Nomenclature

Depending on your 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 Smalltalk, arguably (and somehow sadly) not the most popular OOP language nowadays.

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 is 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 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 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.

Smalltalk nomenclature

The Logtalk name originates from a combination of the Prolog and Smalltalk names. Smalltalk had a significant influence in 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 same 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 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 in 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 includes 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 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 to 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 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 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 updated 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 Java 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 does not a class abstract.
abstract method
Logtalk uses the term predicate interchangeably with method. Predicates can be declared without being also 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
Predicates 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 can play the role of metaclasses by instantiating other objects that play the role of classes.
method
Logtalk uses the terms method and predicate interchangeably. Predicates canc 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.