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80 This is my lovely web page denotes the body of an HTML document, consisting of a single paragraph (enclosed in the ... tags), with the word "my", enclosed in the ... tags, shown in bold, and the word "lovely" shown in italics. This document has 4 elements, enclosed by the tag start/end pairs: note that the elements can be nested, one inside another. In HTML, the tags have a defined meaning in the HTML language, and largely relate to appearance of the content of the document, i.e. how it should be displayed in a web browser. An XML document also consists of elements enclosed in start/end tags. However, there are two fundamental differences. In XML, the tags have no predefined meaning: it is up to the person marking up a document to choose appropriate tags. Usually, 'appropriate' means that the tags would have been agreed by some community of people that will be sharing the documents. Second, the tags in XML usually relate to content rather than appearance. To illustrate both points, consider how the following sentence The bank vole, Clethrionomys glareolus, lives in hedgerows and eats berries, nuts and seeds. might be marked up in an XML document: The bank vole, Clethrionomys glareolus, lives in hedgerows and eats berries, nuts and seeds. Here we have 4 types of element, species_description, english_name, latin_name, and food. These have no pre-defined meaning: presumably a group of people interested in transferring species descriptions got together and agreed on them. The elements do not tell us how the document should appear, but rather add meaning to the content of the document: for example, they enable some program to extract all foods. And that's more-or-less all you need to know about XML. A few extra rules apply: you must have an XML header line; you must only one top-level element; all start-tags must have a matching end-tag. Also, the values of any attributes (inside the start tag) must be enclosed in quote marks. We can use XML notation in 3 ways: Marking up continuous prose This has been illustrated by the example above. The markup is used to structures (such as the Results section of a journal paper) and individual items (such as species name) in normal text. Marking up of metadata A text document can have a set of keywords at the start, which can be marked up in XML to enable targetted retrieval by e.g. computer cataloguing systems. Marking up of structured data This is emerging as the most common use of XML, and is the way it is used in almost all the XML-based markup languages being developed. It is also what is being proposed in this paper for using XML to represent ecosystem models. As an example, the above species description can be presented in a more structured way, as:
81 bank vole Clethrionomys glareolus hedgerows berries nuts seeds This is rather close to the way that the information would be structured in a database. In fact, there are proposals for representing relational database information in XML, and conversely most XML structured documents can be expressed in a relational database format. However, XML provides rather more flexibility than standard relational databases. A2.2 XSLT XSLT - the eXtensible Stylesheet Language for Transformations - is a standard for defining transformations on XML documents. What this actually means is best illustrated by an example. Take the species description given immediately above, and now consider this as part of a catalogue of species. The following box gives a small but complete XML document for two species: bank vole Clethrionomys glareolus hedgerows berries nuts seeds wood mouse Apodemus sylvaticus woodland berries nuts seeds insects Now let us say that we would like to display information on the habitat of each species inside your web browser using the following HTML: The bank vole lives in hedgerows. The wood mouse lives in woodlands. When you view this HTML in your browser, you will see:
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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
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15 LAMOS: Landscape Modelling Shell
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17 5. What does 'declarative' mean?
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19 and write a generic program whic
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21 functions that can calculate vol
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23 5.5 What are the benefits of gre
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25 6.2 Mathematical description The
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27 6.4 Representation of the model
Page 29 and 30: 29 6.6 Representation of the model
Page 31 and 32: 31 6.8 Representation of the model
Page 33 and 34: 33 7. Architecture for a declarativ
Page 35 and 36: 35 7.2 Designing the model-represen
Page 37 and 38: 37 Figure 7.1 is a partial view of
Page 39 and 40: 39 context of spatial modelling). T
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
Page 47 and 48: 47 generates a procedural version o
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
Page 57 and 58: 57 Note the difference between a de
Page 59 and 60: 59 process must be clarity and rigo
Page 61 and 62: 61 product. The other, considered i
Page 63 and 64: 63 11. Critical evaluation of the d
Page 65 and 66: 65 Restriction to scientific freedo
Page 67 and 68: 67 are, for example, initiatives to
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: 79 Appendix 2. A brief introduction
Page 83 and 84: 83 eats(bank_vole,seeds). eats(bank
Page 85: 85 Modelica .......................
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