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66 11.2 Integrating declarative and component-based approaches The situation we find ourselves faced with is this: • There are many models in the ecological and environmental domains currently in use, implemented in a conventional programming language; and there are many groups developing modelling frameworks, mainly component-based. • A declarative modelling approach represents a radically different approach, based on treating the modelling process as a design activity and on the development of independent tools for processing models represented in a common language. Is any form of reconciliation or harmonisation possible between the two approaches? I believe that there are three ways in which this is possible. A declarative modelling approach can be used to build modules (components) for a component-based framework In a conventional component-based approach, a component is a discrete programmed entity. These are normally programmed by human programmers. However, we can also generate such programs automatically, from a declarative representation of the model structure, as Simile demonstrates. We could therefore use declarative modelling environments such as Simile as a front end for making individual components for a component-based framework. Such components would live alongside and be interchangeable with components programmed by hand. Moreover, since developers of this program-generating tool have complete control over the generated text, they can easily wrap the text up in whatever is needed to make it into a module for some component-based system - exactly corresponding to the wrapper a human programmer would have produced. This approach would simplify the task of creating components, since they could be developed in a visual modelling environment. A module (submodel ) in a declarative modelling framework could be a programmed component When Simile makes a runnable version of a model, by default it generates a single compiled unit (a DLL, or Dynamic Linked Library). However, Simile allows you to specify that some submodels are to be handled as separate DLLs, in which case the complete runnable version consists of several DLLs. (This is transparent to the Simile user). It is therefore easy to envisage that a Simile model might include DLLs produced externally, so long as they conform to Simile DLL interface. The model diagram would then show these as 'black boxes' - you can't see inside them, because they have been implemented by programming, not using Simile's diagrammatic language. Taking 'Simile' as shorthand for any declaratively-based modelling environment, we can thus envisage hybrid models, with some parts represented declaratively, and some parts by reference to a programmed module. An extreme case of this would involve using the declarative modelling environment purely as a way of joining together programmed modules, in a manner proposed by Villa (2001) in IMA (the Integrated Modeling Architecture). This approach would provide a means of making use of existing (sub)models, while operating in a visual modelling environment. For example, there are numerous rigorously-tested models of hydrological or atmospheric processes, which could be used as they are, without reimplementation. However, it is likely that the process of integrating (interfacing) such components into a declarative modelling environment will in general not be straightforward. Increasing declarativeness of component-based methods Whether or not there are initiatives to develop declarative modelling within ecosystem research, it is certain that component-based methods themselves will become increasingly declarative. There
67 are, for example, initiatives to develop XML-based notations for specifying the interfaces of COM components in a modelling framework (see http://www.cs.man.ac.uk/cnc-bin/cnc_softiam.pl). Also, many component-based approaches draw on object-oriented methodologies, for which UML (the Unified Modeling Language) [see URLs] is the standard design methodology. There now exist various CAD (computer-aided design) packages that support UML, enabling at least parts of the programming to be done without writing code. So, conceivably, those developing component-based frameworks will gravitate towards declarative methods. It would be unfortunate if this duplicated the effort of initiatives starting from a declarative-modelling standpoint. However, it should also be borne in mind that some of the criticisms of component-based approaches, presented in Section 3.3, do not depend on the programming involved, but on inherent limitations of the approach. Simply adopting a declarative style will not in itself overcome these problems.
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1 Position Paper on Declarative Mod
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3 8.3 Exploiting the declarative na
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5 15. It is argued that the efficie
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7 another for comparing the structu
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9 • Programs for research-grade m
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11 3.2 Modular modelling, component
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13 into the correct order. In some
Page 15 and 16: 15 LAMOS: Landscape Modelling Shell
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Page 23 and 24: 23 5.5 What are the benefits of gre
Page 25 and 26: 25 6.2 Mathematical description The
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Page 41 and 42: 41 ensure that the various packages
Page 43 and 44: 43 and run as a stand-alone model (
Page 45 and 46: 45 Desktop temperature = graph(time
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Page 49 and 50: 49 Example 2: Generating audio desc
Page 51 and 52: 51 8.4 Conclusions The aims of this
Page 53 and 54: 53 Sector Ecosystem services Indica
Page 55 and 56: 55 The XML model documents will be
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Page 61 and 62: 61 product. The other, considered i
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Page 69 and 70: 69 Acknowledgements and declaration
Page 71 and 72: 71 Forrester, J.W. (1971) World Dyn
Page 73 and 74: 73 URLs: (Web links) ACSL Advanced
Page 75 and 76: 75 Glossary See "URLs: Web links" a
Page 77 and 78: 77 Appendix 1. The Modelica, MODCOM
Page 79 and 80: 79 Appendix 2. A brief introduction
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