Subtyping

Overview

Defs are organized into a tree structured taxonomy via the is tag. A def is a subtype if it defines a subset or narrowing of a broader term. For example, we say that water is a subtype of liquid because it is a specific type of liquid. The converse is that liquid is a supertype of water.

In the context of natural language, a subtype is a hyponym and a supertype is a hypernym. If you have ever played Mad Libs, then subtyping is intuitively like filling in the blanks. For example if we say that a pipe conveys a fluid, then the term fluid encompasses specific types of fluids including water, steam, gasoline, or fuel oil.

We can also evaluate subtyping in the context of set theory. If we say that elec-meter is a subtype of meter, then we are saying the set of all things that are elec meters is wholly contained by the set of all meters.

Subtyping is a powerful knowledge modeling tool. However, remember that modeling the real world is a messy business. It's impossible to design a taxonomy that fits all situations. Consider the platypus and its difficulty to fit into a biological taxonomy. Haystack attempts to define a pragmatic approach between simplicity and the common use cases. But, you will likely find use cases that aren't a perfect fit for Haystack's ontology.

Transitivity

Subtyping is defined to be a transitive relationship. If B is a subtype of A and C is a subtype of B, then it is implied that C is a subtype of A. A concrete example: water is a subtype of liquid and liquid is a subtype of fluid, therefore water is inferred to be a subtype of fluid.

Is Tag

Subtyping is declared via the is tag on a def. The value of the is tag is a symbol or list of symbols. Since most defs subtype from a single def, we typically use just a symbol:

def: ^water
is: ^liquid

In cases where multiple supertypes are required, use a list of symbols:

def: ^site
is: [^entity, ^geoPlace]

All terms are required to have an is tag with the exception of the root terms, which are marker, val, and feature. Feature keys have implied subtyping rules and must not declare an is tag.

It is invalid to create cyclic subtyping relationships. Subtyping must result in a strict tree structure.

Inheritance

Defs are themselves specified as a set of tag name/value pairs. Subtypes automatically inherit these tags into their own definitions. We call this process inheritance, and it works similar to inheritance in traditional object-oriented languages. The inheritance process is discussed in detail under normalization.