Logtalk is an object-oriented logic programming language that extends and leverages Prolog with modern code encapsulation and code reuse mechanisms without compromising its declarative programming features. Logtalk is implemented in highly portable code and can use most modern and standards compliant Prolog implementations as a back-end compiler.
To keep its size reasonable, this tutorial necessarily assumes that the reader have a working knowledge of Prolog and is biased towards describing Logtalk object-oriented features.
# Syntax
Logtalk uses standard Prolog syntax with the addition of a few operators and directives for a smooth learning curve and wide portability. One important consequence is that Prolog code can be easily encapsulated in objects with little or no changes. Moreover, Logtalk can transparently interpret most Prolog modules as Logtalk objects.
*`::/1` - sending a message to _self_ (i.e. to the object that received the message being processed)
*`^^/1` - _super_ call (of an inherited or imported predicate)
Some of the most important entity and predicate directives will be introduced in the next sections.
# Entities and roles
Logtalk provides _objects_, _protocols_, and _categories_ as first-class entities. Relations between entities define _patterns of code reuse_ and the _roles_ played by the entities. For example, when an object _instantiates_ another object, the first object plays the role of an instance and the second object plays the role of a class. An _extends_ relation between two objects implies that both objects play the role of prototypes, with one of them extending the other, its parent prototype.
An object encapsulates predicate declarations and definitions. Objects can be created dynamically but are usually static and defined in source files. A single source file can contain any number of entity definitions. A simple object, defining a list member public predicate:
Assuming that the code above for the `list` object is saved in a `list.lgt` file, it can be compiled and loaded using the `logtalk_load/1` built-in predicate or its abbreviation, `{}/1`, with the file path as argument (the extension can be omitted):
Encapsulation is enforced. A predicate can be declared _public_, _protected_, or _private_. It can also be _local_ when there is no scope directive for it. For example:
A subtle point is that predicate scope directives specify predicate _calling_ semantics, not _definition_ semantics. For example, if an object playing the role of a class declares a predicate private, the predicate can be defined in subclasses and instances *but* can only be called in its instances _from_ the class.
An object without an _instantiation_ or _specialization_ relation with another object plays the role of a prototype. A prototype can _extend_ another object, its parent prototype.
% fred, another elephant, is like clyde, except that he's white
:- object(fred,
extends(clyde)).
color(white).
:- end_object.
```
When answering a message sent to an object playing the role of a prototype, we validate the message and look for an answer first in the prototype itself and, if not found, we delegate to the prototype parents if any:
A message is valid if the corresponding predicate is declared (and the sender is within scope) but it will fail, rather then throwing an error, if the predicate is not defined. This is called the _closed-world assumption_. For example, consider the following object, saved in a `foo.lgt` file:
Loading the file and trying to call the `bar/0` predicate fails as expected. Note that this is different from calling an _unknown_ predicate, which results in an error:
In order to define objects playing the role of classes and/or instances, an object must have at least an instantiation or a specialization relation with another object. Objects playing the role of meta-classes can be used when we need to see a class also as an instance. We use the following example to also illustrate how to dynamically create new objects at runtime:
When answering a message sent to an object playing the role of an instance, we validate the message by starting in its class and going up to its class superclasses if necessary. Assuming that the message is valid, then we look for an answer starting in the instance itself:
A category is a fine grained unit of code reuse, used to encapsulate a _cohesive_ set of predicate declarations and definitions, implementing a _single_ functionality, that can be imported into any object. A category can thus be seen as the dual concept of a protocol. In the following example, we define categories representing car engines and then import them into car objects:
Categories are independently compiled and thus allow importing objects to be updated by simple updating the imported categories without requiring object recompilation. Categories also provide _runtime transparency_. I.e. the category protocol adds to the protocol of the objects importing the category:
Categories can be also be used for hot-patching objects. A category can add new predicates to an object and/or replace object predicate definitions. For example, consider the following object:
If the object source code is not available and we need to fix an application running the object code, we can simply define a category that fixes the buggy predicate:
As hot-patching forcefully breaks encapsulation, there is a `complements` compiler flag that can be set (globally or on a per-object basis) to allow, restrict, or prevent it.
Objects and categories can be parameterized by using as identifier a compound term instead of an atom. Object and category parameters are _logical variables_ shared with all encapsulated predicates. An example with geometric circles:
Parametric objects also provide a simple way of associating a set of predicates with a plain Prolog predicate. Prolog facts can be interpreted as _parametric object proxies_ when they have the same functor and arity as the identifiers of parametric objects. Handy syntax is provided to for working with proxies. For example, assuming the following clauses for a `circle/2` predicate:
Logtalk supports _event-driven programming_ by allowing defining events and monitors for those events. An event is simply the sending of a message to an object. Interpreting message sending as an atomic activity, a _before_ event and an _after_ event are recognized. Event monitors define event handler predicates, `before/3` and `after/3`, and can query, register, and delete a system-wide event registry that associates events with monitors. For example, a simple tracer for any message being sent using the `::/2` control construct can be defined as:
Assuming that the `tracer` object and the `list` object defined earlier are compiled and loaded, we can observe the event handlers in action by sending a message:
Events can be set and deleted dynamically at runtime by calling the `define_events/5` and `abolish_events/5` built-in predicates.
Event-driven programming can be seen as a form of _computational reflection_. But note that events are only generated when using the `::/2` message-sending control construct.
Logtalk supports lambda expressions. Lambda parameters are represented using a list with the `(>>)/2` infix operator connecting them to the lambda. Some simple examples using library meta-predicates:
Terms and goals in source files can be _expanded_ at compile time by specifying a _hook object_ that defines term-expansion and goal-expansion rules. For example, consider the following simple object, saved in a `source.lgt` file:
