Predicates¶
Predicate directives and clauses can be encapsulated inside objects and categories. Protocols can only contain predicate directives. From the point-of-view of a traditional imperative object-oriented language, predicates allows both object state and object behavior to be represented. Mutable object state can be represented using dynamic object predicates.
Reserved predicate names¶
For practical and performance reasons, a few predicates have a fixed interpretation. These predicates are declared in the built-protocols. They are: goal_expansion/2 and term_expansion/2, declared in the expanding protocol; before/3 and after/3, declared in the monitoring protocol; and forward/1, declared in the forwarding protocol. By default, the compiler prints a warning when a definition for one of these predicates is found but the reference to the corresponding built-in protocol is missing.
Declaring predicates¶
All object (or category) predicates that we want to access from other objects (or categories) must be explicitly declared. A predicate declaration must contain, at least, a scope directive. Other directives may be used to document the predicate or to ensure proper compilation of the predicate definitions.
Scope directives¶
A predicate scope directive specifies from where the predicate can be
called, i.e. its visibility. Predicates can be public, protected,
private, or local. Public predicates can be called from any object.
Protected predicates can only be called from the container object or
from a container descendant. Private predicates can only be called from
the container object. Local predicates, like private predicates, can
only be called from the container object (or category) but they are
invisible to the reflection built-in methods (current_op/3,
current_predicate/1, and predicate_property/2)
and to the message error handling mechanisms (i.e. sending a message corresponding
to a local predicate results in a predicate_declaration
existence
error instead of a scope error).
The scope declarations are made using the directives public/1, protected/1, and private/1. For example:
:- public(init/1).
:- protected(valid_init_option/1).
:- private(process_init_options/1).
If a predicate does not have a (local or inherited) scope declaration, it is assumed that the predicate is local. Note that we do not need to write scope declarations for all defined predicates. One exception is local dynamic predicates: declaring them as private predicates may allow the Logtalk compiler to generate optimized code for asserting and retracting clauses.
Note that a predicate scope directive doesn’t specify where a predicate is, or can be, defined. For example, a private predicate can only be called from an object holding its scope directive. But it can be defined in descendant objects. A typical example is an object playing the role of a class defining a private (possibly dynamic) predicate for its descendant instances. Only the class can call (and possibly assert/retract clauses for) the predicate but its clauses can be found/defined in the instances themselves.
Mode directive¶
Often predicates can only be called using specific argument patterns. The valid arguments and instantiation modes of those arguments can be documented by using the mode/2 directive. For example:
:- mode(member(?term, ?list), zero_or_more).
The first directive argument describes a valid calling mode. The minimum information will be the instantiation mode of each argument. The first four possible values are described in [ISO95]). The remaining two can also be found in use in some Prolog systems.
+
- Argument must be instantiated (but not necessarily ground).
-
- Argument should be a free (non-instantiated) variable (when bound, the call will unify the returned term with the given term).
?
- Argument can either be instantiated or free.
@
- Argument will not be further instantiated (modified).
++
- Argument must be ground.
--
- Argument must be unbound.
These six mode atoms are also declared as prefix operators by the
Logtalk compiler. This makes it possible to include type information
for each argument like in the example above. Some possible type
values are: event
, object
, category
, protocol
,
callable
, term
, nonvar
, var
, atomic
, atom
,
number
, integer
, float
, compound
, and list
. The
first four are Logtalk specific. The remaining are common Prolog types.
We can also use our own types that can be either atoms or ground
compound terms.
The second directive argument documents the number of proofs (but not necessarily distinct solutions) for the specified mode. Note that different modes for the same predicate often have different determinism. The possible values are:
zero
- Predicate always fails.
one
- Predicate always succeeds once.
zero_or_one
- Predicate either fails or succeeds.
zero_or_more
- Predicate has zero or more proofs.
one_or_more
- Predicate has one or more proofs.
one_or_error
- Predicate either succeeds once or throws an error (see below).
error
- Predicate will throw an error (see below).
Mode declarations can also be used to document that some call modes will
throw an error. For instance, regarding the arg/3
and open/3
ISO
Prolog built-in predicates, we may write:
:- mode(arg(-, -, +), error).
:- mode(open(@, @, --), one_or_error).
Note that most predicates have more than one valid mode implying several
mode directives. For example, to document the possible use modes of the
atom_concat/3
ISO built-in predicate we would write:
:- mode(atom_concat(?atom, ?atom, +atom), one_or_more).
:- mode(atom_concat(+atom, +atom, -atom), zero_or_one).
Some old Prolog compilers supported some sort of mode directives to improve performance. To the best of my knowledge, there is no modern Prolog compiler supporting this kind of directive for that purpose. The current Logtalk version simply parses this directive for collecting its information for use in the reflection API (assuming the source_data is turned on). But see also see the description on synchronized predicates in the multi-threading programming section). In any case, the use of mode directives is a good starting point for documenting your predicates.
Meta-predicate directive¶
Some predicates may have arguments that will be called as goals or interpreted as closures that will be used for constructing goals. To ensure that these goals will be executed in the correct context (i.e. in the calling context, not in the meta-predicate definition context) we need to use the meta_predicate/1 directive. For example:
:- meta_predicate(findall(*, 0, *)).
The meta-predicate mode arguments in this directive have the following meaning:
0
- Meta-argument that will be called as a goal.
N
- Meta-argument that will be a closure used to construct a call by
appending
N
arguments. The value ofN
must be a positive integer. ::
- Argument that is context-aware but that will not be used as a goal or a closure.
^
- Goal that may be existentially quantified (
Vars^Goal
). *
- Normal argument.
The following meta-predicate mode arguments are for use only when writing backend Prolog adapter files to deal with proprietary built-in meta-predicates and meta-directives:
/
- Predicate indicator (
Name/Arity
), list of predicate indicators, or conjunction of predicate indicators. //
- Non-terminal indicator (
Name//Arity
), list of predicate indicators, or conjunction of predicate indicators. [0]
- List of goals.
[N]
- List of closures.
[/]
- List of predicate indicators.
[//]
- List of non-terminal indicators.
To the best of my knowledge, the use of non-negative integers to specify closures has first introduced on Quintus Prolog for providing information for predicate cross-reference tools.
As each Logtalk entity is independently compiled, this directive must be included in every object or category that contains a definition for the described meta-predicate, even if the meta-predicate declaration is inherited from another entity, to ensure proper compilation of meta-arguments.
Discontiguous directive¶
The clause of an object (or category) predicate may not be contiguous. In that case, we must declare the predicate discontiguous by using the discontiguous/1 directive:
:- discontiguous(foo/1).
This is a directive that we should avoid using: it makes your code harder to read and it is not supported by some Prolog compilers.
As each Logtalk entity is compiled independently of other entities, this directive must be included in every object or category that contains a definition for the described predicate (even if the predicate declaration is inherited from other entity).
Dynamic directive¶
An object predicate can be static or dynamic. By default, all object predicates are static. To declare a dynamic predicate we use the dynamic/1 directive:
:- dynamic(foo/1).
This directive may also be used to declare dynamic grammar rule non-terminals. As each Logtalk entity is compiled independently from other entities, this directive must be included in every object that contains a definition for the described predicate (even if the predicate declaration is inherited from other object or imported from a category). If we omit the dynamic declaration then the predicate definition will be compiled static. In the case of dynamic objects, static predicates cannot be redefined using the database built-in methods (despite being internally compiled to dynamic code).
Dynamic predicates can be used to represent persistent mutable object state. Note that static objects may declare and define dynamic predicates.
Operator directive¶
An object (or category) predicate can be declared as an operator using the familiar op/3 directive:
:- op(Priority, Specifier, Operator).
Operators are local to the object (or category) where they are declared. This means that, if you declare a public predicate as an operator, you cannot use operator notation when sending to an object (where the predicate is visible) the respective message (as this would imply visibility of the operator declaration in the context of the sender of the message). If you want to declare global operators and, at the same time, use them inside an entity, just write the corresponding directives at the top of your source file, before the entity opening directive.
When the same operators are used on several entities within the same source file, the corresponding directives must appear before any entity that uses them. However, this results in a global scope for the operators. If you prefer the operators to be local to the source file, just undefine them at the end of the file. For example:
% before any entity that uses the operator
:- op(400, xfx, results).
...
% after all entities that used the operator
:- op(0, xfx, results).
Uses directive¶
When a predicate makes heavy use of predicates defined on other objects, its predicate clauses can be verbose due to all the necessary message sending goals. Consider the following example:
foo :-
...,
findall(X, list::member(X, L), A),
list::append(A, B, C),
list::select(Y, C, R),
...
Logtalk provides a directive, uses/2, which allows us to simplify the code above. The usage template for this directive is:
:- uses(Object, [
Name1/Arity1, Name2/Arity2, ...
]).
Rewriting the code above using this directive results in a simplified and more readable predicate definition:
:- uses(list, [
append/3, member/2, select/3
]).
foo :-
...,
findall(X, member(X, L), A),
append(A, B, C),
select(Y, C, R),
...
Logtalk also supports an extended version of this directive that allows
the declaration of predicate aliases using the notation
Predicate as Alias
(or the alternative notation
Predicate::Alias
). For example:
:- uses(btrees, [new/1 as new_btree/1]).
:- uses(queues, [new/1 as new_queue/1]).
You may use this extended version for solving conflicts between
predicates declared on several uses/2
directives or just for giving
new names to the predicates that will be more meaningful on their using
context.
The uses/2
directive allows simpler predicate definitions as long as
there are no conflicts between the predicates declared in the directive
and the predicates defined in the object (or category) containing the
directive. A predicate (or its alias if defined) cannot be listed in
more than one uses/2
directive. In addition, a uses/2
directive
cannot list a predicate (or its alias if defined) which is defined in
the object (or category) containing the directive. Any conflicts are
reported by Logtalk as compilation errors.
Alias directive¶
Logtalk allows the definition of an alternative name for an inherited or imported predicate (or for an inherited or imported grammar rule non-terminal) through the use of the alias/2 directive:
:- alias(Entity, [
Predicate1 as Alias1,
Predicate2 as Alias2,
...
]).
This directive can be used in objects, protocols, or categories. The
first argument, Entity
, must be an entity referenced in the opening
directive of the entity containing the alias/2
directive. It can be
an extended or implemented protocol, an imported category, an extended
prototype, an instantiated class, or a specialized class. The second
argument is a list of pairs of predicate indicators (or grammar rule
non-terminal indicators) using the as
infix operator as connector.
A common use for the alias/2
directive is to give an alternative
name to an inherited predicate in order to improve readability. For
example:
:- object(square,
extends(rectangle)).
:- alias(rectangle, [width/1 as side/1]).
...
:- end_object.
The directive allows both width/1
and side/1
to be used as
messages to the object square
. Thus, using this directive, there is
no need to explicitly declare and define a “new” side/1
predicate.
Note that the alias/2
directive does not rename a predicate, only
provides an alternative, additional name; the original name continues to
be available (although it may be masked due to the default inheritance
conflict mechanism).
Another common use for this directive is to solve conflicts when two
inherited predicates have the same functor and arity. We may want to
call the predicate which is masked out by the Logtalk lookup algorithm
(see the Inheritance section) or we may need to
call both predicates. This is simply accomplished by using the
alias/2
directive to give alternative names to masked out or
conflicting predicates. Consider the following example:
:- object(my_data_structure,
extends(list, set)).
:- alias(list, [member/2 as list_member/2]).
:- alias(set, [member/2 as set_member/2]).
...
:- end_object.
Assuming that both list
and set
objects define a member/2
predicate, without the alias/2
directives, only the definition of
member/2
predicate in the object list
would be visible on the
object my_data_structure
, as a result of the application of the
Logtalk predicate lookup algorithm. By using the alias/2
directives,
all the following messages would be valid (assuming a public scope for
the predicates):
% uses list member/2
| ?- my_data_structure::list_member(X, L).
% uses set member/2
| ?- my_data_structure::set_member(X, L).
% uses list member/2
| ?- my_data_structure::member(X, L).
When used this way, the alias/2
directive provides functionality
similar to programming constructs of other object-oriented languages
that support multi-inheritance (the most notable example probably being
the renaming of inherited features in Eiffel).
Note that the alias/2
directive never hides a predicate which is
visible on the entity containing the directive as a result of the
Logtalk lookup algorithm. However, it may be used to make visible a
predicate which otherwise would be masked by another predicate, as
illustrated in the above example.
The alias/2
directive may also be used to give access to an
inherited predicate, which otherwise would be masked by another
inherited predicate, while keeping the original name as follows:
:- object(my_data_structure,
extends(list, set)).
:- alias(list, [member/2 as list_member/2]).
:- alias(set, [member/2 as set_member/2]).
member(X, L) :-
::set_member(X, L).
...
:- end_object.
Thus, when sending the message member/2
to my_data_structure
,
the predicate definition in set
will be used instead of the one
contained in list
.
Documenting directive¶
A predicate can be documented with arbitrary user-defined information by using the info/2 directive:
:- info(Name/Arity, List).
The second argument is a list of Key is Value
terms. See the
Documenting section for details.
Multifile directive¶
A predicate can be declared multifile by using the multifile/1 directive:
:- multifile(Name/Arity).
This allows clauses for a predicate to be defined in several objects
and/or categories. This is a directive that should be used with care.
Support for this directive have been added to Logtalk primarily to
support migration of Prolog module code. Spreading clauses for a
predicate among several Logtalk entities can be handy in some cases but
can also make your code difficult to understand. Logtalk precludes using
a multifile predicate for breaking object encapsulation by checking that
the object (or category) declaring the predicate (using a scope
directive) defines it also as multifile. This entity is said to contain
the primary declaration for the multifile predicate. In addition, note
that the multifile/1
directive is mandatory when defining multifile
predicates.
Consider the following simple example:
:- object(main).
:- public(a/1).
:- multifile(a/1).
a(1).
:- end_object.
After compiling and loading the main
object, we can define other
objects (or categories) that contribute with clauses for the multifile
predicate. For example:
:- object(other).
:- multifile(main::a/1).
main::a(2).
main::a(X) :-
b(X).
b(3).
b(4).
:- end_object.
After compiling and loading the above objects, you can use queries such as:
| ?- main::a(X).
X = 1 ;
X = 2 ;
X = 3 ;
X = 4
yes
Entities containing primary multifile predicate declarations must always be compiled before entities defining clauses for those multifile predicates. The Logtalk compiler will print a warning if the scope directive is missing.
Multifile predicates may also be declared dynamic using the same
Entity::Name/Arity
notation (multifile predicates are static by
default).
When a clause of a multifile predicate is a rule, its body is compiled
within the context of the object or category defining the clause. This
allows clauses for multifile predicates to call local object or category
predicates. But the values of the sender, this, and self in the
implicit execution context are passed from the clause head to the clause
body. This is necessary to ensure that these values are always valid and
to allow multifile predicate clauses to be defined in categories. A call
to the parameter/2
execution context methods, however, retrieves
parameters of the entity defining the clause, not from the entity for
which the clause is defined. The parameters of the entity for which the
clause is defined can be accessed by simple unification at the clause
head.
Local calls to the database methods from multifile predicate clauses
defined in an object take place in the object own database instead of
the database of the entity holding the multifile predicate primary
declaration. Similarly, local calls to the expand_term/2
and
expand_goal/2
methods from a multifile predicate clause look for
clauses of the term_expansion/2
and goal_expansion/2
hook
predicates starting from the entity defining the clause instead of the
entity holding the multifile predicate primary declaration. Local calls
to the current_predicate/1
, predicate_property/2
, and
current_op/3
methods from multifile predicate clauses defined in an
object also lookup predicates and their properties in the object own
database instead of the database of the entity holding the multifile
predicate primary declaration.
Coinductive directive¶
A predicate can be declared coinductive by using the coinductive/1 directive. For example:
:- coinductive(comember/2).
Logtalk support for coinductive predicates is experimental and requires a
backend Prolog compiler with minimal support for cyclic terms. The
value of the read-only coinduction flag is set to
supported
for the backend Prolog compilers providing that support.
Defining predicates¶
Object predicates¶
We define object predicates as we have always defined Prolog predicates,
the only difference be that we have four more control structures (the
three message sending operators plus the external call operator) to play
with. For example, if we wish to define an object containing common
utility list predicates like append/2
or member/2
we could write
something like:
:- object(list).
:- public(append/3).
:- public(member/2).
append([], L, L).
append([H| T], L, [H| T2]) :-
append(T, L, T2).
member(H, [H| _]).
member(H, [_| T]) :-
member(H, T).
:- end_object.
Note that, abstracting from the opening and closing object directives and the scope directives, what we have written is also valid Prolog code. Calls in a predicate definition body default to the local predicates, unless we use the message sending operators or the external call operator. This enables easy conversion from Prolog code to Logtalk objects: we just need to add the necessary encapsulation and scope directives to the old code.
Category predicates¶
Because a category can be imported by multiple objects, dynamic private predicates must be called either in the context of self, using the message to self control structure, ::/1, or in the context of this (i.e. in the context of the object importing the category). For example, if we want to define a category implementing variables using destructive assignment where the variable values are stored in self we could write:
:- category(variable).
:- public(get/2).
:- public(set/2).
:- private(value_/2).
:- dynamic(value_/2).
get(Var, Value) :-
::value_(Var, Value).
set(Var, Value) :-
::retractall(value_(Var, _)),
::asserta(value_(Var, Value).
:- end_category.
In this case, the get/2
and set/2
predicates will always
access/update the correct definition, contained in the object receiving
the messages. The alternative, storing the variable values in this,
such that each object importing the category will have its own
definition for the value_/2
private predicate is simple: just omit
the use of the ::/1
control construct in the code above.
A category can only contain clauses for static predicates. Nevertheless, as the example above illustrates, there are no restrictions in declaring and calling dynamic predicates from inside a category.
Meta-predicates¶
Meta-predicates may be defined inside objects (and categories) as any other predicate. A meta-predicate is declared using the meta_predicate/1 directive as described earlier on this section. When defining a meta-predicate, the arguments in the clause heads corresponding to the meta-arguments must be variables. All meta-arguments are called in the context of the entity calling the meta-predicate.
Some meta-predicates have meta-arguments which are not goals but closures. Logtalk supports the definition of meta-predicates that are called with closures instead of goals as long as the definition uses the call/1-N built-in predicate to call the closure with the additional arguments. For example:
:- public(all_true/2).
:- meta_predicate(all_true(1, *)).
all_true(_, []).
all_true(Closure, [Arg| Args]) :-
call(Closure, Arg),
all_true(Closure, Args).
Note that in this case the meta-predicate directive specifies that the closure will be extended with exactly one extra argument.
When calling a meta-predicate, a closure can correspond to a user-defined predicate, a built-in predicate, a lambda expression, or a control construct.
Lambda expressions¶
The use of lambda expressions as meta-predicate goal and closure arguments often saves writing auxiliary predicates for the sole purpose of calling the meta-predicates. A simple example of a lambda expression is:
| ?- meta::map([X,Y]>>(Y is 2*X), [1,2,3], Ys).
Ys = [2,4,6]
yes
In this example, a lambda expression, [X,Y]>>(Y is 2*X)
, is used as
an argument to the map/3
list mapping predicate, defined in the
library object meta
, in order to double the elements of a list of
integers. Using a lambda expression avoids writing an auxiliary
predicate for the sole purpose of doubling the list elements. The lambda
parameters are represented by the list [X,Y]
, which is connected to
the lambda goal, (Y is 2*X)
, by the (>>)/2
operator.
Currying is supported. I.e. it is possible to write a lambda expression whose goal is another lambda expression. The above example can be rewritten as:
| ?- meta::map([X]>>([Y]>>(Y is 2*X)), [1,2,3], Ys).
Ys = [2,4,6]
yes
Lambda expressions may also contain lambda free variables. I.e. variables that are global to the lambda expression. For example, using GNU Prolog as the backend compiler, we can write:
| ?- meta::map({Z}/[X,Y]>>(Z#=X+Y), [1,2,3], Zs).
Z = _#22(3..268435455)
Zs = [_#3(2..268435454),_#66(1..268435453),_#110(0..268435452)]
yes
The ISO Prolog construct {}/1
for representing the lambda free
variables as this representation is often associated with set
representation. Note that the order of the free variables is of no
consequence (on the other hand, a list is used for the lambda parameters
as their order does matter).
Both lambda free variables and lambda parameters can be any Prolog term. Consider the following example by Markus Triska:
| ?- meta::map([A-B,B-A]>>true, [1-a,2-b,3-c], Zs).
Zs = [a-1,b-2,c-3]
yes
Lambda expressions can be used, as expected, in non-deterministic queries as in the following example using SWI-Prolog as the backend compiler and Markus Triska’s CLP(FD) library:
| ?- meta::map({Z}/[X,Y]>>(clpfd:(Z#=X+Y)), Xs, Ys).
Xs = [],
Ys = [] ;
Xs = [_G1369],
Ys = [_G1378],
_G1369+_G1378#=Z ;
Xs = [_G1579, _G1582],
Ys = [_G1591, _G1594],
_G1582+_G1594#=Z,
_G1579+_G1591#=Z ;
Xs = [_G1789, _G1792, _G1795],
Ys = [_G1804, _G1807, _G1810],
_G1795+_G1810#=Z,
_G1792+_G1807#=Z,
_G1789+_G1804#=Z ;
...
As illustrated by the above examples, lambda expression syntax reuses
the ISO Prolog construct {}/1
and the standard operators (/)/2
and (>>)/2
, thus avoiding defining new operators, which is always
tricky for a portable system such as Logtalk. The operator (>>)/2
was chosen as it suggests an arrow, similar to the syntax used in other
languages such as OCaml and Haskell to connect lambda parameters with
lambda functions. This syntax was also chosen in order to simplify
parsing, error checking, and compilation of lambda expressions. The full
specification of the lambda expression syntax can be found in the
Lambda expressions section of the language grammar.
The compiler checks whenever possible that all variables in a lambda expression are either classified as free variables or as lambda parameters. Non-classified variables in a lambda expression should be regarded as a programming error. Unfortunately, the dynamic features of the language and lack of sufficient information at compile time may prevent the compiler of checking all uses of lambda expressions. The compiler also checks if a variable is classified as both a free variable and a lambda parameter. An optimizing meta-predicate and lambda expression compiler, based on the term-expansion mechanism, is provided for practical performance by the standard library.
Definite clause grammar rules¶
Definite clause grammar rules provide a convenient notation to represent the rewrite rules common of most grammars in Prolog. In Logtalk, definite clause grammar rules can be encapsulated in objects and categories. Currently, the ISO/IEC WG17 group is working on a draft specification for a definite clause grammars Prolog standard. Therefore, in the mean time, Logtalk follows the common practice of Prolog compilers supporting definite clause grammars, extending it to support calling grammar rules contained in categories and objects. A common example of a definite clause grammar is the definition of a set of rules for parsing simple arithmetic expressions:
:- object(calculator).
:- public(parse/2).
parse(Expression, Value) :-
phrase(expr(Value), Expression).
expr(Z) --> term(X), "+", expr(Y), {Z is X + Y}.
expr(Z) --> term(X), "-", expr(Y), {Z is X - Y}.
expr(X) --> term(X).
term(Z) --> number(X), "*", term(Y), {Z is X * Y}.
term(Z) --> number(X), "/", term(Y), {Z is X / Y}.
term(Z) --> number(Z).
number(C) --> "+", number(C).
number(C) --> "-", number(X), {C is -X}.
number(X) --> [C], {0'0 =< C, C =< 0'9, X is C - 0'0}.
:- end_object.
The predicate phrase/2 called
in the definition of predicate parse/2
above is a Logtalk built-in
method, similar to the predicate with the same name found on most Prolog
compilers that support definite clause grammars. After compiling and
loading this object, we can test the grammar rules with calls such as
the following one:
| ?- calculator::parse("1+2-3*4", Result).
Result = -9
yes
In most cases, the predicates resulting from the translation of the
grammar rules to regular clauses are not declared. Instead, these
predicates are usually called by using the built-in methods
phrase/2 and phrase/3 as shown in the
example above. When we want to use the built-in methods phrase/2
and
phrase/3
, the non-terminal used as first argument must be within the
scope of the sender. For the above example, assuming that we want the
predicate corresponding to the expr//1
non-terminal to be public,
the corresponding scope directive would be:
:- public(expr//1).
The //
infix operator used above tells the Logtalk compiler that the
scope directive refers to a grammar rule non-terminal, not to a
predicate. The idea is that the predicate corresponding to the
translation of the expr//1
non-terminal will have a number of
arguments equal to one plus the number of additional arguments necessary
for processing the implicit difference list of tokens.
In the body of a grammar rule, we can call rules that are inherited from ancestor objects, imported from categories, or contained in other objects. This is accomplished by using non-terminals as messages. Using a non-terminal as a message to self allows us to call grammar rules in categories and ancestor objects. To call grammar rules encapsulated in other objects, we use a non-terminal as a message to those objects. Consider the following example, containing grammar rules for parsing natural language sentences:
:- object(sentence,
imports(determiners, nouns, verbs)).
:- public(parse/2).
parse(List, true) :-
phrase(sentence, List).
parse(_, false).
sentence --> noun_phrase, verb_phrase.
noun_phrase --> ::determiner, ::noun.
noun_phrase --> ::noun.
verb_phrase --> ::verb.
verb_phrase --> ::verb, noun_phrase.
:- end_object.
The categories imported by the object would contain the necessary grammar rules for parsing determiners, nouns, and verbs. For example:
:- category(determiners).
:- private(determiner//0).
determiner --> [the].
determiner --> [a].
:- end_category.
Along with the message sending operators (::/1
, ::/2
, and
^^/1
), we may also use other control constructs such as \+/1
,
!/0
, ;/2
, ->/2
, and {}/1
in the body of a grammar. In
addition, grammar rules may contain meta-calls (a variable taking the
place of a non-terminal), which are translated to calls of the built-in
method phrase/3
.
You may have noticed that Logtalk defines {}/1 as a control construct for bypassing the compiler when compiling a clause body goal. As exemplified above, this is the same control construct that is used in grammar rules for bypassing the expansion of rule body goals when a rule is converted into a clause. Both control constructs can be combined in order to call a goal from a grammar rule body, while bypassing at the same time the Logtalk compiler. Consider the following example:
bar :-
write('bar predicate called'), nl.
:- object(bypass).
:- public(foo//0).
foo --> {{bar}}.
:- end_object.
After compiling and loading this code, we may try the following query:
| ?- logtalk << phrase(bypass::foo, _, _).
bar predicate called
yes
This is the expected result as the expansion of the grammar rule into a
clause leaves the {bar}
goal untouched, which, in turn, is converted
into the goal bar
when the clause is compiled.
A grammar rule non-terminal may be declared as dynamic or discontiguous,
as any object predicate, using the same Name//Arity
notation
illustrated above for the scope directives. In addition, grammar rule
non-terminals can be documented using the info/2
directive, as in the following example:
:- public(sentence//0).
:- info(sentence//0, [
comment is 'Rewrites sentence into noun and verb phrases.']).
Built-in object predicates (methods)¶
Logtalk defines a set of built-in object predicates or methods to access message execution context, to find sets of solutions, to inspect objects, for database handling, for term and goal expansion, and for printing messages. Similar to Prolog built-in predicates, these built-in methods should not be redefined.
Execution context methods¶
Logtalk defines five built-in private methods to access an object execution context. These methods are in the common usage scenarios translated to a single unification performed at compile time with a clause head context argument. Therefore, they can be freely used without worrying about performance penalties. When called from inside a category, these methods refer to the execution context of the object importing the category. These methods are private and cannot be used as messages to objects.
To find the object that received the message under execution we may use the self/1 method. We may also retrieve the object that has sent the message under execution using the sender/1 method.
The method this/1 enables us to retrieve the name of the object for which the predicate clause whose body is being executed is defined instead of using the name directly. This helps to avoid breaking the code if we decide to change the object name and forget to change the name references. This method may also be used from within a category. In this case, the method returns the object importing the category on whose behalf the predicate clause is being executed.
Here is a short example including calls to these three object execution context methods:
:- object(test).
:- public(test/0).
test :-
this(This),
write('Calling predicate definition in '),
writeq(This), nl,
self(Self),
write('to answer a message received by '),
writeq(Self), nl,
sender(Sender),
write('that was sent by '),
writeq(Sender), nl, nl.
:- end_object.
:- object(descendant,
extends(test)).
:- end_object.
After compiling and loading these two objects, we can try the following goal:
| ?- descendant::test.
Calling predicate definition in test
to answer a message received by descendant
that was sent by user
yes
Note that the goals self(Self)
, sender(Sender)
, and
this(This)
, being translated to unifications with the clause head
context arguments at compile time, are effectively removed from the
clause body. Therefore, a clause such as:
predicate(Arg) :-
self(Self),
atom(Arg),
... .
is compiled with the goal atom(Arg)
as the first condition on the
clause body. As such, the use of these context execution methods do not
interfere with the optimizations that some Prolog compilers perform when
the first clause body condition is a call to a built-in type-test
predicate or a comparison operator.
For parametric objects and categories, the method parameter/2 enables us to retrieve current parameter values (see the section on parametric objects for a detailed description). For example:
:- object(block(_Color)).
:- public(test/0).
test :-
parameter(1, Color),
write('Color parameter value is '),
writeq(Color), nl.
:- end_object.
An alternative to the parameter/2
predicate is to use
parameter variables:
:- object(block(_Color_)).
:- public(test/0).
test :-
write('Color parameter value is '),
writeq(_Color_), nl.
:- end_object.
After compiling and loading either version of the object, we can try the following goal:
| ?- block(blue)::test.
Color parameter value is blue
yes
Calls to the parameter/2
method are translated to a compile time
unification when the second argument is a variable. When the second
argument is bound, the calls are translated to a call to the built-in
predicate arg/3
.
When type-checking predicate arguments, it is often useful to include
the predicate execution context when reporting an argument error. The
context/1 method provides
access to that context. For example, assume a predicate foo/2
that
takes an atom and an integer as arguments. We could type-check the
arguments by writing (using the library type
object):
foo(A, N) :-
% type-check arguments
context(Context),
type::check(atom, A, Context),
type::check(integer, N, Context),
% arguments are fine; go ahead
... .
Error handling and throwing methods¶
Besides the catch/3 and throw/1 methods inherited from
Prolog, Logtalk also provides a set of convenience methods to throw
standard error/2
exception terms:
instantiation_error/0,
type_error/2,
domain_error/2,
existence_error/2,
permission_error/3,
representation_error/1,
evaluation_error/1,
resource_error/1,
syntax_error/1, and
system_error/0.
Database methods¶
Logtalk provides a set of built-in methods for object database handling
similar to the usual database Prolog predicates:
abolish/1,
asserta/1,
assertz/1,
clause/2,
retract/1, and
retractall/1. These
methods always operate on the database of the object receiving the corresponding
message. When called locally, these predicates take into account any
uses/2 or use_module/2 directives that refer
to the dynamic predicate being handled. For example, in the following object, the
clauses for the data/1
predicate are retracted and asserted in user due to
the uses/2
directive:
:- object(an_object).
:- uses(user, [data/1]).
:- public(some_predicate/1).
some_predicate(Arg) :-
retractall(data(_)),
assertz(data(Arg)).
:- end_object.
When working with dynamic grammar rule non-terminals, you may use the built-in method expand_term/2 convert a grammar rule into a clause that can then be used with the database methods.
Meta-call methods¶
Logtalk supports the generalized call/1-N meta-predicate. This built-in private meta-predicate must be used in the implementation of meta-predicates which work with closures instead of goals. In addition, Logtalk supports the built-in private meta-predicates ignore/1, once/1, and \+/1. These methods cannot be used as messages to objects.
All solutions methods¶
The usual all solutions meta-predicates are built-in private methods in Logtalk: bagof/3, findall/3, findall/4, and setof/3. There is also a forall/2 method that implements generate-and-test loops. These methods cannot be used as messages to objects.
Reflection methods¶
Logtalk provides a comprehensive set of built-in predicates and built-in methods for querying about entities and predicates. Some of the information, however, requires that the source files are compiled with the source_data flag turned on.
The reflection API supports two different views on entities and their contents, which we may call the transparent box view and the black box view. In the transparent box view, we look into an entity disregarding how it will be used and returning all information available on it, including predicate declarations and predicate definitions. This view is supported by the entity property built-in predicates. In the black box view, we look into an entity from a usage point-of-view using built-in methods for inspecting object operators and predicates that are within scope from where we are making the call: current_op/3, which returns operator specifications, predicate_property/2, which returns predicate properties, and current_predicate/1, which enables us to query about user-defined predicate definitions. See below for a more detailed description of these methods.
Definite clause grammar parsing methods and non-terminals¶
Logtalk supports two definite clause grammar parsing built-in private methods, phrase/2 and phrase/3, with definitions similar to the predicates with the same name found on most Prolog compilers that support definite clause grammars. These methods cannot be used as messages to objects.
Logtalk also supports phrase//1, call//1-N, and
eos//0 built-in non-terminals.
The call//1-N
non-terminals takes a closure (which can be a lambda
expression) plus zero or more additional arguments and are processed by
appending the input list of tokens and the list of remaining tokens to
the arguments.
Predicate properties¶
We can find the properties of visible predicates by calling the predicate_property/2 built-in method. For example:
| ?- bar::predicate_property(foo(_), Property).
Note that this method respects the predicate’s scope declarations. For instance, the above call will only return properties for public predicates.
An object’s set of visible predicates is the union of all the predicates declared for the object with all the built-in methods and all the Logtalk and Prolog built-in predicates.
The following predicate properties are supported:
scope(Scope)
- The predicate scope (useful for finding the predicate scope with a
single call to
predicate_property/2
) public
,protected
,private
- The predicate scope (useful for testing if a predicate have a specific scope)
static
,dynamic
- All predicates are either static or dynamic (note, however, that a dynamic predicate can only be abolished if it was dynamically declared)
logtalk
,prolog
,foreign
- A predicate can be defined in Logtalk source code, Prolog code, or in foreign code (e.g. in C)
built_in
- The predicate is a built-in predicate
multifile
- The predicate is declared multifile (i.e. it can have clauses defined in several entities)
meta_predicate(Template)
- The predicate is declared as a meta-predicate with the specified template
coinductive(Template)
- The predicate is declared as a coinductive predicate with the specified template
declared_in(Entity)
- The predicate is declared (using a scope directive) in the specified entity
defined_in(Entity)
- The predicate definition is looked up in the specified entity (note
that this property does not necessarily imply that clauses for the
predicate exist in
Entity
; the predicate can simply be false as per the closed-world assumption) redefined_from(Entity)
- The predicate is a redefinition of a predicate definition inherited from the specified entity
non_terminal(NonTerminal//Arity)
- The predicate resulted from the compilation of the specified grammar rule non-terminal
alias_of(Predicate)
- The predicate (name) is an alias for the specified predicate
alias_declared_in(Entity)
- The predicate alias is declared in the specified entity
synchronized
- The predicate is declared as synchronized (i.e. it’s a deterministic predicate synchronized using a mutex when using a backend Prolog compiler supporting a compatible multi-threading implementation)
Some properties are only available when the entities are defined in source files and when those source files are compiled with the source_data flag turned on:
inline
- The predicate definition is inlined
auxiliary
- The predicate is not user-defined but rather automatically generated by the compiler or the term-expansion mechanism
mode(Mode, Solutions)
- Instantiation, type, and determinism mode for the predicate (which can have multiple modes)
info(ListOfPairs)
- Documentation key-value pairs as specified in the user-defined
info/2
directive number_of_clauses(N)
- The number of clauses for the predicate existing at compilation time (note that this property is not updated at runtime when asserting and retracting clauses for dynamic predicates)
number_of_rules(N)
- The number of rules for the predicate existing at compilation time (note that this property is not updated at runtime when asserting and retracting clauses for dynamic predicates)
declared_in(Entity, Line)
- The predicate is declared (using a scope directive) in the specified entity in a source file at the specified line (if applicable)
defined_in(Entity, Line)
- The predicate is defined in the specified entity in a source file at the specified line (if applicable)
redefined_from(Entity, Line)
- The predicate is a redefinition of a predicate definition inherited from the specified entity, which is defined in a source file at the specified line (if applicable)
alias_declared_in(Entity, Line)
- The predicate alias is declared in the specified entity in a source file at the specified line (if applicable)
The properties declared_in/1-2
, defined_in/1-2
, and
redefined_from/1-2
do not apply to built-in methods and Logtalk or
Prolog built-in predicates. Note that if a predicate is declared in a
category imported by the object, it will be the category name — not the
object name — that will be returned by the property declared_in/1
.
The same is true for protocol declared predicates.
Finding declared predicates¶
We can find, by backtracking, all visible user predicates by calling the current_predicate/1 built-in method. This method respects the predicate’s scope declarations. For instance, the following call will only return user predicates that are declared public:
| ?- some_object::current_predicate(Name/Arity).
The predicate property non_terminal/1
may be used to retrieve all
grammar rule non-terminals declared for an object. For example:
current_non_terminal(Object, Name//Args) :-
Object::current_predicate(Name/Arity),
functor(Predicate, Functor, Arity),
Object::predicate_property(Predicate, non_terminal(Name//Args)).
Usually, the non-terminal and the corresponding predicate share the same functor but users should not rely on this always being true.
Calling Prolog predicates¶
Logtalk is designed for both robusteness and portability. In the context of calling Prolog predicates, robusteness requires that the compilation of Logtalk source code must not have accidental dependencies on Prolog code that happens to be loaded at the time of the compilation. One immediate consequence is that only Prolog built-in predicates are visible from within objects and categories. But Prolog systems provide a widely diverse set of built-in predicates, easily rising portability issues. Relying on non-standard predicates is often unavoidable, however, due to the narrow scope of Prolog standards. Logtalk applications may also require calling user-defined Prolog predicates, either in user or in Prolog modules.
Calling Prolog built-in predicates¶
In predicate clauses and object initialization/1
directives, predicate
calls that are not prefixed with a message sending, super call, or module
qualification operator (::
, ^^
, or :
), are compiled to either
calls to local predicates or as calls to Logtalk/Prolog built-in predicates.
A predicate call is compiled as a call to a local predicate if the object (or
category) contains a scope directive, a definition for the called predicate,
or a dynamic declaration for it. When that is not the case, the compiler
checks if the call corresponds to a Logtalk or Prolog built-in predicate.
Consider the following example:
foo :-
...,
write(bar),
...
The call to the write/1
predicate will be compiled as a call to the
corresponding Prolog standard built-in predicate unless the object (or
category) containing the above definition also contains a predicate
named write/1
or a dynamic directive for the predicate.
When calling non-standard Prolog built-in predicates or using non-standard
Prolog arithmetic functions, we may run into portability problems while
trying your applications with different backend Prolog compilers. We can
use the compiler portability flag to generate
warnings for calls to non-standard predicates and arithmetic functions.
We can also document those calls using the uses/2
directive. For example, a few Prolog systems provide an atom_string/2
non-standard predicate. We can write (in the object or category calling the
predicate):
:- uses(user, [atom_string/2])
This directive is based on the fact that built-in predicates are visible in
plain Prolog (i.e. in user
). Besides helping to document the dependency
on a non-standard, this directive will also silence the compiler portability
warning.
Calling Prolog non-standard meta-predicates¶
Prolog built-in meta-predicates may only be called locally within
objects or categories, i.e. they cannot be used as messages. Compiling
calls to non-standard, Prolog built-in meta-predicates can be tricky,
however, as there is no standard way of checking if a built-in predicate
is also a meta-predicate and finding out which are its meta-arguments.
But Logtalk supports overriding the original meta-predicate template
when not programmatically available or usable. For example, assume a
det_call/1
Prolog built-in meta-predicate that takes a goal as
argument. We can add to the object (or category) calling it the
directive:
:- meta_predicate(user::det_call(0)).
Another solution is to explicitly declare all non-standard Prolog
meta-predicates in the corresponding adapter file using the internal
predicate '$lgt_prolog_meta_predicate'/3
. For example:
'$lgt_prolog_meta_predicate'(det_call(_), det_call(0), predicate).
The third argument can be either the atom predicate
or the atom
control_construct
, a distinction that is useful when compiling in
debug mode.
Calling Prolog user-defined plain predicates¶
Prolog user-defined plain predicates can be called from within objects or
categories by sending the corresponding message to user
. For example:
foo :-
...,
user::bar,
...
In alternative, we can use the uses/2 directive and write:
:- uses(user, [bar/0]).
foo :-
...,
bar,
...
Note that user
is a pseudo-object in Logtalk containing all predicate
definitions that are not encapsulated (either in a Logtalk entity or a
Prolog module).
When the Prolog predicate is not a meta-predicate, we can also use the {}/1 compiler bypass control construct. For example:
foo :-
...,
{bar},
...
But note that in this case the reflection API will not record the
dependency of the foo/0
predicate on the Prolog bar/0
predicate
as we are effectively bypassing the compiler.
Calling Prolog module predicates¶
Prolog module predicates can be called from within objects or categories by using explicit qualification. For example:
foo :-
...,
module:bar,
...
You can also use in alternative the use_module/2 directive to call the module predicates using implicit qualification:
:- use_module(module, [bar/0]).
foo :-
...,
bar,
...
Note that the first argument of the use_module/2
, when used within
an object or a category, is a module name, not a file specification
(also be aware that Prolog modules are sometimes defined in files with
names that differ from the module names). As loading a Prolog module varies
between Prolog systems, the actual loading directive or goal is preferably
done by the application loader file. An advantage of this approach
is that it contributes to a clean separation between loading and using
a resource with the loader file being the central point that loads all
application resources (complex applications often use a hierarchy of
loader files but the main idea remains the same).
Warning
The actual module code must be loaded prior to compilation of Logtalk source code that uses it. In particular, programmers should not expect that the module be auto-loaded (including when using a backend Prolog compiler that supports an autoloading mechanism).
Calls to module meta-predicates may require providing a missing meta-predicate template or overriding an existing meta-predicate template due to lack of Prolog standardization as discussed earlier in this section.