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18 Declarative Describes what is Data Requires separate problem solvers Can be manipulated and reasoned with What Sequence of statements (generally) not important Facts, concepts Examples include databases, logic programming languages (e.g Prolog), functional programming languages (e.g. Haskell), designs, spreadsheets, models built in visual modelling environments, XSLT "diameter = 6.28*radius" defines a functional relationship between diameter and radius In humans, declarative knowledge is generally conscious, and expressed as words Declarative memory: facts, knowledge, events. Procedural Says what to do Program Solves a specific problem Can be executed How Sequence of statements is critical Skills Examples include programs written in conventional programming languages (Basic, Fortran, C, C++), control files for statistical packages "diameter = 6.28*radius" means "calculate 3.14*radius, and assign the result to the variable called diameter" In humans, procedural knowledge is generally automatic and subconscious, and expressed in actions Procedural memory: how to do actions. Table 5.1 Comparison of the characteristics of declarative and procedural approaches 5.2 Moving the declarative:procedural boundary The distinction between declarative and procedural, between data and program, seems pretty obvious and barely worth discussing. What is not obvious - in fact, for many people is a novel and difficult concept - is that the boundary between data and program is not fixed. For any given problem, the software engineer can decide what should be represented as data and what should be programmed in a procedural language like Fortran or C++. Consider a non-computing example. You're having a friend around for the evening. You could either give her instructions on how to get to your house from hers: turn left, walk 50 m,... (equivalent to a procedural program); or you could give her a map, and rely on her generic mapinterpretation program (in her brain) to work out the route to take. You are replacing a specific set of instructions with some data (the map) and a more generic set of instructions (the brain's map processor). Or take a computing example. In preparing the invitation, you could write a program which prints the text and draws some lines. Or: you could make a data file containing data like: "text",10,20,"Please come around on Friday" (There is some text at coordinates 10,20) "line",10,25,50,25 (There is a line from coords 10,25 to 50,25)
19 and write a generic program which is able to interpret the data in the file and produce the appropriate output for the printer. The specialised, one-off program that executes print and draw instructions is replaced by a more generic one, plus a data file containing some information. We can venture the following principle: Any given procedural computer program can be replaced by a declarative bit (data) and a more generic procedural program. Understanding this concept is absolutely fundamental to appreciating the concept of declarative modelling. This also involves shifting information (the model specification) from a procedural program (the way models are currently implemented) to data in a data file, to be processed by a more generic program. 5.3 Two examples of shifting the declarative:procedural boundary We will now explore these concepts in a bit more detail, by considering 2 examples of moving the declarative/procedural boundary: 1 - a simple data-processing program; 2 - a simple function-evaluation program. We will then consider why one should want to do this: what are the benefits of increasing the declarative content of a solution to a problem? A simple data-processing program. You'd probably be pretty horrified if you asked someone to work out the total of some numbers, and they came up with the following program: a(1)=27.1; a(2)=53.2; a(3)=41.9...... a(157)=19.2 sum=0; for i = 1 to 157; sum = sum+a(i); next i print sum Why? Because this one program contains both the data as program assignment statements, and the instructions for analysing them. Instead, you would expect the data to be in one file (data1), and a more generic version of the program in another (prog1). File data1 157 27.1, 53.2, 41.9, .... 19.2 File prog1 input n for i = 1 to n; input a(i); next i sum = 0; for i = 1 to n; sum = sum+ a(i); next i print sum Separating data and program gives huge benefits: the same data can now be analysed by different programs; and the same program can be applied to different data sets. A simple function-evaluation program Consider the following program, to work out the diameter of a circle from its radius: input radius diameter = 6.28*radius print "radius="; radius; " diameter="; diameter Here, the data (well, datum: the value for radius) has already been separated out (as one would expect, in the light of the previous discussion).
Page 1 and 2: 1 Position Paper on Declarative Mod
Page 3 and 4: 3 8.3 Exploiting the declarative na
Page 5 and 6: 5 15. It is argued that the efficie
Page 7 and 8: 7 another for comparing the structu
Page 9 and 10: 9 • Programs for research-grade m
Page 11 and 12: 11 3.2 Modular modelling, component
Page 13 and 14: 13 into the correct order. In some
Page 15 and 16: 15 LAMOS: Landscape Modelling Shell
Page 17: 17 5. What does 'declarative' mean?
Page 21 and 22: 21 functions that can calculate vol
Page 23 and 24: 23 5.5 What are the benefits of gre
Page 25 and 26: 25 6.2 Mathematical description The
Page 27 and 28: 27 6.4 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
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69 Acknowledgements and declaration
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71 Forrester, J.W. (1971) World Dyn
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73 URLs: (Web links) ACSL Advanced
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75 Glossary See "URLs: Web links" a
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77 Appendix 1. The Modelica, MODCOM
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79 Appendix 2. A brief introduction
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81 bank vole Clethrionomys glareol
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83 eats(bank_vole,seeds). eats(bank
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85 Modelica .......................
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