NOTES.md

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This file is part of Logtalk https://logtalk.org/
SPDX-FileCopyrightText: 1998-2025 Paulo Moura [email protected]
SPDX-License-Identifier: Apache-2.0

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

http://www.apache.org/licenses/LICENSE-2.0

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.

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

Each example folder contains a NOTES.md 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 NOTES.md files inside the library folder, plus the NOTES.md 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 examples have been adapted from public Prolog code or from Prolog text books and tutorials 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 indented using tabs (a common 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/NOTES.md 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 backend 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

• around_methods
  example of defining a complementing category that uses the experimental @/1 goal annotation to define an "around method"

• aspects
  example of defining aspects (as in Aspect-Oriented Programming)

• assign_parameters
  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 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 to compare the performance of Logtalk message-sending when using different backend Prolog compilers and for comparing the 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

• books
  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

• 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

• cascade
  example of using expected terms to call a conjunction of goals where any of them may cause an error condition without using the traditional catch/throw mechanism

• 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)

• closed_world_assumption
  example illustrating the difference between declaring a predicate and defining a predicate and the Closed World Assumption (CWA) semantics as implemented in Logtalk when calling predicates and sending messages

• 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 backend 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

• defaulty
  example comparing defaulty and tagged data representations

• delegates
  simple implementation of the delegation design pattern

• design_patterns sample implementation of common object-oriented behavioral, creational, and structural design patterns

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

• document_converter
  example of using a Java library for converting documents to text

• dynpred
  example of using the built-in database handling methods 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 a backend supporting at least UTF-8 encoding)

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

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

• expansion
  example illustrating the term and goal expansion mechanisms

• 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

• ncl
  Net-Clause Language (NCL) examples (including the figures individual example of network modeling for recognizing polyhedra represented as graphs)

• futures
  simple example of working with futures, a common concurrent programming idiom, using the high-level multi-threading predicates

• hailstone
  example of computing Hailstone sequences

• haunted_wasteland
  solution for the Advent of Code 2023 Day 8 problem; illustrates the use of the dictionaries and grammars libraries plus cyclic terms

• 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

• jpl
  examples adapted from SWI-Prolog/YAP JPL library; illustrates how to use the java 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

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

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

• 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

• missing_data
  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

• modules
  simple example of compiling Prolog modules as objects

• module_aliases
  simple examples illustrating the use of module aliases

• 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

• neo4j
  example of using the Java library to interface with Neo4j

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

• object_aliases
  simple examples illustrating the use of object aliases

• operators
  example of using operators local to objects and categories

• 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

• pengines
  simple example of using SWI-Prolog pengines from objects

• people
  simple example of defining object constructors

• permutations
  benchmarks based on list permutations

• 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

• predicate_lookups
  example illustrating the predicate declaration and predicate definition lookup algorithms used when sending a message to an object

• process_modeling
  example of using parametric objects to represent and restore shared variables between sets of constraints that are stored in different objects

• 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 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

• scratchcards
  solution for the Advent of Code 2023 Day 4 problem; illustrates the use of the dictionaries and grammars libraries

• 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"

• self_vs_super
  simple example illustrating the semantics of calling an inherited meta-predicate using a message to "self" versus using a "super" call

• self_vs_this
  simple example illustrating the difference between "self" and "this"

• serialization
  simple example of serializing objects to a file

• 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

• slides
  example of using the library list zippers

• 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 a backend supporting tabling)

• tcltk
  example illustrating how to add a portable GUI to an application using Tcl and Tk

• testing
  some examples of writing unit tests

• tests_dsl
  example illustrating how to define a Domain Specific Language (DSL) for writing 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 a multi-threaded backend)

• trebuchet
  solution for the Advent of Code 2023 Day 1 problem; illustrates the use of push-back lists in DCGs

• units
  Logtalk version of a GNU Prolog/CX parametric unit example

• viewpoints
  example of 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