Protocols enable the separation between interface and implementation: several objects can implement the same protocol and an object can implement several protocols. Protocols may contain only predicate declarations. In some languages the term interface is used with similar meaning. Logtalk allows predicate declarations of any scope within protocols, contrary to some languages that only allow public declarations.

Logtalk defines three built-in protocols, monitoring, expanding, and forwarding, which are described at the end of this section.

Defining a new protocol

We can define a new protocol in the same way we write Prolog code: by using a text editor. Logtalk source files may contain one or more objects, categories, or protocols. If you prefer to define each entity in its own source file, it is recommended that the file be named after the protocol. By default, all Logtalk source files use the extension .lgt but this is optional and can be set in the adapter files. Intermediate Prolog source files (generated by the Logtalk compiler) have, by default, a _lgt suffix and a .pl extension. Again, this can be set to match the needs of a particular Prolog compiler in the corresponding adapter file. For example, we may define a protocol named listp and save it in a listp.lgt source file that will be compiled to a Prolog file (depending on the backend compiler, the names of the intermediate Prolog files may include a directory hash and a process identifier to prevent file name clashes when embedding Logtalk applications or running parallel Logtalk processes).

Protocol names must be atoms. Objects, categories and protocols share the same namespace: we cannot have a protocol with the same name as an object or a category.

Protocol directives are textually encapsulated by using two Logtalk directives: protocol/1-2 and end_protocol/0. The most simple protocol will be one that is self-contained, not depending on any other Logtalk entity:

:- protocol(Protocol).
:- end_protocol.

If a protocol extends one or more protocols, then the opening directive will be:

:- protocol(Protocol,
    extends([Protocol1, Protocol2, ...])).
:- end_protocol.

In order to maximize protocol reuse, all predicates specified in a protocol should relate to the same functionality. Therefore, the only recommended use of protocol extension is when you need both a minimal protocol and an extended version of the same protocol with additional, convenient predicates.

Finding defined protocols

We can find, by backtracking, all defined protocols by using the current_protocol/1 built-in predicate with a unbound argument:

| ?- current_protocol(Protocol).

This predicate can also be used to test if a protocol is defined by calling it with a valid protocol identifier (an atom).

Creating a new protocol in runtime

We can create a new (dynamic) protocol at runtime by calling the Logtalk built-in predicate create_protocol/3:

| ?- create_protocol(Protocol, Relations, Directives).

The first argument should be either a variable or the name of the new protocol (a Prolog atom, which must not match an existing entity name). The remaining two arguments correspond to the relations described in the opening protocol directive and to the protocol directives.

For instance, the call:

| ?- create_protocol(ppp, [extends(qqq)], [public([foo/1, bar/1])]).

is equivalent to compiling and loading the protocol:

:- protocol(ppp,

    :- dynamic.

    :- public([foo/1, bar/1]).

:- end_protocol.

If we need to create a lot of (dynamic) protocols at runtime, then is best to define a metaclass or a prototype with a predicate that will call this built-in predicate in order to provide more sophisticated behavior.

Abolishing an existing protocol

Dynamic protocols can be abolished using the abolish_protocol/1 built-in predicate:

| ?- abolish_protocol(Protocol).

The argument must be an identifier of a defined dynamic protocol, otherwise an error will be thrown.

Protocol directives

Protocol directives are used to define protocol properties and documentation.

Dynamic protocols

As usually happens with Prolog code, a protocol can be either static or dynamic. A protocol created during the execution of a program is always dynamic. A protocol defined in a file can be either dynamic or static. Dynamic protocols are declared by using the dynamic/0 directive in the protocol source code:

:- dynamic.

The directive must precede any predicate directives. Please be aware that using dynamic code results in a performance hit when compared to static code. We should only use dynamic protocols when these need to be abolished during program execution.

Protocol documentation

A protocol can be documented with arbitrary user-defined information by using the info/1 entity directive. See the Documenting section for details.

Loading files into a protocol

The include/1 directive can be used to load the contents of a file into a protocol. See the Objects section for an example of using this directive.

Protocol relationships

Logtalk provides two sets of built-in predicates that enable us to query the system about the possible relationships that a protocol have with other entities.

The extends_protocol/2-3 built-in predicates return all pairs of protocols so that the first one extends the second:

| ?- extends_protocol(Protocol1, Protocol2).

or, if we also want to know the extension scope:

| ?- extends_protocol(Protocol1, Protocol2, Scope).

To find which objects or categories implement which protocols we can call the implements_protocol/2-3 built-in predicates:

| ?- implements_protocol(ObjectOrCategory, Protocol).

or, if we also want to know the implementation scope:

| ?- implements_protocol(ObjectOrCategory, Protocol, Scope).

Note that, if we use a non-instantiated variable for the first argument, we will need to use the current_object/1 or current_category/1 built-in predicates to identify the kind of entity returned.

Protocol properties

We can find the properties of defined protocols by calling the protocol_property/2 built-in predicate:

| ?- protocol_property(Protocol, Property).

A protocol may have the property static, dynamic, or built_in. Dynamic protocols can be abolished in runtime by calling the abolish_protocol/1 built-in predicate. Depending on the backend Prolog compiler, a protocol may have additional properties related to the source file where it is defined.

The following protocol properties are supported:


The protocol is static


The protocol is dynamic (and thus can be abolished in runtime by calling the abolish_category/1 built-in predicate)


The protocol is a built-in protocol (and thus always available)


Source data available for the protocol


Absolute path of the source file defining the protocol (if applicable)

file(Basename, Directory)

Basename and directory of the source file defining the protocol (if applicable); Directory always ends with a /

lines(BeginLine, EndLine)

Source file begin and end lines of the protocol definition (if applicable)

directive(BeginLine, EndLine)

Source file begin and end lines of the protocol opening directive (if applicable)


List of public predicates and operators declared by the protocol


List of protected predicates and operators declared by the protocol


List of private predicates and operators declared by the protocol

declares(Predicate, Properties)

List of properties for a predicate declared by the protocol

alias(Predicate, Properties)

List of properties for a predicate alias declared by the protocol (the properties include for(Original), from(Entity), non_terminal(NonTerminal), and line_count(Line) with Line being the begin line of the alias directive)

Some of the properties such as line numbers are only available when the protocol is defined in a source file compiled with the source_data flag turned on.

Implementing protocols

Any number of objects or categories can implement a protocol. The syntax is very simple:

:- object(Object,
:- end_object.

or, in the case of a category:

:- category(Object,
:- end_category.

To make all public predicates declared via an implemented protocol protected or to make all public and protected predicates private we prefix the protocol’s name with the corresponding keyword. For instance:

:- object(Object,
:- end_object.


:- object(Object,
:- end_object.

Omitting the scope keyword is equivalent to writing:

:- object(Object,
:- end_object.

The same rules applies to protocols implemented by categories.

Built-in protocols

Logtalk defines a set of built-in protocols that are always available for any application.

The built-in protocol expanding

The built-in expanding protocol declares the term_expansion/2 and goal_expansion/2 predicates. See the description of the hook compiler flag for more details.

The built-in protocol monitoring

The built-in monitoring protocol declares the before/3 and after/3 public event handler predicates. See the Event-driven programming section for more details.

The built-in protocol forwarding

The built-in forwarding protocol declares the forward/1 user-defined message forwarding handler, which is automatically called (if defined) by the runtime for any message that the receiving object does not understand. See also the []/1 control construct.