Choices

Overview

Choices define exclusive relationship between a set of marker tags. They are used as "adjectives" in an ontology to add additional constraints that work in conjunction with the primary inheritance hierarchy. For example, a sensor measures the ph::Quantity of a specific ph::Phenomenon. Both quantity and phenomenon are mutually exclusive. A given sensor point cannot report both air temperature and air humidity - it has a single current value that is one or the other.

Syntax

Choices leverage spec inheritance to build out a taxonomy of the choices. The root of the taxonomy extends sys::Choice. Choice specs can define only marker type slots. For example to build a exclusive set of markers to model color:

Color: Choice
Red: Color { red }
Green: Color { green }
Blue: Color { blue }

When an entity defines the 'Color' choice it creates a validation rule that instances must have exactly one of the marker tags: 'red', 'blue', or 'green'. Here is an example:

// color is required
Car: {
  color: Color
}

// instances
Car {red}         // valid
Car {}            // invalid, color is required
Car {red, blue}   // invalid, red and blue are mutually exclusive

You can use a maybe choice to define zero or one choice selection:

// color is required
Car: {
  color: Color?
}

// instances
Car {red}          // valid
Car {}             // valid, color is maybe type
Car {red, blue}    // invalid, red and blue are mutually exclusive

MultiChoice

If the choice is not mutually exclusive then add the 'multiChoice' marker to call site. This allows instances to make multiple selections of the choice. Extending our car example from above:

// color is required multi choice
Car: {
  color: Color <multiChoice>
}

// instances
Car {red}          // valid
Car {}             // invalid, at least one color is required
Car {red, blue}    // valid

Scope

Choices are by design open ended. Any lib in the namespace can add additional choices via inheritance. This means that the range of choices might vary based on what libs are loaded into the namespace. If a choice should be a closed set, then add the 'sealed' metadata:

Color: Choice <sealed>

The sealed marker indicates that only specs within the parent lib can extend the choice.