Learning by examples

Logtalk is a programming language that extends and subsumes Prolog. Thus, a good knowledge of Prolog paves the way for a quick learning of Logtalk programming. Basic knowledge of object-oriented programming also helps, specially if from a generalized, first principles, approach. It is highly recommended that you start with the tutorial available at:

Learn X in Y minutes Where X=Logtalk

Although this short tutorial is a recommended starting point for new users for a quick overview, be aware that some of the examples will not work as-is in Logtalk 3.x (a pull request have been submitted). There’s also an up-to-date Jupyter notebook version of this tutorial that you can run with a subset of the Logtalk supported Prolog backends. You can also run it online: Binder

Note that the Logtalk distribution includes HTML versions of the user manual, the reference manual, and the APIs documentation. Documentation can also be browsed at:

Logtalk documentation resources

The Logtalk distribution also includes a large number of well documented programming examples that introduce most of the language features. The examples included are fully documented with source code comments and sample goals:

Programming examples (described here).

All examples include a loader.lgt file that loads the example and any dependencies, a NOTES.md file with a description of the example, and a SCRIPT.txt file with sample queries (including how to load the example) for you to try. Most examples also include a tests.lgt file with unit tests and a tester.lgt loader file to run them. Being programming examples, it is expected that you run them side-by-side with the source code open in your favorite editor after reading their documentation.

Learning and development tools

To help new users understand and act on compiler and runtime warning and error messages, load the tutor tool at startup. To help new users trace query execution, also load the debugger tool at startup. These and other developer tools (e.g. for testing and documentation) can be automatically loaded at startup by defining a default settings file.

  • hello_world - mandatory example; cannot be avoided
  • roles - learn about the different roles that an object can play
  • scopes - learn about predicate scopes
  • self_messages - learn how to use messages to self to call redefined predicates
  • super_calls - learn how to use super calls to specialize inherited predicates
  • elephants - learn about the concept of prototypes (videocast)
  • prototypes - another example on prototypes
  • closed_world_assumption - learn about the Closed World Assumption (CWA) semantics
  • self_vs_this - learn the difference between self and this
  • planets - learn how to define and use protocols and categories
  • carengines - learn more about categories
  • shapes - learn about the differences between class and prototype hierarchies
  • predicate_lookups - learn about the predicate lookups used when sending a message to an object
  • inheritance - learn about public, protected, and private inheritance
  • family - no self respecting logic programming language can do without a family relations example

Need a break after playing with the above examples? Have fun with the following ones:

  • adventure - text-based adventure games; beware of the monsters!
  • puzzles - logical puzzles

The Logtalk compiler does extended lint checks. Trouble understanding compiler output? Want to learn more about compilation errors and warnings? Load the tutor tool and then run the following example:

  • errors - learn about compiler errors and warnings

Want to learn about parametric objects (and categories)? See the following examples:

Logtalk support for composition using categories provides an alternative to inheritance. Two good examples are:

Another distinctive Logtalk feature is the support for event-driven programing. Some illustrative examples are:

Looking for how to express traditional OOP class concepts from other languages in Logtalk? See:

Now that you got the basics of OOP in Logtalk, time to take it to the next level:

But Logtalk is a declarative language supporting all those features that drive people to logic programming. Some good examples are:

Looking for extending Logtalk with your constructs? Logtalk supports a term-expansion mechanism (aka as macros):

Happy about your progress? Time to celebrate with:

Running example queries in the debugger

Running a query using the debugger can help understand the inference process. An example session, using the planets example:

?- {planets(loader)}.
% [ /Users/pmoura/logtalk/examples/planets/planets.lgt loaded ]
% [ /Users/pmoura/logtalk/examples/planets/loader.lgt loaded ]
% (0 warnings)

In the context of the top-level interpreter, the {}/1 goal is just a handy shortcut for the logtalk_load/1 predicate. The argument uses library notation to specify the file to load: planets is the library name (an alias to a directory of source files) and loader is the file name (with the extension omitted).

Let’s try a query:

?- mars::weight(m2, W2).
W2 = 14.88304.

How do Logtalk arrived to this solution? Let’s recompile the example in debug mode:

?- {+d}.
% Reloading files in debug mode
% Redefining protocol physical_properties
% Redefining category planet
% Redefining object m1
% Redefining object m2
% Redefining object earth
% Redefining object mars
% [ /Users/pmoura/logtalk/examples/planets/planets.lgt reloaded ]
% [ /Users/pmoura/logtalk/examples/planets/loader.lgt reloaded ]
% (0 warnings)
% Reloaded all Logtalk source files modified or that required
% recompilation due to a change to the compilation mode

The {+d} goal is a top-level shortcut for the logtalk_make/1 predicate that recompiles loaded files in debug mode.

We also need to load the debugger (hint: you can load the debugger and other tools automatically at startup by defining a default settings file):

?- {debugger(loader)}.
% [ /Users/pmoura/logtalk/tools/debugger/debuggerp.lgt loaded ]
% [ /Users/pmoura/logtalk/tools/debugger/debugger.lgt loaded ]
% [ /Users/pmoura/logtalk/tools/debugger/debugger_messages.lgt loaded ]
% [ /Users/pmoura/logtalk/tools/debugger/loader.lgt loaded ]
% (0 warnings)

We can now trace the query:

?- debugger::trace.
   Debugger switched on: tracing everything for all objects compiled in debug mode.

?- mars::weight(m2, W2).
   Call: (1) mars::weight(m2,_32108) ? 
   Call: (2) weight(m2,_32108) ? 
   Rule: (2) weight(m2,_32108) ? x
     Entity:            planet
     Sender:            user
     This:              mars
     Self:              mars
     Meta-call context: []
     Coinduction stack: []
   Rule: (2) weight(m2,_32108) ? 
   Call: (3) m2::mass(_34428) ? 
   Call: (4) mass(_34428) ? 
   Fact: (4) mass(4) ? 
   Exit: (4) mass(4) ? 
   Exit: (3) m2::mass(4) ? 
   Call: (5) ::gravitational_acceleration(_37184) ? 
   Call: (6) gravitational_acceleration(_37184) ? 
   Fact: (6) gravitational_acceleration(3.72076) ? .
     File:          /Users/pmoura/logtalk/examples/planets/planets.lgt
     Line number:   97
     Entity:        mars
     Predicate:     gravitational_acceleration/1
     Clause number: 1
   Fact: (6) gravitational_acceleration(3.72076) ? 
   Exit: (6) gravitational_acceleration(3.72076) ? 
   Exit: (5) ::gravitational_acceleration(3.72076) ? 
   Call: (7) _32108 is 4*3.72076 ? 
   Exit: (7) 14.88304 is 4*3.72076 ? 
   Exit: (2) weight(m2,14.88304) ? 
   Exit: (1) mars::weight(m2,14.88304) ? 
W2 = 14.88304.

The debugger supports a several handy options. Type h or ? at its prompt for inline help. Consult the user manual section on debugging for further information.

To terminate the debugging session, you can turn off the debugger and recompile the files in normal (or optimized mode):

?- debugger::nodebug.
   Debugger switched off.

?- {+n}.
% Reloading files in normal mode
% Redefining protocol physical_properties
% Redefining category planet
% Redefining object m1
% Redefining object m2
% Redefining object earth
% Redefining object mars
% [ /Users/pmoura/logtalk/examples/planets/planets.lgt reloaded ]
% [ /Users/pmoura/logtalk/examples/planets/loader.lgt reloaded ]
% (0 warnings)
% Reloaded all Logtalk source files modified or that required
% recompilation due to a change to the compilation mode

Modifying and reloading an example

But don’t just run the provided examples. Experiment also modifying them. Continuing to use the planets example, let’s add another planet, jupiter, by editing the planets.lgt file. At the end of the file, we can write:

:- object(jupiter,


:- end_object.

As we just modified a loaded file, we can use the make tool to reload it:

?- {*}.
% Redefining protocol physical_properties
% Redefining category planet
% Redefining object m1
% Redefining object m2
% Redefining object earth
% Redefining object mars
% [ /Users/pmoura/logtalk/examples/planets/planets.lgt reloaded ]
% (0 warnings)
% Reloaded all Logtalk source files modified or that required
% recompilation due to a change to the compilation mode

?- jupiter::weight(m2, W2).
W2 = 103.54.

The {*} goal is a top-level shortcut to the logtalk_make goal (and the logtalk_make(all) goal).

Running example tests

Next we could also modify the example unit tests to account for the newly added jupiter object by editing the tests.lgt file. As we added a new object, jupiter, we want to add a cover/1 clause for it to that we can collect cove coverage data:


After saving the changes to the tests.lgt file, we can run the tests by simply loading the tester.lgt file:

?- {planets(tester)}.
% tests started at 2019/2/20, 14:34:43
% running tests from object tests
% file: /Users/pmoura/logtalk/examples/planets/tests.lgt
% planets_01: success
% planets_02: success
% planets_03: success
% planets_04: success
% 4 tests: 0 skipped, 4 passed, 0 failed
% completed tests from object tests
% clause coverage ratio and covered clauses per entity predicate
% earth: gravitational_acceleration/1 - 1/1 - (all)
% earth: 1 out of 1 clause covered, 100.000000% coverage
% jupiter: gravitational_acceleration/1 - 0/1 - []
% jupiter: 0 out of 1 clause covered, 0.000000% coverage
% m1: mass/1 - 1/1 - (all)
% m1: volume/1 - 0/1 - []
% m1: 1 out of 2 clauses covered, 50.000000% coverage
% m2: mass/1 - 1/1 - (all)
% m2: volume/1 - 0/1 - []
% m2: 1 out of 2 clauses covered, 50.000000% coverage
% mars: gravitational_acceleration/1 - 1/1 - (all)
% mars: 1 out of 1 clause covered, 100.000000% coverage
% planet: weight/2 - 1/1 - (all)
% planet: gravitational_acceleration/1 - 0/0 - (all)
% planet: 1 out of 1 clause covered, 100.000000% coverage
% 6 entities declared as covered containing 8 clauses
% 5 out of 6 entities covered, 83.333333% entity coverage
% 5 out of 8 clauses covered, 62.500000% clause coverage
% tests ended at 2019/12/13, 14:29:25

As expected, there is no code coverage for the new jupiter object. To fix it, we can take the query we used above and add it as a test:

test(planets_05) :-
    jupiter::weight(m2, W2),
    W2 =~= 103.54.

Consult the documentation of the lgtunit testing tool to learn more about testing, including more expressive test dialects.

At this point, you should have a good understanding of the basics of Logtalk programming. But there are more features to explore and many more examples for you to play with. We suggest that you look into the examples summary and choose the next ones according to your interests.

Enjoy and remember: the discussion forums and the chat room welcome your participation. Don’t be a stranger.