实化

实化（英語：reification）在计算机科学中是指把抽象的想法（idea）转化为明确的数据模型或程序设计语言创建的其他对象的过程。通过实化，以前是隐式的、未表示的、可能是不可表示的被显式形成可概念（逻辑或计算）操作的。非正式地，实化也常被称作在特定系统范围内把某些东西变为“头等公民”。 实化是概念分析与知识表示中最常用的技术。

在程序设计语言中的实化与反射

在程序设计语言中，把用户程序或者程序设计语言某一部分本来在编译期与运行期是隐式的，表示为语言自身，这一过程称为实化。这使得其对程序可当作普通数据来利用。在具有反射的程序设计语言中，实化的数据表示了相关的被实化的领域。实化的数据常被称作头等对象。

• C语言中，把内存地址的底层细节实化为指针。例如：char* buffer = (char*) 0xB800000;
• 基于λ演算的函数式程序设计语言，把过程抽象（procedure abstraction）与过程应用（procedure application）的概念实化为λ表达式。
• Scheme语言实化了延续性 (粗略说近似于运行期上下文环境)。
• C#实化了参数多态为多态，成为语言的头等特征。
• Java中存在“可实化的类型”（reifiable types），其编译期信息在运行期保留并可完全利用。
• REBOL实化了代码与数据的相互转换。
• 许多语言，如Curl, JavaScript, 与Lisp等提供了eval或evaluate过程使其有解释器的功能。
• Prolog语言的Logtalk框架提供了逻辑程序设计上下文的实化。
• Smalltalk与参与者模式的语言实化了块（block）与messages。
• Smalltalk-80把源代码编译为字节码，这是首例。

数据实化

数据实化涉及在形式规范中找到抽象数据类型的更为具体的表示。 例如，抽象数据类型集合（set）可以用map或者array来实化。

概念建模中的实化

概念建模中广泛使用了实化。实化一个关系（relationship）意味着把它视作一个实体。实化一个关系的目的是令其显式，必要时可以增加额外的信息。例如，关系IsMemberOf(member:Person, Committee)表示某人是某委员会的成员。下图左侧用表的形式列出了人P1是委员会C1与C2的成员；人P2是委员会C1成员。

Example population of 关系IsMemberOf的实化。

这一关系，也可看作实体。见图示的右侧。称作该实体实化了这个关系。这个实体类型为Membership。现在，可以给这个关系实体增加新的信息，如某人属于某委员会是由谁来提名加入的，可表示为IsNominatedBy(Membership, Person)。

与其密切相关的用法见实化 (知识表示)。

实化在UML中

UML的类图表示Membership关系例子。

UML提供了关联类（association class）用于构建被实化的关系类型。关联类既是关联又是类。

已隱藏部分未翻譯内容，歡迎參與翻譯。

Reification on Semantic Web

RDF and OWL

In Semantic Web languages, such as Resource Description Framework (RDF) and Web Ontology Language (OWL), a statement is a binary relation. It is used to link two individuals or an individual and a value. Applications sometimes need to describe other RDF statements, for instance, to record information like when statements were made, or who made them, which is sometimes called "provenance" information. As an example, we may want to represent properties of a relation, such as our certainty about it, severity or strength of a relation, relevance of a relation, and so on.

The example from the conceptual modeling section describes a particular person with URIref person:p1, who is a member of the committee:c1. The RDF triple from that description is

  person:p1   committee:isMemberOf   committee:c1 .

Consider to store two further facts: (i) to record who nominated this particular person to this committee (a statement about the membership itself), and (ii) to record who added the fact to the database (a statement about the statement).

The first case is a case of classical reification like above in UML: reify the membership and store its attributes and roles etc.:

 committee:Membership        rdf:type              owl:Class .
 committee:membership12345   rdf:type              committee:Membership .
 committee:membership12345   committee:ofPerson    person:p1 .
 committee:membership12345   committee:inCommittee committee:c1 .
 person:p2                   committee:nominated   committee:membership12345 .

Additionally, RDF provides a built-in vocabulary intended for describing RDF statements. A description of a statement using this vocabulary is called a reification of the statement. The RDF reification vocabulary consists of the type rdf:Statement, and the properties rdf:subject, rdf:predicate, and rdf:object.

Using the reification vocabulary, a reification of the statement about the person's membership would be given by assigning the statement a URIref such as committee:membership12345 so that describing statements can be written as follows:

 committee:membership12345Stat   rdf:type        rdf:Statement .
 committee:membership12345Stat   rdf:subject     person:p1 .
 committee:membership12345Stat   rdf:predicate   committee:isMemberOf . 
 committee:membership12345Stat   rdf:object      committee:c1 .

These statements say that the resource identified by the URIref committee:membership12345Stat is an RDF statement, that the subject of the statement refers to the resource identified by person:p1, the predicate of the statement refers to the resource identified by committee:isMemberOf, and the object of the statement refers to the resource committee:c1. Assuming that the original statement is actually identified by committee:membership12345, it should be clear by comparing the original statement with the reification that the reification actually does describe it. The conventional use of the RDF reification vocabulary always involves describing a statement using four statements in this pattern. Therefore, they are sometimes referred to as the "reification quad".

Using reification according to this convention, we could record the fact that person:p3 added the statement to the database by

  person:p3    committee:addedToDatabase    committee:membership12345Stat .

It is important to note that in the conventional use of reification, the subject of the reification triples is assumed to identify a particular instance of a triple in a particular RDF document, rather than some arbitrary triple having the same subject, predicate, and object. This particular convention is used because reification is intended for expressing properties such as dates of composition and source information, as in the examples given already, and these properties need to be applied to specific instances of triples. Note that the described triple (subject predicate object) itself is not implied by such a reification quad (and it is not necessary that it actually exists in the database). This allows also to use this mechanism to express which triples do not hold.

The power of the reification vocabulary in RDF is restricted by the lack of a built-in means for assigning URIrefs to statements, so in order to express "provenance" information of this kind in RDF, one has to use some mechanism (outside of RDF) to assign URIs to individual RDF statements, then make further statements about those individual statements, using their URIs to identify them.

Reification in Topic Maps

In an XML Topic Map (XTM), only a topic can have a name or play a role in an association. One may use an association to make an assertion about a topic, but one cannot directly make assertions about that assertion. However, it is possible to create a topic that reifies a non-topic construct in a map, thus enabling the association to be named and treated as a topic itself.

Reification and n-ary relations

In Semantic Web languages, such as RDF and OWL, a property is a binary relation used to link two individuals or an individual and a value. However, in some cases, the natural and convenient way to represent certain concepts is to use relations to link an individual to more than just one individual or value. These relations are called n-ary relations. Examples are representing relations among multiple individuals, such as a committee, a person who is a committee member and another person who has nominated the first person to become the committee member, or a buyer, a seller, and an object that was bought when describing a purchase of a book.

A more general approach to reification is to create an explicit new class and n new properties to represent an n-ary relation, making an instance of the relation linking the n individuals an instance of this class. This approach can also be used to represent provenance information and other properties for an individual relation instance.

:p1
     a       :Person ;
     :has_membership _:membership_12345 .
_:membership_12345
     a       :Membership ;
     :committee :c1;
     :nominated_by :p2 .

Reification vs. quotation

It is also important to note that the reification described here is not the same as "quotation" found in other languages. Instead, the reification describes the relationship between a particular instance of a triple and the resources the triple refers to. The reification can be read intuitively as saying "this RDF triple talks about these things", rather than (as in quotation) "this RDF triple has this form." For instance, in the reification example used in this section, the triple:

  committee:membership12345   rdf:subject   person:p1 .

describing the rdf:subject of the original statement says that the subject of the statement is the resource (the person) identified by the URIref person:p1. It does not state that the subject of the statement is the URIref itself (i.e., a string beginning with certain characters), as quotation would.

参见

• 指称语义
• 形式语义学
• 操作语义学