Syntactical Conventions

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Purpose

This page is meant as a reference for syntactical details and conventions, reserved words, etc.

Learning the syntax

The most effective way to learn Ampersand's syntax is to copy from existing scripts. This is learning by examples. This reference chapter is suitable to check things, and less suitable for learning.

Symbols

Ampersand has reserved words, such as RELATION, CONTEXT, CONTAINS. All reserved words are written in capital letters. They are introduced on the fly. You will find an exhaustive list of reserved words at the end of this page.

Untyped atoms are written between double quotes, e.g. "Peter" or "KD-686-D". If you want to introduce a double quote inside an atom, escape it with a backslash, e.g. "the symbol \" is called double quote". Numeric atoms always start with a digit, e.g. 4711 or 75.88E3. The boolean atoms are TRUE and FALSE. Dates and timestamps follow the Excel-syntax, e.g. ??? The atom _SESSION indicates the current user session, and is an instance of concept SESSION. It is used in services.

Brackets must always match. For terms, we use round brackets ( and ). For populations and services we use square brackets [ and ].

Constructs that contain ampersand statements are contexts and patterns. They always come in pairs: PATTERN and ENDPATTERN, and CONTEXT and ENDCONTEXT.

White space characters (spaces, tabs, CRLF) are meaningless. You can use them freely to layout your script in a way that helps you to recognize its structure.

A comment on a single line starts with --. Everything after a -- symbol is ignored until the line ends. Multiline comments are wrapped between comment brackets {- and -}. Multiline comments may be nested.

Identifiers always start with a letter. Concepts start with a capital letter, as in Person, Case, A, and Order. Relation names start with a lower case letter, as in contains, attr, sessionLogin, or r.

terms

terms are combined with operators. Binary operators may require brackets to avoid ambiguity. To save writing unneccessary brackets, some precedence rules are in place.

operator category

precedence

operators

logic

1 (weakest)

|- (subset), = (equal)

binary boolean

2

\/ (union), /\ (intersect), - (difference)

binary relational

3

; (composition), \ (right residual), / (left residual)

unary prefix, unary postfix

4 (strongest)

- (complement), ~ (converse)

Within an operator category, you must place brackets to disambiguate. E.g. r/\s\/t is not allowed. You have to write either (r/\s)\/t or r/\(s\/t). Across categories, you may omit brackets because a higher precedence binds stronger. So r;s\/t means (r;s)\/t. (Note that (r;s)\/t and r;(s\/t) have different meanings). Associative operators (\/, /\, ;) need not be disambiguated with brackets. So r\/s\/t and (r\/s)\/t and r\/(s\/t) all mean exactly the same.

List of reserved words

Keywords in Ampersand are always written in CAPITALS.

  • Keywords for the main structure of the code

  • Keywords for relations

  • Keywords for rules

    • RULE

    • MESSAGE

    • VIOLATION

    • TXT

    • SRC

    • TGT

    • I

    • V

    • ONE

    • ROLE

    • MAINTAINS

  • Keywords for documentation

  • Keywords for services

    • INTERFACE

    • FOR

    • LINKTO

    • BOX

    • ROWS

    • TABS

    • COLS

  • Keywords for identities

  • Keywords for views

    • VIEW

    • ENDVIEW

    • DEFAULT

    • TEMPLATE

    • HTML

  • Keywords for generalisations:

    • CLASSIFY

    • ISA

    • IS

  • Keywords for TType:

    • REPRESENT

    • TYPE

    • ALPHANUMERIC

    • BIGALPHANUMERIC

    • HUGEALPHANUMERIC

    • PASSWORD

    • BINARY

    • BIGBINARY

    • HUGEBINARY

    • DATE

    • DATETIME

    • BOOLEAN

    • INTEGER

    • FLOAT

    • AUTOINCREMENT

  • Reserved words for values of atoms:

    • TRUE

    • FALSE --for booleans

    • _SESSION

  • Reserved words for concepts

    • ONE

    • SESSION

  • Experimental keywords:

    • SERVICE

    • EDITS

  • Deprecated keywords:

    • SPEC

    • KEY

    • PROCESS

    • ENDPROCESS