Coding in yscript

tags

yscript

Notes

Chapter 1: Introduction

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yscript1 (pronounced ‘why-script’) is a computer language for representing and manipulating propositions.

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When yscript code is executed, it results in a dialog or consultation.

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develop a form of representation that looked as much like natural language as possible.

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yscript code consists of rules that deal with facts.

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case-based reasoning

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yscript applications can generate documents

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directly adopt the wording of a set of rules that you are trying to represent.

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isomorphism (that is, one- to-mapping) of real-word rules into yscript code

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A major environment that uses yscript is AustLII’s DataLex platform

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Chapter 2: Getting Started

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Chapter 3: Syntax, Types, Constants and Expressions

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very limited use of non-alphanumeric symbols.

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characters such as ‘=’, ‘+’ or ‘&’ have no special meaning

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keyword is a reserved term that is always written in upper case

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A descriptor is a sequence of letters and symbols that does not include a keyword and is used to refer to a constant, fact or text.

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Sequences of blanks (spaces, tabs, and newlines) containing more than one newline character are treated as a single newline. All other blank sequences are processed as though they were a single space.

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Upper and lower case is significant.

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Text may optionally be enclosed in single or double quotes.

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may be escaped by preceding it with a backslash

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Multi-line comments are introduced with the characters /* and finish with */

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comments nest

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You can also use C++ style single line // comments.

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Facts are dynamically typed and only need to be formally declared if their type is not implicit from their usage.

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special type of facts called - named subjects

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The basic types and associated constant forms

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GENDER facts may refer to any string value.

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A MONEY value cannot be associated with a non-currency unit

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The DATE type is used to refer to calendar dates.

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supported range of years is from AD 1 to 9999

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good practice to use a date format with a non-numerical month

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TIME type is used to refer to time.

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Numbers can be written in a variety of formats.

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Facts and constants with a numerical value can be associated with a unit of measurement or an international currency.

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An expression consists of fact or constant references, connected by operators.

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The PLUS and MINUS expression operators can be used to perform date arithmetic.

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where the day falls on the last day of the month, the resulting date will fall on the last day of the month

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Times are stored as the number of seconds since midnight in local time.

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The value of all facts and constants is stored in base-unit format.

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Chapter 4: Facts

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Facts are used to hold a value during execution and are referred to by a descriptor that is not a constant and that is not text.

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Facts cannot be declared as being local to a rule or procedure and are accessible from all rules in the context.

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yscript code generally involves manipulating facts in propositional form.

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fact names are also used to build elements of English dialogs

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information can be treated symbolically but with no requirement to provide a separate textual representation

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yscript uses a light-weight natural language parser

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heuristic rules that identify different parts of speech based around generally used word forms

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Before a fact is assigned a value or one is provided by the user, the fact value is unknown.

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FACT declaration can include a RANGE restriction.

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representation as contained in facts in the code to be different from the text that is used to communicate with the user.

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Explanations are text descriptions that add further explanation about a fact

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Aliases are also used in document assembly and provide a way to specify a short name for a fact as used in document templates.

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Rules and facts within each context have their own separate namespace.

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if identically named facts or rules exist in two or more contexts, all are treated as referring to different facts.

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Chapter 5: Statements

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Assignments are used to set the value of a fact

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One of the side-effects of assignments is that they associate facts that are used to calculate the assigned value, as reasons

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Chapter 6: Rules

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Once the goal rule has started execution (or has been fired), the rule statements are executed in order.

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Rule names are in a different namespace from fact names

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explanation as to how each of these facts was determined

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Where the goal rule does not determine the value of any facts, the report will explain all determined facts in reverse order

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yscript will normally fire each rule in the order that they were declared

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The order in which rules are fired can be explicitly set

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yscript is a declarative language. The order of execution is normally driven by goal-directed backward chaining or result-directed forward firing of rules. Rules and expressions themselves, however, are imperative.

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The rules engine will block attempts by rules to invoke themselves to stop loops and infinite recursion.

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executed from the starting rule to re-establish the current state and to use any new information

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When a CALL or NEXT statement attempts to execute a rule that is already being executed, the interpreter will automatically call FORGET to bring the system back to the state that it was in when the rule was originally called. The effect of this is that it is always safe to use CALL or NEXT to invoke any rule, and if necessary, the system will backtrack to produce a sensible result.

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a fact effectively becomes a very simple object in the sense that it can hold a value and that it also has associated code.

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Chapter 7: Examples

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yscript also supports simple analogous reasoning

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When yscript is trying to infer a value for a fact and no further rules can be found to assist, it will look to see if the fact is contained in an example set.

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Chapter 8: Document Assembly

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Chapter 9: Style Guide

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Relying upon the order of rule execution can lead to code that is cryptic and difficult to understand.

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Appendix 1: Units, Currencies and Time Zones

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Appendix 2: The cyscript Interpreter

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1. Usage

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The -x flag produces a fact cross-reference.

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2. Interactive Sessions

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Appendix 3: yscript 2.x Language Changes

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Appendix 4: Formal Grammar

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Appendix 5: The yscript API

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Appendix 6: Using cyscript as a Server

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Appendix 7: yscript Code Examples

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1. Commonwealth Constitution s44

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2. Modern Slavery Act 2018 (Cth)

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3. Community Gaming Regulations 2020 (NSW)

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Index

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