“I assume that a precisely defined, verifiable, executable, and translatable UML is a Good Thing and leave it to others to make that case… In the summer of 1999, the UML has definitions for the semantics of its components. These definitions address the static structure of UML, but they do not define an execution semantics. They also address (none too precisely) the meaning of each component, but there are "semantic variation points" which allow a component to have several different meanings. Multiple views are defined, but there is no definition of how the views fit together to form a complete model. When alternate views conflict, there is no definition of how to resolve them. There are no defined semantics for actions…
To determine what requires formalization, the UML must distinguish clearly between essential, derived, auxiliary, and deployment views. An essential view models precisely and completely some portion of the behavior of a subject matter, while a derived view shows some projection of an essential view…
All we need now is to make the market aware that all this is possible, build tools around the standards defined by the core, executable UML, and make it so…”

—  Stephen J. Mellor

Mellor in Andy Evans et al. (1999) " Advanced methods and tools for a precise UML http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.115.2039&rep=rep1&type=pdf." UML’99—The Unified Modeling Language. Springer Berlin Heidelberg. p. 709-714.

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Stephen J. Mellor13
British computer scientist 1952

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“In its current form UML is designed to support a wide variety of different modelling techniques and formalisms. This is evident, for example, in the state machine formalism which allows both Moore and Mealy formalism with hierarchical states including concurrent sub-states and both synchronous and asynchronous calling semantics. The result of this is not only that almost any state modelling style can be supported but also that many combinations of elements have no defined execution semantics. It is now widely recognised within the UML community, however, that considerable benefit can be gained by forming subsets of the UML with well defined execution semantics. Such subsets can form an “executable UML” which would enable the simulation, execution, testing and ultimately translation of UML models into target code. As part of this movement, work is progressing under the auspices of the OMG towards the definition of “profiles” that define such subsets and towards the more detailed definition of the contents of “actions” including a more precise definition of the execution semantics of UML models.”

Stephen J. Mellor (1952) British computer scientist

Mellor and Ian Wilkie (1999). A mapping from Shlaer-Mellor to UML http://www.ooatool.com/docs/SMUML99.pdf. Technical report, Projtech Inc. and Kennedy Carter Limited, 1999.

“Today some evidence arises that UML will more and more be used not as a specification language but as a high level programming language. This has some advantages, as if the concepts of UML are executable, they can immediately be animated and tested, or the generated code even be used as implementation. Thus UML probably will have an implementation-oriented semantics describing this animation.”

Bernhard Rumpe (1967) German computer scientist

Bernhard Rumpe (1998) " A Note on Semantics (with an Emphasis on UML) http://sse-tubs.de/~rumpe/publications/papers/RUM98a/RUM98a.pdf." Proceedings of Second ECOOP Workshop on Precise Behavioral Semantics. 1998.

“Creating a modeling language that is also an executable language has long been a goal of the software community. Many years ago, in 1968 to be exact, while working with software components to successfully develop a telecommunications system, we created a modeling language that was the forerunner to UML. To model components we used sequence diagrams, collaboration diagrams, and state transition diagrams (a combination of state charts and activity diagrams). Our modeling language then seamlessly translated the component models into code. Each code component was in its turn compiled into an executable component that was deployed in our computer system. The computer system was a component management system—thus we had "components all the way down."”

Stephen J. Mellor (1952) British computer scientist

Source: Executable Uml: A Foundation for Model-Driven Architecture, 2002, p. xxiii: Foreword.

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“A proposed profile of UML—Executable UML—supports creating a complete and implementation-neutral self-contained expression of application functionality. Steven J. Mellor and Marc J. Balcer define this profile in their book Executable UML: A Foundation for Model-Driven Architecture”

Grady Booch (1955) American software engineer

The Limits of Software

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“Meaning… is to be regarded as a complex of contextual relations, and phonetics, grammar, lexicography, and semantics each handles its own components of the complex in its appropriate context.”

John Rupert Firth (1890–1960) English linguist

Source: "The Technique of Semantics." 1935, p. 54

“I know that I disagree with many other UML experts, but there is no magic about UML. If you can generate code from a model, then it is programming language. And UML is not a well-designed programming language.
The most important reason is that it lacks a well-defined point of view, partly by intent and partly because of the tyranny of the OMG standardization process that tries to provide everything to everybody. It doesn't have a well-defined underlying set of assumptions about memory, storage, concurrency, or almost anything else. How can you program in such a language?
The fact is that UML and other modelling language are not meant to be executable. The point of models is that they are imprecise and ambiguous. This drove many theoreticians crazy so they tried to make UML "precise", but models are imprecise for a reason: we leave out things that have a small effect so we can concentrate on the things that have big or global effects. That's how it works in physics models: you model the big effect (such as the gravitation from the sun) and then you treat the smaller effects as perturbation to the basic model (such as the effects of the planets on each other). If you tried to solve the entire set of equations directly in full detail, you couldn't do anything.”

James Rumbaugh (1947) Computer scientist, software engineer

James Rumbaugh in Federico Biancuzzi and Shane Warden eds. (2009) Masterminds of Programming. p. 339; cited in " Quote by James Rumbaugh http://www.ptidej.net/course/cse3009/winter13/resources/james" on ptidej.net. Last updated 2013-04-09 by guehene; Rumbaugh is responding to the question: "What do you think of using UML to generate implementation code?"

“Originally UML was intended to serve as a specification language. But a specification is primarily intended to describe properties of systems that the system developers want to be valid, but to leave open other properties that are not clear already. Today this is partly achieved by having a semantics that is rather vague (and here we mean imprecise as opposed to not detailed). However, this is not an advantage, as the developer cannot fix this kind of impreciseness within UML, but can adapt the individual interpretation only. Furthermore, to get complete (and therefore executable) UML descriptions, often certain details have to be specified, which the developer does not yet know or wants to leave open to a later phase of development or even implementation. It is an intrinsic problem of executable languages that this kind of over-specification frequently occurs. Instead it would be of some help to have flexible concepts of under-specification to postpone detail decisions to situations, where the decisions can and must be made.”

Bernhard Rumpe (1967) German computer scientist

Bernhard Rumpe (1998) " A Note on Semantics (with an Emphasis on UML) http://sse-tubs.de/~rumpe/publications/papers/RUM98a/RUM98a.pdf." Proceedings of Second ECOOP Workshop on Precise Behavioral Semantics. 1998.

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“For Bréal, semantics was the science the subject matter of which was study of the cause and structure of the processes of changes in meanings of words: expansion and contraction of meanings, transfer of meanings, elevation and degradation of their value, etc.
Such a delineation of semantics as a branch of linguistics is maintained to this day, for all the differences between the various schools in linguistics. Such degree of uniformity is not confined to the definition of semantics alone. Not all authors give such a definition; some of them approach the issue from a different point of view as regards general classification… but all schools of linguistics engage in the study of the meanings of words and their changes. Thus all of them, in one way or another, engage in semantics as understood by Breal.”

Adam Schaff (1913–2006) Polish Marxist philosopher and theorist

Source: Introduction to semantics, 1962, p. 4

“What's really going on is that Executable UML is a concurrent specification language.”

Stephen J. Mellor (1952) British computer scientist

Source: MDA Distilled. Principles of Model-Driven Architecture, 2003, p. 96.

“Executable UML is at the next higher layer of abstraction, abstracting away both specific programming languages and decisions about the organization of the software so that a specification built in Executable UML can be deployed in various software environments without change.”

Stephen J. Mellor (1952) British computer scientist

Source: Executable Uml: A Foundation for Model-Driven Architecture, 2002, p. 5.

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