Editing Object Constraint Language

{{Short description|Declarative language}}
{{Update|date=April 2018}}

The '''Object Constraint Language''' ('''OCL''') is a [[declarative language]] describing rules applying to [[Unified Modeling Language]] (UML) models developed at [[IBM]] and is now part of the UML standard. Initially, OCL was merely a formal specification language extension for UML.<ref>Object Management Group (OMG); ''Object Constraint Language Specification'', Chapter 7 of ''OMG Unified Modeling Language Specification'', Version 1.3, March 2000 (first edition)</ref> OCL may now be used with any [[Meta-Object Facility]] (MOF) [[Object Management Group]] (OMG) [[meta-model]], including UML.<ref>Object Management Group (OMG); ''Object Constraint Language OMG Available Specification Version 2.0'', May 2006</ref> 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 (Software)|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.<ref name="FOLDOC">{{foldoc|Object+Constraint+Language}}</ref>

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.<ref name="FOLDOC" /> OCL is also a navigation language for graph-based models.

=== OCL and MOF ===
OCL makes a [[Meta-Object Facility]] 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 [[Object Management Group|OMG]] has defined a specific standard for model transformation called '''MOF/QVT''' or in short [[QVT]]. Several model transformation languages like [[GReAT]], [[VIATRA]], 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 tree|(XML) tree]]s while OCL makes it possible to navigate [[Meta-Object Facility|MOF]]-based models and meta-models (i.e. XML Metadata Interchange ([[XML Metadata Interchange|XMI]]) trees). In other words, OCL relates to UML or MOF similarly to how Schematron relates to [[XML]]. (Note that Schematron uses [[XPath]] to navigate inside the XML trees.)<br>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 [[Alloy language|Alloy]]. Automated OCL generation is in principle possible from natural language.<ref name="NL2OCL">{{cite journal| author=Imran Sarwar Bajwa|title=OCL Constraints Generation from Natural Language Specification, 2010|publisher =[[IEEE]]|date=October 2010|doi=10.1109/EDOC.2010.33|s2cid=7495256}}</ref>

{| class="wikitable"
|+ Examples
!Constraint
!OCL Equivalent
|-
|The age of a person is not negative.
|<code>'''context''' Person '''inv''': self.age >=0</code>
|-
|A person is younger than its parents.
|<code>'''context''' Person '''inv'''<nowiki>: self.parents->forAll(p|p.age>self.age)</nowiki></code>
|-
|After a birthday, a person becomes one year older.
|<code>'''context''' Person::hasBirthday() '''post''': self.age=self.age@pre+1</code>
|-
|A Person has 2 parents at max.
|<code>'''context''' Person '''inv''': self.parents->size()<=2</code>
|-
|After somebody has a child, his/her child-set is not empty, and it is larger than before.
|<code>'''context''' Person::getsChild() '''post''': self.childs->notEmpty() and self.childs->size() > self.childs@pre->size()</code>
|-
|Only an adult can be owner of a car.
|<code>'''context''' Person '''inv''': self.age<18 '''implies''' self.cars->isEmpty()</code>
|-
|The first registration of a car can not be before it is built.
|<code>'''context''' Auto '''inv''': self.registration>=self.constructionYear</code>
|-
|Every Person that has a car has at least one car which is younger than the Person.
|<code>'''context''' Person '''inv''': self.cars->notEmpty() '''implies'''<nowiki> self.cars->exists( c | </nowiki>'''Calendar.YEAR''' - c.constructionYear < self.age)</code>
|-
|Nobody can be his/her own parent.
|<code>'''context''' Person '''inv''': self.parents->excludes(self)</code>
|-
|There's at least one Person which owns a car.
|<code style="white-space: nowrap">'''context''' Person '''inv'''<nowiki>: Person.allInstances()->exists(p | p.cars->size() > 0)</nowiki></code>
|}
<br />

== Extensions ==

Eclipse OCL <ref>{{cite web | url=https://projects.eclipse.org/projects/modeling.mdt.ocl | title=Eclipse OCL™ (Object Constraint Language) | date=31 January 2013 }}</ref> extends Standard OCL with additional operators and a type for Maps. AgileUML extends Standard OCL with Map and Function types.<ref>{{cite book | chapter-url=https://link.springer.com/chapter/10.1007/978-3-030-89247-0_8 | doi=10.1007/978-3-030-89247-0_8 | chapter=Extending OCL with Map and Function Types | title=Fundamentals of Software Engineering | series=Lecture Notes in Computer Science | year=2021 | last1=Lano | first1=Kevin | last2=Kolahdouz-Rahimi | first2=Shekoufeh | volume=12818 | pages=108–123 | isbn=978-3-030-89246-3 | s2cid=239029860 }}</ref> These extensions are consistent with the map and function types present in modern programming languages such as Python and Swift.

== See also ==
* [[Computer model]]
* [[Data mapping]]
* [[Domain Specific Language]]  (DSL)
* [[Domain-specific modelling]] (DSM)
* [[Eclipse (software)|Eclipse]] [https://web.archive.org/web/20060407092135/http://www.eclipse.org/gmt/ GMT Project]
* [[Gello Expression Language]]
* [[Glossary of Unified Modeling Language terms]]
* [[List of UML tools]]
* [[Meta-modeling]]
* [[Meta-modeling technique]]
* [[Meta-Object Facility]] (MOF)
* [[Metadata]]
* [[Model-based testing]] (MBT) 
* [[Model-driven architecture]] (MDA)
* [[Model Driven Engineering]] (MDE)
* [[Model Transformation Language]] (MTL)
* [[Modeling language]]
* [[Modeling perspectives]]
* [[MOFM2T]]
* [[Object-oriented analysis and design]] (OOAD) 
* [[QVT|MOF Queries/Views/Transformations]] (QVT)
* [[Semantic translation]] 
* [[Transformation language]] (TL)
* [[UML tool]]
* [[Vocabulary-based transformation]]
* [[XML Metadata Interchange|XMI]]
* [[XML transformation language]] (XTL)

== References ==
{{Reflist}}

== External links ==
* [http://www.omg.org/spec/OCL/ OMG OCL specification]
* [http://www-st.inf.tu-dresden.de/ocl/ OCL Portal - The center for OCL related information]
* [http://www.csci.csusb.edu/dick/samples/ocl.html OCL page of Computer Science Dept. of CSUSB] (brief OCL 2.0 syntax)
* [http://octopus.sourceforge.net/ Octopus: OCL Tool for Precise Uml Specifications] (OCL checker)
* {{usurped|1=[https://archive.today/20121225102952/http://www.dresden-ocl.org/index.php/DresdenOCL Dresden OCL Toolkit]}} (OCL Toolkit, various OCL related publications)
* [http://www.brucker.ch/projects/hol-ocl/ HOL-OCL] (An interactive theorem proof environment for OCL, various OCL related publications)
* [https://web.archive.org/web/20060220003552/http://www.parlezuml.com/tutorials/umlforjava/java_ocl.pdf OCL for Java tutorial on ParlezUML] 
* [http://www.eclipse.org/articles/article.php?file=Article-EMF-Codegen-with-OCL/index.html Article on using EMF's OCL in Java code]
* [https://web.archive.org/web/20060831031409/http://www.cetus-links.org/oo_uml.html UML link page on cetus-links.org]
* [http://sourceforge.net/projects/useocl/ USE (UML-based Specification Environment)] (OCL Tool for model validation, various OCL related publications)
* [http://modeling-languages.com/ocl-tutorial/ OCL tutorial]
* [https://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=5630350/ NL2OCL] (OCL Tool for invariant generation from NL)

{{Query languages}}
{{ISO standards}}

[[Category:Unified Modeling Language]]
[[Category:Formal specification languages]]
[[Category:ISO standards]]