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List predicates
===============

In this example, we will illustrate the use of:

-  objects
-  protocols

by using common list utility predicates.

.. _object:

Defining a list object
----------------------

We will start by defining an object, ``list``, containing predicate
definitions for some common list predicates like ``append/3``,
``length/2``, and ``member/2``:

::

   :- object(list).

       :- public([
           append/3, length/2, member/2
       ]).

       append([], List, List).
       append([Head| Tail], List, [Head| Tail2]) :-
           append(Tail, List, Tail2).

       length(List, Length) :-
           length(List, 0, Length).

       length([], Length, Length).
       length([_| Tail], Acc, Length) :-
           Acc2 is Acc + 1,
           length(Tail, Acc2, Length).

       member(Element, [Element| _]).
       member(Element, [_| List]) :-
           member(Element, List).

   :- end_object.

What is different here from a regular Prolog program? The definitions of
the list predicates are the usual ones. We have two new directives,
:ref:`directives_object_1_5` and
:ref:`directives_end_object_0`, that
encapsulate the object's code. In Logtalk, by default, all object
predicates are private; therefore, we have to explicitly declare all
predicates that we want to be public, that is, that we want to call from
outside the object. This is done using the
:ref:`directives_public_1` scope directive.

After we copy the object code to a text file and saved it under the name
``list.lgt``, we need to change the Prolog working directory to the one
used to save our file (consult your Prolog compiler reference manual).
Then, after starting Logtalk (see the
:ref:`Installing and running Logtalk <installing_installing>` section on
the User Manual), we can compile and load the object using the
:ref:`predicates_logtalk_load_1` Logtalk built-in predicate:

.. code-block:: text

   | ?- logtalk_load(list).

   object list loaded
   yes

We can now try goals like:

.. code-block:: text

   | ?- list::member(X, [1, 2, 3]).

   X = 1;
   X = 2;
   X = 3;
   no

or:

.. code-block:: text

   | ?- list::length([1, 2, 3], L).

   L = 3
   yes

The infix operator
:ref:`control_send_to_object_2` is used in
Logtalk to send a message to an object. The message must match a public
object predicate. If we try to call a non-public predicate such as the
``length/3`` auxiliary predicate an exception will be generated:

.. code-block:: text

   | ?- list::length([1, 2, 3], 0, L).

   uncaught exception:
       error(
           existence_error(predicate_declaration, length/3),
           logtalk(list::length([1,2,3],0,_), ...)
       )   

The exception term describes the type of error and the context
where the error occurred.

.. _protocol:

Defining a list protocol
------------------------

As we saw in the above example, a Logtalk object may contain predicate
directives and predicate definitions (clauses). The set of predicate
directives defines what we call the object's *protocol* or interface. An
interface may have several implementations. For instance, we may want to
define a new object that implements the list predicates using difference
lists. However, we do not want to repeat the predicate directives in the
new object. Therefore, what we need is to split the object's protocol
from the object's implementation by defining a new Logtalk entity known
as a protocol. Logtalk protocols are compilation units, at the same
level as objects and categories. That said, let us define a ``listp``
protocol:

::

   :- protocol(listp).

       :- public([
           append/3, length/2, member/2
       ]).

   :- end_protocol.

Similar to what we have done for objects, we use the 
:ref:`directives_protocol_1_2` and :ref:`directives_end_protocol_0`
directives to encapsulate the predicate directives. We can improve
this protocol by documenting the call/return modes and the
number of proofs of each predicate using the :ref:`directives_mode_2`
directive:

::

   :- protocol(listp).

       :- public(append/3).
       :- mode(append(?list, ?list, ?list), zero_or_more).

       :- public(length/2).
       :- mode(length(?list, ?integer), zero_or_more).

       :- public(member/2).
       :- mode(member(?term, ?list), zero_or_more).

   :- end_protocol.

We now need to change our definition of the ``list`` object by removing
the predicate directives and by declaring that the object implements the
``listp`` protocol:

::

   :- object(list,
       implements(listp)).

       append([], List, List).
       append([Head| Tail], List, [Head| Tail2]) :-
           append(Tail, List, Tail2).
       ...

   :- end_object.

The protocol declared in ``listp`` may now be alternatively implemented
using difference lists by defining a new object, ``difflist``:

::

   :- object(difflist,
       implements(listp).

       append(L1-X, X-L2, L1-L2).
       ...

   :- end_object.

Summary
-------

*  It is easy to define a simple object: just write your Prolog code
   inside starting and ending object directives and add the necessary
   scope directives. The object will be self-defining and ready to use.

*  Define a protocol when you may want to provide or enable several
   alternative definitions to a given set of predicates. This way we
   avoid needless repetition of predicate directives.
