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Getting rid of Meta ...

  • Artigo

We’re currently doing a major cleanup and revision of the APIs of the DSL Tools.  An aspect of this is a systematic renaming so that the API is a better match to the concepts we are dealing with.  One thing we decided to do is get rid of the Meta… prefix altogether.

In my experience around UML and MOF, the Meta… prefix causes endless confusion, especially the concept of a “meta-instance”: M3 is an instance of M4, M2 is an instance of M3, and so on.  I became notorious for insisting that “M0 is a fiction”.

The issue here is the relationship between a model, and the stuff that it is a model of; and what this has to do with building a model of the concepts of a modelling language.

Sometimes people say “a metamodel is a model of a model”.  This is simply wrong.  If X is a model of Y, then X systematically represents Y for some purpose, leaving out unnecessary detail. That is not the relationship between a metamodel and a model.

Sometimes people say “a model is an instance of a metamodel”.  This begs the question of what it means to be an instance of a model.  If I have a model of a program, what is an instance of that model?  A copy of the source code? The DLL?  The DLL loaded into memory? An execution of the program? A stacktrace?  A mathematical model of the program’s semantics? It is an ill-defined concept.

So, to avoid all of this (which in any case reminds me of the issue of how many angels can dance on the head of a pin), we decided to call the models of the concepts in our DSLs Domain Models; which contain Domain Classes, Domain Relationships, Domain Properties, Domain Roles and so on.  These models are used to generate tools, and serialized in files.  We also have a model which we use to generate the tool and formats that we use to manipulate Domain Models – we call this the Domain Model Domain Model, or DMDM.

Comments

  • Anonymous
    September 22, 2005
    Likely you will now get confusion on what you mean with "domain" :)
  • Anonymous
    September 27, 2005
    So, what in your view is the relationship between the metamodel and a model? Oh, BTW how different is the "metamodel of the metamodel" from the "metamodel of a model"?
  • Anonymous
    September 27, 2005
    A metamodel specifies the concepts of the language in which the model is defined.

    -- Steve
  • Anonymous
    September 30, 2005
    The comment has been removed
  • Anonymous
    October 01, 2005
    A modeler is a tool that makes models. A metamodeler is a tool that makes modelers.
  • Anonymous
    October 01, 2005
    Hi Steve, does this mean that a global replace of "meta" by "domain" will do the job? I agree that this will resolve some confusion, but I do agree with Martijn that "domain" will cause confusion. People are already cofusing business domains and technical domains and will now have "meta-(sorry to use the word)-domains" to worry about as well. Let's hope the change in terminology helps.
  • Anonymous
    October 06, 2005
    I had a conversation at breakfast today during which all of the following words were used: model, metamodel, domain model, metalanguage, and domain language. We do need to resolve some confusion. In response to Stefan Tilkov ( http://www.innoq.com/blog/st/2005/10/02/not_getting_rid_of_meta.html ): a domain model specifies the concepts of a domain-specific language.
  • Anonymous
    October 10, 2005
    Steve,
    some of the questions on meta-ness you raised can be attributed to the difference between type and token model as elaborated by Thomas Kühne (see http://drops.dagstuhl.de/opus/volltexte/2005/23/). I think these central issues ("what is an instance of what in which sense") need to be clarified, not obscured by yet another name change.
  • Anonymous
    October 12, 2005
    Another thing you might consider getting rid of is "model". A model is incomplete. However when you define something in a DSL you are not, in general, leaving anything out (assuming the syntax and semantics of the DSL are well-defined). For example if the domain of the DSL is user interfaces then a definition of something in the DSL is an executable definition of a user interface, not a model of a user interface. If the DSL is a mark-up language for documents then a definition in the DSL is a renderable definition of a document, not a model of a document. If the domain is DSLs then a representation in the [DSL]DSL is a complete definition of the syntax and semantics of a DSL, not a model of a DSL. There might be two different DSLs targetting the same domain. In that case you could have two different representations of the same domain thing (where a representation is an expression in a DSL). The representations are different from the thing. That does not make them models, though. The representations are complete, unlike models. A recipe in English for Aunt Jane's muffins and a recipe in Spanish for Aunt Jane's muffins are two representations of the same recipe, not two models of the same recipe.