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This file is part of Logtalk
Copyright 1998-2018 Paulo Moura

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

This folder contains several examples of Logtalk programs. A brief description of each example is included below.

Each example folder contains a file and a loader helper file (usually named loader.lgt) that can be used to compile and load the example source code. Most examples also contain a SCRIPT.txt file with instructions on how to load the example and sample queries for you to try.

Most of these examples require objects, protocols, and categories that are defined in the Logtalk standard library or in other examples. See the files inside the library folder, plus the and SCRIPT.txt files inside each example folder.

Some examples may redefine objects already loaded from other examples. It might be necessary to restart Logtalk after playing with some examples.

Some of the examples have been adapted from public Prolog code or from Prolog text books and are copyrighted by the respective authors.

These are programming examples, meaning that you should study the source files to fully understand them. Note, however, that some examples main purpose is to illustrate general principles rather than being adequate, efficient solutions for deployment code.

All examples are formatted using tabs (the recommended setting is a tab width equivalent to 4 spaces).

Most example folders contain two files, tests.lgt and tester.lgt. The file tests.lgt contains unit tests for the example. These unit tests are based on the sample queries found on the example SCRIPT.txt file. The file tester.lgt is a loader file that, when loaded, will automatically run all the example unit tests. You can automate running all these tests by calling the logtalk_tester shell script from the command-line (see the scripts/ file for details on how to use with POSIX and Windows operating-systems). Type man logtalk_tester or logtalk_tester -h for usage details, including how to select the back-end Prolog compiler.

Follows a short description of each included example (in alphabetical order):

  • ack
    implementation of the Ackermann arithmetic function

  • adventure
    some simple examples of text adventures

  • aliases
    example of using the alias/2 predicate directive to provide alternative names to inherited predicates in order to improve readability or to solve multi-inheritance conflicts

  • assignvars
    example of using assignable variables in the context of parametric objects in order to represent object state

  • assumptions
    simple example of implementation of ground linear and intuitionistic assumptions

  • attvars
    experimental example of of using attributed variables within Logtalk objects and categories (requires Logtalk to be run with B-Prolog, SWI-Prolog, XSB, or YAP)

  • bench
    classic set of plain Prolog benchmark programs and Logtalk wrappers for those programs

  • benchmarks
    simple benchmarks for helping comparing the performance of Logtalk message sending when using different backend Prolog compilers and for comparing performance of message sending calls with predicate calls in plain Prolog and explicitly-qualified Prolog module calls (when applicable)

  • birds
    bird identification expert system (example adapted from the "Adventure in Prolog" Amzi! book)

  • blocks
    simpler version of the bricks example; illustrates how to use events to avoid breaking object encapsulation when handling object relations

  • bottles
    99 bottles of beer on the wall! Sing along!

  • bricks
    example of representation and handling of relations using events; illustrates how to use events to avoid breaking object encapsulation

  • carengines
    example of extending categories using car engines

  • cc
    example of using conditional compilation directives to write code portable across several back-end Prolog compilers

  • classmethods
    example of defining "class methods" as found on some class-based object-oriented programming languages

  • classvars
    example of implementation of class variables (as found in Smalltalk; i.e. shared instance variables)

  • clustering
    example of using a Java library for performing clustering of a set of numbers

  • coinduction
    experimental example of coinductive predicates (requires Logtalk to be run with CxProlog, ECLiPSE, SICStus Prolog, SWI-Prolog, or YAP)

  • complements
    examples of using a category to explicitly complement an existing object, either for hot patching or for adding new functionality

  • constraints
    several examples of using constraints within objects and categories when using constraint domain solvers found in back-end Prolog compilers

  • dcgs
    examples of using DCG rules inside objects and categories

  • debug_hooks
    simple example of using compilation hooks and term expansion for conditional compilation of debug statements

  • delegates
    simple implementation of the delegation design pattern

  • diamonds
    examples of problems and solutions for the "diamond problem" (multi-inheritance conflicts and ambiguities)

  • dynpred
    example of using some of the built-in database handling methods in order to implement dynamic object state

  • edcgs
    several examples of Extended Definite Clause Grammars (EDCGs)

  • encodings
    very simple example of using the experimental encoding/1 directive (requires Logtalk to be run with YAP, SWI-Prolog, CxProlog, JIProlog, SICStus Prolog, or Lean Prolog)

  • engines
    examples of using threaded engines to implement fluents, lazy meta-predicates, interactors, and simple agents (requires Logtalk to be run with SWI-Prolog)

  • errors
    example showing the Logtalk compiler warning and error reporting for common programming errors

  • expansion
    example illustrating the term and goal expansion mechanisms

  • expecteds
    illustrates using the expected terms library to decouple data acquisition, which must be resilient to unexpected events, from data processing, which decides how to handle those events

  • family
    classical family relations example

  • family_alt
    alternative version of the family example using a family registry implemented using multifile predicates to avoid dynamic binding

  • hailstone
    example of computing Hailstone sequences

  • hello_world
    the unavoidable "hello world" programming example

  • hooks
    simple example of using compiler hook objects and predicates

  • includes
    simple example of using the include/1 directive as both a file directive and an entity directive

  • inheritance
    examples of public, protected, and private inheritance using both prototypes and classes/instances

  • inlining
    simple example for illustrating and testing inlining of predicate definitions

  • instmethods
    example of instance defined methods; also illustrates the use of "super calls" to call overridden method definitions

  • instvars
    example of defining instance variables, default variable values, and setter and getter methods

  • java
    example of using the library minimal abstraction for calling Java from Logtalk using familiar message sending syntax (requires Logtalk to be run with SWI-Prolog, YAP, or JIProlog as the backend compiler)

  • lambdas
    example of using lambda expressions

  • lambdas_compiled
    pseudo-example for testing compilation of calls to library meta-predicates with lambda expressions as meta-arguments

  • laptops
    example of defining an object as a composition of other objects in order to contrast with category-based composition

  • lo
    examples adapted from the Francis G. McCabe L&O system

  • localizations
    simple example of supporting application localization in multiple languages

  • logging
    example of using a category to define a simple logging support for objects

  • logic
    example of a translator of first-order predicate logic propositions to conjunctive normal form and to clausal form

  • lpa
    examples adapted from the LPA Prolog++ system (an expert system for automobile fault diagnosis and a timetables example)

  • metaclasses example of using classes and metaclasses

  • metainterpreters
    some examples of simple meta-interpreters defined as categories that can be imported by "database" objects

  • metapredicates
    example of using meta-predicates in Logtalk objects

  • metapredicates_compiled
    pseudo-example for testing compilation of calls to library meta-predicates

  • mi
    simple multi-inheritance examples

  • miscellaneous
    unsorted examples

  • modules
    simple example of compiling Prolog modules as objects

  • msglog
    example of using events and monitors for recording, replaying, and printing user messages

  • multifile
    example illustrates how to use multifile predicates within Logtalk objects and categories

  • my_types
    example of defining new types using the user-extensible type library object

  • named_databases
    example of an implementation of the Lean Prolog API for named databases for Prolog compilers with a module system

  • now_you_see_me
    example illustrating requirements dictated by inheritance semantics for the implementation of dynamic predicates

  • operators
    example of using operators local to objects and categories

  • optionals
    illustrates using the optional terms library to decouple data acquisition, which must be able to represent optional values, from data processing, which decides how to handle those values and their absence

  • parametric
    simple examples of parametric objects

  • pardicts
    simple SWI-Prolog only example of using a dictionary term for representing object parameters

  • parvars
    variant of the parametric example using parameter variables

  • patches
    example of using complementing categories to patch broken object code

  • patching
    another example of using complementing categories to patch broken object code

  • people
    simple example of defining object constructors

  • persistency
    illustrates a very simple solution for persisting an object dynamic state across sessions

  • planets
    simple example illustrating the concepts of protocol and category

  • poem
    examples adapted from the Ben Staveley-Taylor POEM system

  • points
    example adapted from SICStus Objects documentation; defines a simple class hierarchy of points illustrating how to use categories as object components

  • polygons
    example of representation and handling of relations using events

  • profiling
    examples of using of events and monitors to implement profilers

  • prototypes
    example illustrating the concept of prototypes

  • proxies
    example of using parametric object proxies for an efficient representation of objects with read-only state

  • puzzles
    several examples of solving logical puzzles

  • quick_check
    example of using the QuickCheck support provided by the lgtunit tool both for interactive testing at the top-level interpreter and for defining unit tests

  • recipes
    example of a possible solution for representing structured data and also of hot patching of running code

  • reflection
    example of a simple class-based reflective system

  • relations
    objects implementing predicates for dealing with relations and constrained relations between objects; used by other examples

  • roles
    simple example illustrating the different roles that can be played by an object

  • roots
    objects, protocols, and categories used by some of the other examples; illustrates how you can define object creation and abolishing methods, complete with initialization and termination options

  • scopes
    simple example illustrating predicate scope semantics

  • searching
    state-space searching framework (this example includes some code adapted from Ivan Bratko's "Prolog Programming for Artificial Intelligence" book)

  • securemp
    a set of source files for testing Logtalk secure implementation of meta-predicates

  • self_messages
    simple example illustrating the semantics of messages to "self"

  • shapes
    simple geometric shapes implemented as both a prototype hierarchy and a class hierarchy for comparing both approaches

  • sicstus
    examples adapted from SICStus Objects documentation

  • super_calls
    simple example illustrating that "super" calls preserve the value of "self"

  • symbiosis examples of using Prolog non-standard built-in meta-predicates and module meta-predicates that take closures as arguments

  • symdiff
    example of using parametric objects to implement symbolic expression differentiation and simplification

  • tabling
    simple example of using tabling directives within objects (requires Logtalk to be run with B-Prolog, SWI-Prolog, XSB, or YAP)

  • testing
    some examples of writing unit tests

  • threads
    several simple examples of multi-threading programming, some of them intended only for benchmarking multi-threading Prolog compilers (requires Logtalk to be run with YAP, SWI-Prolog, or XSB)

  • viewpoints
    example on how to implement property sharing and value sharing with prototypes

  • wrappers
    example of using the begin_of_file term generated when compiling a source file to define object wrappers for plain Prolog code

  • xpce
    SWI-Prolog only example of using XPCE from Logtalk