Skip to main content

Syntactical conventions

This section is a reference for meta syntax, syntactical conventions, reserved words, etc.

To keep this documentation readable, the documentation omits some details that we deem irrelevant for most Ampersand modelers. The definitive syntax definition is the source code of the parser, where all EBNF statements are fully detailed in comments.

How to read syntax statements​

Sometimes, in describing the syntax, we use EBNF-like notation with the following meaning:

Operatormeaning
<foo>?Zero or one occurrence of <foo>
<foo>+One or more occurrences of <foo>
<foo>*Zero or more occurrences of <foo>

Syntactical Conventions​

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 [here](## List of reserved words).

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 interfaces.

Brackets must always match. For terms, we use round brackets ( and ). For populations and interfaces 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 specify calculations with relations. They combine relations with operators to produce new relations. There are unary and binary operators. Binary operators may require brackets to avoid ambiguity. To save writing unneccessary brackets, some precedence rules are in place.

operator categoryprecedenceoperators
logic1 (weakest)|- (subset), = (equal)
binary boolean2\/ (union), /\ (intersect), - (difference)
binary relational3; (composition), ! (relational addition), \ (right residual), / (left residual), <> (diamond operator)
unary prefix, unary postfix4 (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.