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Glossary

.. _glossary:

.. glossary:: :sorted:

ancestor A :term:class or a parent :term:prototype that contributes (via inheritance) to the definition of an object. For class-based hierarchies, the ancestors of an instance are its class(es) and all the superclasses of its class(es). For prototype-based hierarchies, the ancestors of a prototype are its parent(s) and the ancestors of its parent(s).

category A set of :term:`predicates directives <predicate directive>` and clauses that can be (virtually) imported by any object. Categories support composing objects using fine-grained units of code reuse and also :term:`hot patching` of existing objects. A category should be functionally-cohesive, defining a single functionality.

complementing category A category used for :term:`hot patching` an existing object (or a set of objects).

parametric category See :term:`parametric entity`.

class An :term:object that specializes another object, interpreted as its superclass. A class defines the common predicates of a set of objects that instantiate it. An object can also be interpreted as a class when it instantiates itself.

abstract class A :term:class that cannot be instantiated by sending it a message. Usually used to contain common predicates that are inherited by other classes.

metaclass The :term:class of a class, when interpreted as an :term:instance. Metaclass instances are themselves classes. Metaclasses are optional, except for the root class, and can be shared by several classes.

subclass A :term:class that is a specialization, direct or indirectly, of another class. A class may have multiple subclasses.

superclass A :term:class from which another class is a specialization (directly or indirectly via another class). A class may have multiple superclasses.

closed-world assumption The assumption that what cannot be proved true is false. Therefore, sending a :term:message corresponding to a declared but not defined predicate, or calling a declared predicate with no clauses, fails. But messages or calls to undeclared predicates generate an error.

directive A source file term that affects the interpretation of source code. Directives use the (:-)/1 prefix operator as functor.

entity directive A directive that affects how Logtalk :term:`entities <entity>` are used or compiled.

predicate directive A directive that specifies a predicate property that affects how predicates are called or compiled.

source file directive A :term:directive that affects how a :term:`source file` is compiled.

encapsulation The hiding of an object implementation. This promotes software reuse by isolating the object clients from its implementation details. Encapsulation is enforced in Logtalk by using :term:`predicate scope directives <predicate scope directive>`.

entity Generic name for Logtalk compilation units: :term:`objects <object>`, :term:`categories <category>`, and :term:`protocols <protocol>`. Entities share a single namespace (i.e., entity :term:`identifiers <identity>` must be unique) and can be static (the default) or dynamic. Static entities are defined in source files. Dynamic entities can also be defined in source files but are usually created and abolished at runtime using the language built-in predicates.

parametric entity An :term:object or :term:category whose :term:`identifier <identity>` is a compound term possibly containing free variables that can be used to parameterize the entity predicates. Parameters are logical variables implicitly shared by all the entity clauses. Note that the identifier of a parametric entity is its functor, irrespective of the possible values of its arguments (e.g., foo(bar) and foo(baz) are different parameterizations of the same parametric entity, foo/1).

entity alias An alternative name for an entity. Entity aliases can be defined using the :ref:directives_uses_1 and :ref:directives_use_module_1 directives. Entity aliases can be used to improve code clarity by using alternative names that are more meaningful in the calling context, to bind parametric entity parameters, and to simplify experimenting with alternative entities implementing the same protocol.

static entity See :term:entity.

dynamic entity See :term:entity.

event The sending of a :term:message to an object. An event can be expressed as an ordered tuple: (Event, Object, Message, Sender). Logtalk distinguishes between the sending of a message — before event — and the return of control to the sender — after event.

grammar rule An alternative notation for predicates used to parse or generate sentences in some language. This notation hides the arguments used to pass the sequences of tokens being processed, thus simplifying the representation of grammars. Grammar rules are represented using as functor the infix operator (-->)/2 instead of the (:-)/2 operator used with predicate clauses.

grammar rule non-terminal A syntactic category of words or phrases. A non-terminal is identified by its non-terminal indicator, i.e. by its name and number of arguments using the notation Name//Arity.

grammar rule terminal A word or basic symbol of a language.

identity Property of an :term:entity that distinguishes it from every other entity. The identifier of an entity is its functor (i.e., its name and arity), which must be unique. Object and :term:category identifiers can be atoms or compound terms. Protocol identities must be atoms. All Logtalk entities (objects, protocols, and categories) share the same namespace.

inheritance An :term:entity inherits :term:`predicates directives <predicate directive>` and clauses from related entities. In the particular case of objects, when an object extends other object, we have prototype-based inheritance. When an object specializes or instantiates another object, we have class-based inheritance. See also :term:public inheritance, :term:`protected inheritance`, and :term:`private inheritance`.

private inheritance All public and protected predicates are inherited as private predicates. See also :term:public inheritance and :term:`protected inheritance`.

protected inheritance All public predicates are inherited as protected. No scope change for protected or private predicates. See also :term:public inheritance and :term:`private inheritance`.

public inheritance All inherited predicates maintain their declared scope. See also :term:`protected inheritance` and :term:`private inheritance`.

instance An :term:object that instantiates another object, interpreted as its :term:class. An object may instantiate multiple objects (also known as multiple instantiation).

instantiation The process of creating a new :term:class :term:instance. In Logtalk, this does not necessarily imply dynamic creation of an object at runtime; an instance may also be defined as a static object in a source file.

polymorphism Different objects (and categories) can provide different implementations of the same predicate. The predicate declaration can be inherited from a common ancestor, also known as subtype polymorphism. Logtalk implements single dispatch on the receiver of a message, which can be described as single-argument polymorphism. As :term:`message lookup` only uses the predicate functor, multiple predicate implementations for different types of arguments are possible, also known as ad hoc polymorphism. :term:`Parametric objects and categories <parametric entity>` enable implementation of parametric polymorphism by using one or more parameters to pass object identifiers that can be used to parameterize generic predicate definitions.

library A directory containing source files. See also :term:`library alias` and :term:`library notation`.

library alias An atom that can be used as an alias for a :term:library full path. Library aliases and their corresponding paths can be defined using the :ref:predicates_logtalk_library_path_2 predicate. See also :term:`library notation`.

library notation A compound term where the name is a :term:`library alias` and the single argument is a :term:`source file` relative path. Use of library notation simplifies compiling and loading source files and can make an application easily relocatable by defining an alias for the root directory of the application files.

module A Prolog entity characterized by an identity and a set of :term:`predicates directives <predicate directive>` and clauses. Prolog modules are usually static although some Prolog systems allow the creation of dynamic modules at runtime. Prolog modules can be interpreted as prototypes.

message A query sent to an object. In logical terms, a message can be interpreted as a request for proof construction using an object database and the databases of related entities.

message lookup Sending a message to an object requires a lookup for the :term:`predicate declaration`, to check if the message is within the scope of the sender, and a lookup for the :term:`predicate definition` that is going to be called to answer the message. Message lookup can occur at :term:`compile <static binding>` time or at :term:`runtime <dynamic binding>`.

message to self A message sent to the object that received the original message under processing. Messages to self require :term:dynamic binding as the value of self is only know at runtime.

meta-interpreter A program capable of running other programs written in the same language.

method The :term:`predicate definition` used to answer a :term:message sent to an object. Logtalk supports both :term:`static binding` and :term:dynamic binding to find which method to run to answer a message. See also :term:`built-in method`.

abstract method A :term:method implementing an algorithm whose step corresponds to calls to methods defined in the descendants of the object (or :term:category) containing it.

built-in method A predefined :term:method that can be called from within any object or :term:category. I.e. built-in methods are built-in object and category predicates. Built-in methods cannot be redefined.

singleton method A :term:method defined in an :term:instance itself. Singleton methods are supported in Logtalk and can also be found in other object-oriented programming languages.

template method See :term:`abstract method`.

monitor Any object, implementing the :ref:`monitoring apis:monitoring/0` built-in protocol, that is notified by the runtime when a spied event occurs. The spied :term:`events <event>` can be set by the monitor itself or by any other object.

object An entity characterized by an :term:identity and a set of :term:`predicates directives <predicate directive>` and clauses. Logtalk objects can be either static or dynamic. Logtalk objects can play the role of classes, instances, or prototypes. The role or roles an object plays are a function of its relations with other objects.

object database The set of predicates locally defined inside an object.

doclet object An object specifying the steps necessary to (re)generate the API documentation for a project. See the :doc:devtools/doclet and :doc:`devtools/lgtdoc/` tools for details.

hook object An object, implementing the :ref:`expanding apis:expanding/0` built-in protocol, defining term- and goal-expansion predicates, used in the compilation of Logtalk or Prolog source files. A hook object can be specified using the :ref:`hook <flag_hook> flag. It can also be specified using a :ref:directives_set_logtalk_flag_2` directive in the source files to be expanded.

expansion workflow A sequence of term-expansion or goal-expansion steps where each step is usually defined using a :term:`hook object` or a combination of hook objects.

parametric object See :term:`parametric entity`.

parametric object proxy A compound term (usually represented as a plain Prolog fact) with the same name and number of arguments as the identifier of a parametric object.

parameter An argument of a parametric object or a parametric category identifier. Parameters are logical variables implicitly shared by all the entity predicate clauses.

parameter variable A variable used as parameter in a parametric object or a parametric category using the syntax _ParameterName_ (i.e., a variable whose name stars and ends with an underscore). Parameter variables are logical variables shared by all entity terms. Occurrences of parameter variables in :term:`entity directives <entity directive>` and clauses are implicitly unified with the corresponding entity parameters.

parent A prototype that is extended by another prototype.

predicate Predicates describe what is true about the application domain. A predicate is identified by its predicate indicator, i.e. by its name and number of arguments using the notation Name/Arity. See also :term:`built-in predicate` and :term:method.

predicate declaration A predicate declaration is composed by a set of :term:`predicates directives <predicate directive>`, which must include ar least a :term:`scope directive <predicate scope directive>`.

predicate definition The set of clauses for a predicate, contained in an object or category. Predicate definitions can be overridden or specialized in descendant entities.

predicate alias An alternative functor (Name/Arity) for a predicate. Predicate aliases can be defined for any inherited predicate using the :ref:directives_alias_2 directive and for predicates listed in :ref:directives_uses_2 and :ref:directives_use_module_2 directives. Predicate aliases can be used to solve inheritance conflicts, to improve code clarity by using alternative names that are more meaningful in the calling context, and to use a different order of the predicate arguments.

predicate shorthand A :term:`predicate alias` that defines a call template, possibly using a different name, with a reduced number of arguments by hard-coding the value of the omitted arguments in the original call template. Predicate shorthands can be defined using :ref:directives_uses_2 and :ref:directives_use_module_2 directives. They can be used to simplify predicate calls and to ensure consistent call patterns when some of the arguments always use the same fixed values in the calling context.

built-in predicate A predefined predicate that can be called from anywhere. Built-in predicates can be redefined within :term:`objects <object>` and :term:`categories <category>`.

coinductive predicate A predicate whose calls are proved using greatest fixed point semantics. Coinductive predicates allows reasoning about infinite rational entities such as cyclic terms and ω-automata.

local predicate A predicate that is defined in an object (or in a :term:category) but that is not listed in a :term:`scope directive <predicate scope directive>`. These predicates behave like private predicates but are invisible to the reflection :term:`built-in methods <built-in method>`. Local predicates are usually auxiliary predicates and only relevant to the entity where they are defined.

meta-argument A predicate argument that is called as a goal, used as a :term:closure to construct a goal that will be called, or that is handled in a way that requires awareness of the predicate calling context.

meta-predicate A predicate with one or more :term:`meta-arguments <meta-argument>`. For example, :ref:methods_call_N and :ref:methods_findall_3 are built-in meta-predicates.

closure A callable term (i.e., an atom or a compound term) passed to a :term:meta-predicate call where it is extended with additional arguments to form a goal called by the meta-predicate.

predicate scope directive A directive that declares a predicate by specifying its visibility as public, protected, or private.

predicate scope container The object that inherits a :term:`predicate declaration` from an imported :term:category or an implemented :term:protocol.

private predicate A predicate that can only be called from the object that contains its :term:`scope directive <predicate scope directive>`.

protected predicate A predicate that can only be called from the object containing its :term:`scope directive <predicate scope directive>` or from an object that inherits the predicate.

public predicate A predicate that can be called from any object.

primary predicate declaration See :term:multifile predicate.

predicate calling context The object or category from within a predicate is called (either directly or using a control construct such as a message-sending control construct).

predicate definition context The object or category that contains the definition (i.e., clauses) for a predicate.

predicate execution context The implicit arguments (including :term:sender, :term:self, and :term:this) required for the correct execution of a predicate call.

dynamic predicate A predicate whose clauses can be dynamically added or retracted at runtime.

discontiguous predicate A predicate whose clauses are not contiguous in a :term:`source file`. I.e. a predicate whose clauses are mixed with clauses for other predicates.

multifile predicate A predicate whose clauses can be defined in multiple :term:`entities <entity>` and :term:`source files <source file>`. The object or category holding the directive without an entity prefix qualifying the predicate holds the multifile predicate primary declaration, which consists of both a :term:`scope directive <predicate scope directive>` and a :ref:directives_multifile_1 directive for the predicate.

synchronized predicate A synchronized predicate is protected by a mutex ensuring that, in a multi-threaded application, it can only be called by a single thread at a time.

visible predicate A predicate that is within scope, a locally defined predicate, a :term:`built-in method`, a Logtalk built-in predicate, or a Prolog built-in predicate.

hook predicate A predicate, usually declared :term:`multifile <multifile predicate>`, that allows the user to customize another predicate or provide alternative definitions for a default predicate definition.

profiler A program that collects data about other program performance.

protocol An entity that contains :term:`predicate declarations <predicate declaration>`. A predicate is declared using a :term:`scope directive <predicate scope directive>`. It may be further specified by additional predicate directives. Protocols support the separation between interface and implementation, can be implemented by both objects and categories, and can be extended by other protocols. A protocol should be functionally-cohesive, specifying a single functionality. Also known as interface.

interface See :term:protocol.

prototype A self-describing object that may extend or be extended by other objects. An object with no instantiation or specialization relations with other objects is always interpreted as a prototype.

self The object that received the :term:message under processing.

sender An object that sends a :term:message to other object. When a message is sent from within a :term:category, the sender is the object importing the category on which behalf the message was sent.

super call Call of an inherited (or imported) :term:`predicate definition`. Mainly used when redefining an inherited (or imported) predicate to call the overridden definition while making additional calls. Super calls preserve :term:self and may require :term:dynamic binding if the predicate is dynamic.

specialization A :term:class is specialized by defining a new class that inherits its predicates and possibly adds new ones.

source file A text file defining Logtalk and/or Prolog code. Multiple Logtalk entities may be defined in a single source file. Plain Prolog code may be intermixed with Logtalk entity definitions. Depending on the used :term:`backend Prolog compiler`, the text encoding may be specified using an :ref:directives_encoding_1 directive as the first term in the first line in the file.

adapter file A Prolog source file defining a minimal abstraction layer between the Logtalk compiler/runtime and a specific :term:`backend Prolog compiler`.

doclet file A :term:`source file` whose main purpose is to generate documentation for a :term:library or an application.

loader file A :term:`source file` whose main purpose is to load a set of source files (possibly with specific compiler flags) and any library dependencies.

settings file A :term:`source file`, compiled and loaded automatically by default at Logtalk startup, mainly defining default values for compiler flags that override the defaults found on the backend Prolog compiler :term:`adapter files <adapter file>`.

tester file A :term:`source file` whose main purpose is to load and a run a set of unit tests.

component A unique atom or compound term template identifying a library, tool, application, or application sub-system. Component names are notably used by the message printing and question asking mechanisms. Compound terms are used instead of atoms when parameterization is required.

scratch directory The directory used to save the intermediate Prolog files generated by the compiler when compiling :term:`source files <source file>`.

this The object that contains the predicate clause under execution. When the predicate clause is contained in a :term:category, this is a reference to the object importing the category on which behalf the predicate clause is being used to prove the current goal.

dynamic binding Runtime lookup of a :term:`predicate declaration` and :term:`predicate definition` to verify the validity of a :term:message (or a :term:`super call`) and find the predicate definition that will be used to answer the message (or the super call). Also known as late binding. See also :term:`static binding`.

late binding See :term:dynamic binding.

static binding Compile time lookup of a :term:`predicate declaration` and :term:`predicate definition` when compiling a :term:message sending call (or a :term:`super call`). Dynamic binding is used whenever static binding is not possible (e.g., due to the predicate being dynamic or due to lack of enough information at compilation time). Also known as early binding. See also :term:dynamic binding.

early binding See :term:`static binding`.

lambda expression A compound term that can be used in place of a goal or :term:closure meta-argument and that abstracts a :term:`predicate definition` by listing its variables and a callable term that implements the definition. Lambda expressions help avoid the need of naming and defining auxiliary predicates.

lambda parameter A term (usually a variable or a non-ground compound term) that is local to a :term:`lambda expression`. All lambda parameters must be explicitly enumerated in a lambda expression for well-defined semantics.

lambda free variable A variable that is global to a :term:`lambda expression`. All used global variables must be explicitly listed in a lambda expression for well-defined semantics.

hot patching The act of fixing :term:`entity directives <entity directive>` and predicates or adding new entity directives and predicates to loaded :term:`entities <entity>` in a running application without requiring access to the entities source code or restarting the application. Achieved using :term:`complementing categories <complementing category>`.

threaded engine A computing thread running a goal whose solutions can be lazily and concurrently computed and retrieved. A threaded engine also supports a term queue that allows passing arbitrary terms to the engine. This queue can be used to pass e.g. data and new goals to the engine.

backend Prolog compiler The Prolog compiler that is used to host and run Logtalk and that is called for compiling the intermediate Prolog code generated by the Logtalk compiler when compiling source files.

steadfastness A predicate definition is steadfast when it still generates only correct answers when called with unexpected arguments (notably, bound output arguments). Typically, a predicate may not be steadfast when output argument unifications can occur before a cut in a predicate clause body.

top-level interpreter shorthand Aliases for frequently used built-in predicates such as :ref:predicates_logtalk_load_1 and :ref:predicates_logtalk_make_1. These shorthands are not part of the Logtalk language and must only be used at the top-level interpreter.

meta-variable A variable in a :term:meta-argument position that is expected to be unified with a goal or a closure at runtime.

naked meta-variable A :term:meta-variable used as the body of a predicate clause or grammar rule or used in a cut-transparent argument of a control construct. The "naked" designation highlights that the meta-variable is not wrapped by call/1 or phrase//1 goals.

homoiconic A property of programming languages where code and data use the same representation. Logtalk (and Prolog) are examples of homoiconic programming languages. A Logtalk program is a set of terms (clauses and directives) that can be handled as data by e.g. the term-expansion mechanism.

clause reference An opaque term that uniquely identifies a clause. Provided by some backends via alternative database built-in predicates.