The language Ampersand

Last updated 2 months ago

Watch this clip to learn how we use the words atom, concept, and relation. Below is a list of other words with a specific meaning in Ampersand.

Word

Meaning

Example

Purpose

Atom

an indivisible item

"Peter"

to represent a thing

Concept

a name to categorize similar items

Person

Pair

two atoms: a source and a target atom

("Ida",5)

to state that two atoms are related

Relation

a set of pairs that is identifyable in a context by its name and type

r[A*B]

to build true statements and store pairs persistently in an application

Rule

a constraint, which is supposed to remain satisfied.

r;s |- t

to provide meaning in a given context

satisfy

A rule is satisfied (in a context) if the data (in that context) do not cause any violation of that rule.

to calculate violations at run-time helps users do the right things

Pattern

a set of rules

PATTERN

...

ENDPATTERN

to gather rules that belong together for reusing them in different contexts

Population

a set of pairs in a context

POPULATION r[A*B] CONTAINS [ ("Ida",5), ("Bob",1) ]

to represent the facts (i.e. true statements) in an information system

Context

a population together with a set of rules that are satisfied by the population.

CONTEXT

...

ENDCONTEXT

to maintain a consistent representation of a real life situation

View

A set of pairs that can be shown to users in a particular formulation.

to represent facts

Service

A structure meant for "the outside world" to communicate with the system and possibly change the population.

INTERFACE Request FOR Customer

to let "the outside world" communicate with the system in a given context and possibly change its population

Multiplicity

A predefined property of a relation

UNI, TOT, SUR, INJ

to constrain a relation with predefined properties

Term

A combination of relations and operators that satisfy the Ampersand syntax

r;s-t

to express rules

Operator

a symbol used in combining terms into other terms.

-, ~, \/, /\, -, ;, \, /, |-, =

to express more complex rules.

Role

A name for a group of people

ROLE Customer MAINTAINS paymentObligation

to talk about users without having any users

Reading guide

Syntactic definitions are given where the underlying notions (e.g. rule, relation, pattern, etc.) are discussed. The metasyntax is singled out on a separate page. Because terms are defined in relation algebra, their semantics are explained in various ways to suit the background of each individual reader. Terms are the only algebraically defined things.

This section is organized by discussing each notion in isolation. Hyperlinks are added in the text to let the reader navigate on her own. The text is suitable for reference purposes, so there is no preferred order in reading.