goals = [],
ord_intersect([Other], Da) % value in domain?
).
verify_attributes(_, _, []). % unification triggered
% because of attributes
% in other modules
attribute_goal(Var, domain(Var,Dom)) :- % interpretation as goal
get_atts(Var, dom(Dom)).
domain(X, Dom) :-
var(Dom), !,
get_atts(X, dom(Dom)).
domain(X, List) :-
list_to_ord_set(List, Set),
Set = [El|Els], % at least one element
( Els = [] -> % exactly one element
X = El % implied binding
; put_atts(Fresh, dom(Set)),
X = Fresh % may call
% verify_attributes/3
).
~~~~~
Note that the _implied binding_ `Other=El` was deferred until after
the completion of `verify_attribute/3`. Otherwise, there might be a
danger of recursively invoking `verify_attribute/3`, which might bind
`Var`, which is not allowed inside the scope of `verify_attribute/3`.
Deferring unifications into the third argument of `verify_attribute/3`
effectively serializes the calls to `verify_attribute/3`.
Assuming that the code resides in the file domain.yap, we
can use it via:
~~~~~
| ?- use_module(domain).
~~~~~
Let's test it:
~~~~~
| ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]).
domain(X,[1,5,6,7]),
domain(Y,[3,4,5,6]),
domain(Z,[1,6,7,8]) ?
yes
| ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]),
X=Y.
Y = X,
domain(X,[5,6]),
domain(Z,[1,6,7,8]) ?
yes
| ?- domain(X,[5,6,7,1]), domain(Y,[3,4,5,6]), domain(Z,[1,6,7,8]),
X=Y, Y=Z.
X = 6,
Y = 6,
Z = 6
~~~~~
To demonstrate the use of the _Goals_ argument of
verify_attributes/3, we give an implementation of
freeze/2. We have to name it `myfreeze/2` in order to
avoid a name clash with the built-in predicate of the same name.
~~~~~
:- module(myfreeze, [myfreeze/2]).
:- use_module(library(atts)).
:- attribute frozen/1.
verify_attributes(Var, Other, Goals) :-
get_atts(Var, frozen(Fa)), !, % are we involved?
( var(Other) -> % must be attributed then
( get_atts(Other, frozen(Fb)) % has a pending goal?
-> put_atts(Other, frozen((Fa,Fb))) % rescue conjunction
; put_atts(Other, frozen(Fa)) % rescue the pending goal
),
Goals = []
; Goals = [Fa]
).
verify_attributes(_, _, []).
attribute_goal(Var, Goal) :- % interpretation as goal
get_atts(Var, frozen(Goal)).
myfreeze(X, Goal) :- put_atts(Fresh, frozen(Goal)), Fresh = X. ~~~~~
Assuming that this code lives in file myfreeze.yap,
we would use it via:
~~~~~
| ?- use_module(myfreeze).
| ?- myfreeze(X,print(bound(x,X))), X=2.
bound(x,2) % side effect
X = 2 % bindings
~~~~~
The two solvers even work together:
~~~~~
| ?- myfreeze(X,print(bound(x,X))), domain(X,[1,2,3]),
domain(Y,[2,10]), X=Y.
bound(x,2) % side effect
X = 2, % bindings
Y = 2
~~~~~
The two example solvers interact via bindings to shared attributed
variables only. More complicated interactions are likely to be found
in more sophisticated solvers. The corresponding
verify_attributes/3 predicates would typically refer to the
attributes from other known solvers/modules via the module prefix in
Module:get_atts/2`.
@}
@{
### hProlog and SWI-Prolog style Attribute Declarations {#New_Style_Attribute_Declarations}
The following documentation is taken from the SWI-Prolog manual.
Binding an attributed variable schedules a goal to be executed at the
first possible opportunity. In the current implementation the hooks are
executed immediately after a successful unification of the clause-head
or successful completion of a foreign language (built-in) predicate. Each
attribute is associated to a module and the hook attr_unify_hook/2 is
executed in this module. The example below realises a very simple and
incomplete finite domain reasoner.
~~~~~
:- module(domain,
[ domain/2 % Var, ?Domain %
]).
:- use_module(library(ordsets)).
domain(X, Dom) :-
var(Dom), !,
get_attr(X, domain, Dom).
domain(X, List) :-
list_to_ord_set(List, Domain),
v put_attr(Y, domain, Domain),
X = Y.
% An attributed variable with attribute value Domain has been %
% assigned the value Y %
attr_unify_hook(Domain, Y) :-
( get_attr(Y, domain, Dom2)
-> ord_intersection(Domain, Dom2, NewDomain),
( NewDomain == []
-> fail
; NewDomain = [Value]
-> Y = Value
; put_attr(Y, domain, NewDomain)
)
; var(Y)
-> put_attr( Y, domain, Domain )
; ord_memberchk(Y, Domain)
).
% Translate attributes from this module to residual goals %
attribute_goals(X) -->
{ get_attr(X, domain, List) },
[domain(X, List)].
~~~~~
Before explaining the code we give some example queries:
The predicate `domain/2` fetches (first clause) or assigns
(second clause) the variable a <em>domain</em>, a set of values it can
be unified with. In the second clause first associates the domain
with a fresh variable and then unifies X to this variable to deal
with the possibility that X already has a domain. The
predicate attr_unify_hook/2 is a hook called after a variable with
a domain is assigned a value. In the simple case where the variable
is bound to a concrete value we simply check whether this value is in
the domain. Otherwise we take the intersection of the domains and either
fail if the intersection is empty (first example), simply assign the
value if there is only one value in the intersection (second example) or
assign the intersection as the new domain of the variable (third
example). The nonterminal `attribute_goals/3` is used to translate
remaining attributes to user-readable goals that, when executed, reinstate
these attributes.
@}
@{
### Co-routining {#CohYroutining}
Prolog uses a simple left-to-right flow of control. It is sometimes
convenient to change this control so that goals will only execute when
sufficiently instantiated. This may result in a more "data-driven"
execution, or may be necessary to correctly implement extensions such
as negation by failure.
Initially, YAP used a separate mechanism for co-routining. Nowadays, YAP uses
attributed variables to implement co-routining.
Two declarations are supported:
+ block/1
The argument to `block/1` is a condition on a goal or a conjunction
of conditions, with each element separated by commas. Each condition is
of the form `predname( _C1_,..., _CN_)`, where _N_ is the
arity of the goal, and each _CI_ is of the form `-`, if the
argument must suspend until the first such variable is bound, or
`?`, otherwise.
+ wait/1
The argument to `wait/1` is a predicate descriptor or a conjunction
of these predicates. These predicates will suspend until their first
argument is bound.
The following primitives can be used:
- freeze/2
- dif/2
- when/2
- frozen/2
@}
@}

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YAP Core Built-ins {#core}
=================
This chapter describes the core predicates that control the execution of
Prolog programs, provide fundamental functionality such as termm manipulation or arithmetic, and support interaction with external
resources, Many of the predicates described here have been standardised by the ISO. The standartised subset of Prolog also known as ISO-Prolog.
In the description of the arguments of predicates the following
notation will be used:
+ a preceding plus sign will denote an argument as an "input
argument" - it cannot be a free variable at the time of the call;
+ a preceding minus sign will denote an "output argument";
+ an argument with no preceding symbol can be used in both ways.
@copydoc builtins
@{
@defgroup builtins YAP Core Builtins:
@}

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@page Library YAP Library
the library_directory path (set by the
`LIBDIR` variable in the Makefile for YAP). Several files in the
library are originally from the public-domain Edinburgh Prolog library.