# Ampersand Syntax

When writing an Ampersand model, the Ampersand language is used. This chapter contains a reference of the syntax.

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.


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 interfaces.

Brackets must always match. For expressions, 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.


Expressions 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.

How to read the syntax statements used in this chapter

Sometimes, in describing the syntax, EBNF-like notation is used, with the following meaning:

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

To keep this chapter as readable as possible, we have chosen to omit some details that are irrelevant for practically all &-modelers. In the very rare case that these technicalities are of interest, the reader could have a look in the sourcecode of the parser, where all EBNF statements are in comments.

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
    • TXT
    • SRC
    • TGT
    • I
    • V
    • ONE
    • ROLE
  • Keywords for documentation
  • Keywords for interfaces
    • FOR
    • LINKTO
    • BOX
    • ROWS
    • TABS
    • COLS
  • Keywords for identities
  • Keywords for views
    • VIEW
    • HTML
  • Keywords for generalisations:
    • ISA
    • IS
  • Keywords for TType:
    • TYPE
    • BINARY
    • DATE
    • FLOAT
  • Reserved words for values of atoms:
    • TRUE
    • FALSE --for booleans
    • _SESSION
  • Reserved words for concepts
    • ONE
  • Experimental keywords:
    • BYPLUG
    • EDITS
  • Deprecated keywords:
    • SPEC
    • KEY

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