The Object Constraint Language (OCL) is a

declarative language In computer science, declarative programming is a programming paradigm—a style of building the structure and elements of computer programs—that expresses the logic of a computation without describing its control flow. Many languages that app ...

describing rules applying to

Unified Modeling Language The Unified Modeling Language (UML) is a general-purpose visual modeling language that is intended to provide a standard way to visualize the design of a system. UML provides a standard notation for many types of diagrams which can be roughly ...

(UML) models developed at

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and is now part of the UML standard. Initially, OCL was merely a formal specification language extension for UML. OCL may now be used with any

Meta-Object Facility The Meta-Object Facility (MOF) is an Object Management Group (OMG) standard for model-driven engineering. Its purpose is to provide a type system for entities in the CORBA architecture and a set of interfaces through which those types can be cre ...

(MOF)

Object Management Group The Object Management Group (OMG®) is a computer industry Standards Development Organization (SDO), or Voluntary Consensus Standards Body (VCSB). OMG develops enterprise integration and modeling standards for a range of technologies. Busin ...

(OMG) meta-model, including UML. The Object Constraint Language is a precise text language that provides constraint and object query expressions on any MOF model or meta-model that cannot otherwise be expressed by diagrammatic notation. OCL is a key component of the new OMG standard recommendation for transforming models, the Queries/Views/Transformations ( QVT) specification.

Description

OCL is a descendant of Syntropy, a second-generation object-oriented analysis and design method. The OCL 1.4 definition specified a constraint language. In OCL 2.0, the definition has been extended to include general object query language definitions. OCL statements are constructed in four parts: # a context that defines the limited situation in which the statement is valid # a property that represents some characteristics of the context (e.g., if the context is a class, a property might be an attribute) # an operation (e.g., arithmetic, set-oriented) that manipulates or qualifies a property, and # keywords (e.g., if, then, else, and, or, not, implies) that are used to specify conditional expressions.

Relation

OCL and UML

OCL supplements UML by providing expressions that have neither the ambiguities of natural language nor the inherent difficulty of using complex mathematics. OCL is also a navigation language for graph-based models.

OCL and MOF

OCL makes a

model more precise by associating assertions with its meta-elements.

OCL and QVT

Of particular importance to Model Driven Engineering (MDE) or model-driven architecture is the notion of Model transformation. The OMG has defined a specific standard for model transformation called MOF/QVT or in short QVT. Several model transformation languages like

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VIATRA VIATRA is an open-source model transformation framework based on the Eclipse Modeling Framework (EMF) and hosted by the Eclipse Foundation. VIATRA supports the development of model transformations with specific focus on event-driven, reactive ...

, or Tefkat are presently available, with different levels of compliance with the QVT standard. Many of these languages are built on top of OCL, which is the main part of the QVT-compliance.

Alternatives

Being a rule-based validation language, Schematron may be considered an alternative to OCL. However Schematron works for Extensible Markup Language (XML) trees while OCL makes it possible to navigate MOF-based models and meta-models (i.e. XML Metadata Interchange ( XMI) trees). In other words, OCL relates to UML or MOF similarly to how Schematron relates to

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing data. It defines a set of rules for encoding electronic document, documents in a format that is both human-readable and Machine-r ...

. (Note that Schematron uses

XPath XPath (XML Path Language) is an expression language designed to support the query or transformation of XML documents. It was defined by the World Wide Web Consortium (W3C) in 1999, and can be used to compute values (e.g., strings, numbers, or ...

to navigate inside the XML trees.)
Being a model specification language permitting designers to decorate a model or a meta-model with side-effect-free annotations, OCL could be replaced by languages like

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. Automated OCL generation is in principle possible from natural language.

Extensions

Eclipse OCL extends Standard OCL with additional operators and a type for Maps. AgileUML extends Standard OCL with Map and Function types. These extensions are consistent with the map and function types present in modern programming languages such as Python and Swift.

References

External links

OMG OCL specification

OCL Portal - The center for OCL related information

(brief OCL 2.0 syntax)
Octopus: OCL Tool for Precise Uml Specifications
(OCL checker) * (OCL Toolkit, various OCL related publications)
HOL-OCL
(An interactive theorem proof environment for OCL, various OCL related publications)
OCL for Java tutorial on ParlezUML

USE (UML-based Specification Environment)
(OCL Tool for model validation, various OCL related publications)
OCL tutorial

NL2OCL
(OCL Tool for invariant generation from NL) {{ISO standards Unified Modeling Language Formal specification languages ISO standards