# List predicates

In this example, we will illustrate the use of:

- objects

- protocols

by using common list utility predicates.

## 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, object/1-5 and 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 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 Installing and running Logtalk section on the User Manual), we can compile and load the object using the logtalk_load/1 Logtalk built-in predicate:

    | ?- logtalk_load(list).

    object list loaded
    yes

We can now try goals like:

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

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

or:

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

    L = 3
    yes

The infix operator (::)/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:

    | ?- 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.

## 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 protocol/1-2 and 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 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.