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9.2 An example module

For illustrative purposes, here is the definition of a simple module for managing queues: 

:- module queue.
:- interface.

% Declare an abstract data type.

:- type queue(T).

% Declare some predicates which operate on the abstract data type.

:- pred empty_queue(queue(T)).
:- mode empty_queue(out) is det.
:- mode empty_queue(in) is semidet.

:- pred put(queue(T), T, queue(T)).
:- mode put(in, in, out) is det.

:- pred get(queue(T), T, queue(T)).
:- mode get(in, out, out) is semidet.

:- implementation.

% Queues are implemented as lists. We need the `list' module
% for the declaration of the type list(T), with its constructors
% '[]'/0 % and '.'/2, and for the declaration of the predicate
% list.append/3.

:- import_module list.

% Define the queue ADT.

:- type queue(T) == list(T).

% Define the exported predicates.

empty_queue([]).

put(Queue0, Elem, Queue) :-
    list.append(Queue0, [Elem], Queue).

get([Elem | Queue], Elem, Queue).

:- end_module queue.