RIDDL Documentation
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Application Models Users' Tool

This page describes the nature of a RIDDL Application definition.

What is a User?

The term user in this context is a word of art. It doesn’t necessarily mean a human being. A user is, literally any thing that uses the system. This could be a piece of software, a human, an alien, or an artificial intelligence. We can define a user in RIDDL very simply:

user Gemini is "an artifical intelligence provided by Google"

What Is A User Interface?

A user interface is just that: an interface to a system that is geared toward ease of use by the user. We regard the User Interface to be a component of the system being modelled.

What Is An Application?

A RIDDL Application defines a user interface for the system being modeled in RIDDL. Applications are the means by which a user may communicate with a system, and control it. In other words, it is the system component that adapts the system to the user. A system may have multiple applications.

Abstractly, the communication between a User and an Application requires only four kinds of things:

  • Ways to navigate the application’s structure (navigate)
  • Ways to control the system it represents (control)
  • Ways to receive information from the user (input)
  • Ways to show information to the user (output)

User interfaces are made of many kinds of components and new ones are being invented continuously. RIDDL Applications do not intend to model the look, feel, shape, sensory utilization or any other design attribute of an actual user interface. RIDDL Applications do not limit themselves to only the current set of technology currently available as a user interfaces( web sites, mobile applications, telephones, console panels).

RIDDL And The User Experience

RIDDL recognizes that the user experience (UX) is an art and science of its own, and divorces itself from that concern. This allows Applications to refer to rich user interface models that cover all the senses of a human, without having to define all that complexity. Such models are better defined with illustrations, demonstrations, wireframes, and etc. To support that separation well, RIDDL Applications use the shown by <url> syntax to link its components to such illustrations and demonstrations.

Consequently, RIDDL Applications concern themselves with the logical and functional details necessary to support the user’s interaction with the system. A RIDDL Application is simply the system facade component that permits its users to control the system the Application represents, as illustrated here:

graph LR
    User( fas:fa-user-tie 
User) System( fas:fa-microchip
System) Application( fas:fa-tablet-alt
Application) User -- interacts with --> Application Application -- controls --> System

The control of both the system and the application is accomplished by sending and processing messages. Consequently a RIDDL Application definition is merely composed of inputs and outputs, and groups of those things, as shown here:

    APPLICATION ||--|{ GROUP : contains
    GROUP ||--|{ INPUT: contains
    INPUT ||--|{ INPUT: contains
    GROUP ||--|{ OUTPUT: contains
    OUTPUT ||--|{ OUTPUT: contains

The entity-relationship diagram shows the containment hierarchy for a RIDDL Application and its components. All the complexity of modern user interfaces can be broken down into these few ideas.


Inputs are manipulated by the user and consequently send messages to the application for processing:

graph LR
  User( fas:fa-user-tie 
User) Input( far:fa-keyboard
Input) Application( fas:fa-tablet-alt
Application) User -- manipulates --> Input Input -- sends message to --> Application


Outputs receive messages from the application and present them to the user in a form suitable for that user’s perception:

graph LR
  User( fas:fa-user-tie 
User) Output( fas:fa-tv
Output) Application( fas:fa-tablet-alt
Application) Application -- sends message to --> Output Output -- presents information to --> User

Navigation of the user interface occurs when the user makes an input to the UI that causes it to change the resource it is presenting. RIDDL Applications model this by responding to a message in a handler that runs a statement that changes what is displayed to the user. This is shown in the following diagram:

graph LR
    User( far:fa-user-tie User)
    Menu( far:fa-image Menu)
    UI(fas:fa-tablet-alt Application)
    Page( far:fa-image Page)
    User -- selects --> Menu
    Menu -- sends command to --> UI
    UI -- executes `focus` 
statement to
display --> Page

Navigation occurs, as shown above, when

  • the user presses or activatees some input control
  • that control creates a message and sends it to the application
  • in response to that message, the application executes a focus statement
  • the focus statement execution changes what is presented to the user.


Control of the underlying System occurs when a user’s request results in a message being sent from the Application to one or more other components in the same system. Consequently, the Application fully determines to what extent the user may control the system.

Applications Are Processors

Because RIDDL applications process messages, they are considered in RIDDL parlance to be Processors. This means their definition involves defining the following concepts:

  • inlets - the place that takes messages in for processing
  • outlets - the place that the application sends messages
  • handlers - the processor of messages that implements the application’s behavior based on messages received on its inlets and the effects it causes by sending messages to its outlets.

This arrangement abstractly mimics how current user interface software works since operating systems receive events (inputs) and forward them to the application for handling. Similarly, applications generate actions (effects) that can be regarded as messages that yield information updates to users. All of the application logic is specified in the handlers.

A RIDDL application can be a model both a source and a sink of events.

Additionally, because RIDDL Applications must arbitrate the control of the system, they can process messages to and from other system components (e.g. contextsAs processors, the application can model both a source and a sink of events.

Language Syntax

This part presents a set of very small examples to show how the concepts described above can be specified in RIDDL.

To aid readability




Navigation in RIDDL syntax looks like this:

command DoCheckout is { cart: ShoppingCartId, user: UserId }
button CheckOut directs user Shopper with command DoCheckout


System control occurs within the Application’s business logic when an On Clause is . Control occurs whenever a Command message is sent from the application to a system component within the Applic