yap-6.3/pl/attributes.md

@{

@defgroup AttributedVariables Attributed Variables and Co-Routining @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 atts 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 attributes
• @ref New_Style_Attribute_Declarations
• @ref CohYroutining
• @ref AttributeVariables_Builtins

@{

@defgroup attributes SICStus Style attribute declarations. @ingroup AttributedVariables

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 attribute_goal/2 converts from an attribute to a goal.

• The user-defined predicate project_attributes/2 is used from a set of variables into a set of constraints or goals. One application of project_attributes/2 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 name which is private 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 + and - unary operators. The prefix + 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 depends 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`.

@}

@{

@defgroup New_Style_Attribute_Declarations hProlog and SWI-Prolog style Attribute Declarations
@ingroup AttributedVariables

  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), 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.

@}


@{
@defgroup CohYroutining Co-routining
@ingroup AttributedVariables

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


@}

@}