Nomenclature — Logtalk 3.100.1

# Nomenclature

Depending on your logic programming and object-oriented programming background (or lack of it), you may find Logtalk nomenclature either familiar or at odds with the terms used in other languages. In addition, being a superset of Prolog, terms such as predicate and method are often used interchangeably. Logtalk inherits most of its nomenclature from Prolog and Smalltalk.

Note that the same terms can have different meanings in different languages. A good example is class. The support for meta-classes in e.g. Smalltalk translates to a concept of class that is different in key aspects from the concept of class in e.g. Java or C++. Other terms that can have different meanings are delegation and forwarding. There are also cases where the same concept is found under different names in some languages (e.g., self and this) but that can also mean different concepts in Logtalk and other languages. Always be aware of these differences and be cautious with assumptions carried from other programming languages.

In this section, we map nomenclatures from Prolog and popular OOP languages such as Smalltalk, C++, Java, and Python to the Logtalk nomenclature. The Logtalk distribution includes several examples of how to implement common concepts found in other languages, complementing the information in this section. This Handbook also features a Prolog interoperability section and an extensive glossary providing the exact meaning of the names commonly used in Logtalk programming.

## Prolog nomenclature

Being a superset of Prolog, Logtalk inherits its nomenclature. But Logtalk also aims to fix several Prolog shortcomings, thus introducing new concepts and refining existing Prolog concepts. Logtalk object-oriented nature also introduces names and concepts that are not common when discussing logic programming semantics. We mention here the most relevant ones, notably those where semantics or common practice differ. Further details can be found elsewhere in this Handbook.

arbitrary goals as directives

  Although not ISO Prolog Core standard compliant, several Prolog systems accept using arbitrary goal as directives. This is not supported in Logtalk source files. Always use an initialization/1 directive to wrap those goals. This ensures that any initialization goals, which often have side-effects, are only called if the source file is successfully compiled and loaded.

calling a predicate

  Sending a message to an object is similar to calling a goal with the difference that the actual predicate that is called is determined not just by the message term but also by the object receiving the message and possibly its ancestors. This is also different from calling a Prolog module predicate: a message may result e.g. in calling a predicate inherited by the object but calling a module predicate requires the predicate to exist in (or be reexported by) the module.

closed-world assumption semantics

  Logtalk provides clear closed-world assumption semantics: messages or calls for declared but undefined predicates fail. Messages or calls for unknown (i.e., not declared) predicates throw an error. Crucially, this semantics applies to both static and dynamic predicates. But in Prolog workarounds are required to have a static predicate being known by the runtime without it being also defined (so that calling it would fail instead of throwing a predicate existence error).

compiling and loading source files

  Logtalk provides its own built-in predicates for compiling and loading source files. It also provides convenient top-level interpreter shorthands for these and other frequent operations. In general, the traditional Prolog built-in predicates and top-level interpreter shorthands cannot be used to load Logtalk source files.

debugging

  In most (if not all) Prolog systems, debugging support is a built-in feature made available using a set of built-in predicates like `trace/0` and `spy/1`. But in Logtalk the default debugger is a regular application, implemented using a public reflection API. This means that the debugger must be explicitly loaded (either automatically from a settings file at startup or from the top-level). It also means that the debugger can be easily extended or replaced by an alternative application.

directive operators

  Some Prolog systems declare directive names as operators (e.g., `dynamic`, `multifile`, …). This is not required by the ISO Prolog Core standard. It’s a practice that should be avoided as it makes code non-portable.

encapsulation

  Logtalk enforces encapsulation of object predicates, generating a permission error when a predicate is not within the scope of the caller. In contrast, most Prolog module systems allow any module predicate to be called by using explicit qualification, even if not exported. Worse, some Prolog systems also allow defining clauses for a module predicate outside the module, without declaring the predicate as multifile, by simply writing clauses with explicit module-qualified heads.

entity loading

  When using Prolog modules, `use_module/1-2` (or equivalent) directives both load the module files and declare that the (implicitly or explicitly) imported predicates can be used with implicit module qualification. But Logtalk separates entity (object, protocol, category, or module) predicate usage declarations (via uses/1 and uses/2 or its own use_module/1 and use_module/2 directives) from loading goals (using the logtalk_load/1 and logtalk_load/2 predicates), called using an explicit and disciplined approach from loader files.

flags scope

  The set_logtalk_flag/2 directive is always local to the entity or source file that contains it. Only calls to the set_logtalk_flag/2 predicate set the global default value for a flag. This distinction is lacking in Prolog (where directives usually have a global scope) and Prolog modules (where some flags are local to modules in some systems and global in other systems).

meta-predicate call semantics

  Logtalk provides consistent meta-predicate call semantics: meta-arguments are always called in the meta-predicate calling context. This contrasts with Prolog module meta-predicates where the semantics of implicitly qualified calls are different from explicitly qualified calls.

operators scope

  Operators declared inside an entity (object, protocol, or category) are local to the entity. But operators defined in a source file but outside an entity are global for compatibility with existing Prolog code.

predicates scope

  In plain Prolog, all predicates are visible. In a Prolog module, a predicate can be exported or local. In Logtalk, a predicate can be public, protected, private, or local.

predicate declaration

  Logtalk provides a clear distinction between declaring a predicate and defining a predicate. This is a fundamental requirement for the concept of protocol (aka interface) in Logtalk: we must be able to declare a predicate without necessarily defining it. This clear distinction is missing in Prolog and Prolog modules. Notably, it’s a compiler error for a module to try to export a predicate that it does not define.

predicate loading conflicts

  Logtalk does not use predicate import/export semantics. Thus, there are never conflicts when loading entities (objects, protocols, or categories) that declare the same public predicates. But attempting to load two Prolog modules that export the same predicate results in a conflict, usually a compilation error (this is especially problematic when the `use_module/1` directive is used; e.g. adding a new exported predicate can break applications that use the module but not the new predicate).

## Smalltalk nomenclature

The Logtalk name originates from a combination of the Prolog and Smalltalk names. Smalltalk had a significant influence on the design of Logtalk and thus inherits some of its ideas and nomenclature. The following list relates the most commonly used Smalltalk terms with their Logtalk counterparts.

abstract class

  Similar to Smalltalk, an abstract class is just a class not meant to be instantiated by not understanding a message to create instances.

assignment statement

  Logtalk, as a superset of Prolog, uses logic variables and unification and thus provides no equivalent to the Smalltalk assignment statement.

block

  Logtalk supports lambda expressions and meta-predicates, which can be used to provide similar functionality to Smalltalk blocks.

class

  In Logtalk, class is a just a role that an object can play. This is similar to Smalltalk, where classes are also objects.

class method

  Class methods in Logtalk are simply instance methods declared and defined in the class metaclass.

class variable

  Logtalk objects, which can play the roles of class and instance, encapsulate predicates, not state. Class variables, which in Smalltalk are really shared instance variables, can be emulated in a class by defining a predicate locally instead of defining it in the class instances.

inheritance

  While Smalltalk only supports single inheritance, Logtalk supports single inheritance, multiple inheritance, and multiple instantiation.

instance

  While in Smalltalk every object is an instance of some class, objects in Logtalk can play different roles, including the role of a prototype where the concepts of instance and class don’t apply. Moreover, instances can be either created dynamically or defined statically.

instance method

  Instance methods in Logtalk are simply predicates declared and defined in a class and thus inherited by the class instances.

instance variable

  Logtalk being a declarative language, objects encapsulate a set of predicates instead of encapsulating state. But different objects may provide different definitions of the same predicates. Mutable internal state as in Smalltalk can be emulated by using dynamic predicates.

message

  Similar to Smalltalk, a message is a request for an operation, which is interpreted in Logtalk as a logic query, asking for the construction of a proof that something is true.

message selector

  Logtalk uses the predicate template (i.e., the predicate callable term with all its arguments unbound) as a message selector. The actual type of the message arguments is not considered. Like Smalltalk, Logtalk uses single dispatch on the message receiver.

metaclass

  Metaclasses are optional in Logtalk (except for a root class) and can be shared by several classes. When metaclasses are used, infinite regression is simply avoided by making a class an instance of itself.

method

  Same as in Smalltalk, a method is the actual code (i.e., predicate definition) that is run to answer a message. Logtalk uses the words method and predicate interchangeably.

method categories

  There is no support in Logtalk for partitioning the methods of an object into different categories. The Logtalk concept of category (a first-class entity) was, however, partially inspired by Smalltalk method categories.

object

  Unlike Smalltalk, where everything is an object, Logtalk language constructs include both terms (as in Prolog representing e.g. numbers and structures) and three first-class entities: objects, protocols, and categories.

pool variables*

  Logtalk, as a superset of Prolog, uses predicates with no distinction between variables and methods. Categories can be used to share a set of predicate definitions between any number of objects.

protocol

  In Smalltalk, an object protocol is the set of messages it understands. The same concept applies in Logtalk. But Logtalk also supports protocols as first-class entities where a protocol can be implemented by multiple objects and an object can implement multiple protocols.

self

  Logtalk uses the same definition of self found in Smalltalk: the object that received the message being processed. Note, however, that self is not a keyword in Logtalk but is implicit in the (::)/1 message to self control construct.

subclass

  Same definition in Logtalk.

super

  As in Smalltalk, the idea of super is to allow calling an inherited predicate (that is usually being redefined). Note, however, that super is not a keyword in Logtalk, which provides instead a (^^)/1 super call control construct.

superclass

  Same definition in Logtalk. But while in Smalltalk a class can only have a single superclass, Logtalk support for multiple inheritance allows a class to have multiple superclasses.

## C++ nomenclature

There are several C++ glossaries available on the Internet. The list that follows relates the most commonly used C++ terms with their Logtalk equivalents.

abstract class

  Logtalk uses an operational definition of abstract class: any class that does not inherit a method for creating new instances can be considered an abstract class. Moreover, Logtalk supports interfaces/protocols, which are often a better way to provide the functionality of C++ abstract classes.

base class

  Logtalk uses the term superclass with the same meaning.

data member

  Logtalk uses predicates for representing both behavior and data.

constructor function

  There are no special methods for creating new objects in Logtalk. Instead, Logtalk provides a built-in predicate, create_object/4, which can be used as a building block to define more sophisticated object creation predicates.

derived class

  Logtalk uses the term subclass with the same meaning.

destructor function

  There are no special methods for deleting new objects in Logtalk. Instead, Logtalk provides a built-in predicate, abolish_object/1, which is often used to define more sophisticated object deletion predicates.

friend function

  Not supported in Logtalk. Nevertheless, see the User Manual section on meta-predicates.

instance

  In Logtalk, an instance can be either created dynamically at runtime or defined statically in a source file in the same way as classes.

member

  Logtalk uses the term predicate.

member function

  Logtalk uses predicates for representing both behavior and data.

namespace

  Logtalk does not support multiple identifier namespaces. All Logtalk entity identifiers share the same namespace (Logtalk entities are objects, categories, and protocols).

nested class

  Logtalk does not support nested classes.

static member

  Logtalk does not support a `static` keyword. But the equivalent of static members can be declared in a class metaclass.

template

  Logtalk supports parametric objects, which allows you to get the similar functionality of templates at runtime.

this

  Logtalk uses the built-in context method self/1 for retrieving the instance that received the message being processed. Logtalk also provides a this/1 method but for returning the class containing the method being executed. Why the name clashes? Well, the notion of self was inherited from Smalltalk, which predates C++.

virtual member function

  There is no `virtual` keyword in Logtalk. Any inherited or imported predicate can be redefined (either overridden or specialized). Logtalk can use static binding or dynamic binding for locating both method declarations and method definitions. Moreover, methods that are declared but not defined simply fail when called (as per the closed-world assumption).

## Java nomenclature

There are several Java glossaries available on the Internet. The list that follows relates the most commonly used Java terms with their Logtalk equivalents.

abstract class

  Logtalk uses an operational definition of abstract class: any class that does not inherit a method for creating new instances is an abstract class. I.e. there is no `abstract` keyword in Logtalk.

abstract method

  In Logtalk, you may simply declare a method (predicate) in a class without defining it, leaving its definition to some descendant subclass.

assertion

  There is no `assertion` keyword in Logtalk. Assertions are supported using Logtalk compilation hooks and developer tools.

class

  Logtalk objects can play the role of classes, instances, or protocols (depending on their relations with other objects).

extends

  There is no `extends` keyword in Logtalk. Class inheritance is indicated using specialization relations. Moreover, the extends relation is used in Logtalk to indicate protocol, category, or prototype extension.

interface

  Logtalk uses the term protocol with a similar meaning. But note that Logtalk objects and categories declared as implementing a protocol are not required to provide definitions for the declared predicates (closed-world assumption).

callback method

  Logtalk supports event-driven programming, the most common usage context of callback methods. Callback methods can also be implemented using meta-predicates.

constructor

  There are no special methods for creating new objects in Logtalk. Instead, Logtalk provides a built-in predicate, create_object/4, which is often used to define more sophisticated object creation predicates.

final

  There is no `final` keyword in Logtalk. Predicates can always be redeclared and redefined in subclasses (and instances!).

inner class

  Inner classes are not supported in Logtalk.

instance

  In Logtalk, an instance can be either created dynamically at runtime or defined statically in a source file in the same way as classes.

method

  Logtalk uses the term predicate interchangeably with the term method.

method call

  Logtalk usually uses the expression message-sending for method calls, true to its Smalltalk heritage.

method signature

  Logtalk selects the method/predicate to execute in order to answer a method call based only on the method name and number of arguments. Logtalk (and Prolog) are not typed languages in the same sense as Java.

package

  There is no concept of packages in Logtalk. All Logtalk entities (objects, protocols, categories) share a single namespace. But Logtalk does support a concept of library that allows grouping of entities whose source files share a common path prefix.

reflection

  Logtalk features a white box API supporting structural reflection about entity contents, a black box API supporting behavioral reflection about object protocols, and an events API for reasoning about messages exchanged at runtime.

static

  There is no `static` keyword in Logtalk. See the entries below on static method and static variable.

static method

  Static methods may be implemented in Logtalk by using a metaclass for the class and defining the static methods in the metaclass. I.e. static methods are simply instance methods of the class metaclass.

static variable

  Static variables are shared instance variables and can simply be both declared and defined in a class. The built-in database methods can be used to implement destructive updates if necessary by accessing and updating a single clause of a dynamic predicate stored in the class.

super

  Instead of a `super` keyword, Logtalk provides a super operator and control construct, (^^)/1, for calling overridden methods.

synchronized

  Logtalk supports multi-threading programming in selected Prolog compilers, including a synchronized/1 predicate directive. Logtalk allows you to synchronize a predicate or a set of predicates using per-predicate or per-predicate-set mutexes.

this

  Logtalk uses the built-in context method self/1 for retrieving the instance that received the message being processed. Logtalk also provides a this/1 method but for returning the class containing the method being executed. Why the name clashes? Well, the notion of self was inherited from Smalltalk, which predates C++.

## Python nomenclature

The list that follows relates the commonly used Python concepts with their Logtalk equivalents.

abstract class

  Logtalk uses a different definition of abstract class: a class that does not inherit a method for creating new instances. Notably, the presence of abstract methods (i.e., predicates that are declared but not defined) does not make a class abstract.

abstract method

  Logtalk uses the term predicate interchangeably with method. Predicates can be declared without also being defined in an object (or category).

class

  Logtalk objects can play the role of classes, instances, or protocols (depending on their relations with other objects).

dictionary

  There is no native, built-in associative data type. But the library provides several implementations of a dictionary protocol.

function

  The closest equivalent is a predicate defined in `user`, a pseudo-object for predicates not defined in regular objects, and thus callable from anywhere without requiring a scope directive.

function object

  Predicate calls (goals) can be passed or returned from other predicates and unified with other terms (e.g., variables).

import path

  Logtalk uses the term library to refer to a directory of source files and supports defining aliases (symbolic names) to library paths to abstract the actual locations.

lambda

  Logtalk natively supports lambda expressions.

list

  Lists are compound terms with native syntax support.

list comprehensions

  There is no native, built-in support for list comprehensions. But the standard `findall/3` predicate can be used to construct a list by calling a goal that generates the list elements.

loader

  Logtalk uses the term loader to refer to source files whose main or sole purpose is to load other source files.

loop

  There are no native loop control constructs based on a counter. But the library provides implementations of several loop predicates.

metaclass

  Logtalk objects play the role of metaclasses when instantiated by objects that play the role of classes.

method

  Logtalk uses the terms method and predicate interchangeably. Predicates can be defined in objects (and categories). The value of self is implicit, unlike in Python where it is the first parameter of any method.

method resolution order

  Logtalk uses a depth-first algorithm to lookup method (predicate) declarations and definitions. It’s possible to use predicate aliases to access predicate declarations and definitions other than the first ones found by the lookup algorithm.

object

  Objects are first-class entities that can play multiple roles, including prototype, class, instance, and metaclass.

package

  Logtalk uses the term library to refer to a directory of source files defining objects, categories, and protocols.

set

  There is no native, built-in set type. But the library provides set implementations.

string

  The interpretation of text between double-quotes depends on the `double_quotes` flag. Depending on this flag, double-quoted text can be interpreted as a list of characters, a list of character codes, or an atom. Some backend Prolog compilers allow double-quoted text to be interpreted as a string in the Python sense.

tuple

  Compound terms can be used to represent tuples of any complexity.

variable

  Logtalk works with logical variables, which are close to the mathematical concept of variables and distinct from variables in imperative or imperative-based OOP languages where they are symbolic names for memory locations. Logical variables can be unified with any term, including other variables.

while loop

  The built-in `forall/2` predicate implements a generate-and-test loop.