Talking about schemas can be particularly challenging because so many terms (like "type" or even "term") are often overloaded with multiple meanings on different levels.
To minimize confusion, the tasl documentation uses a consistent and opinionated vocabulary.
A namespace is a vocabulary of URI terms. Namespaces don't have a concrete representation or specification; they're an entirely informal abstract resource.
schema.org is an example of a namespace.
Namespaces are identified with a namespace URI, which is a URI that ends in an empty path segment
/ or an empty fragment
#. Namespace URIs are the common "prefix" part of the terms in the namespace.
http://schema.org/ is the namespace URI for the schema.org namespace.
A term is an individual name in a namespace. Terms are URIs that begin with the namespace URI, and have one or more additional nonempty path segments or a nonempty fragment.
http://schema.org/Person is a term in the schema.org namespace.
Since "type" has a specific meaning in tasl, we prefer to use "kind" when referring casually to different types of things. For example, rather than saying "there are five types of types in tasl", we instead say "tasl has five different kinds of types". This is unrelated to the formal concept of kinds in type theory.
A schema is a set of classes. You can equivalently think of a schema as a map from URI terms to types.
A class is one of the top-level entries in a schema. A class has a key and a type.
Classes are roughly analogous to tables in the relational data model. But in tasl, we separate the data describing the structure and layout of the table (a class in a schema) from the data populating the "rows" of the table (elements in an instance).
A type is one of five kinds of types: literal, URI, product, coproduct, or reference. Types in tasl are classic algebraic data types augmented with a special reference type.
An instance of a schema is a set of elements for each of the schema's classes, where each element has a value of the corresponding class's type.
Elements intuitively correspond to "rows" in the relational data model. An instance of a schema has a set of elements for each of the schema's classes, and each element of a class has a value of that class's type.
Each of the five kinds of types are instantiated by a corresponding kind of value. For example, the literal type
integer is instantiated by integer values
"1004", etc, the URI type is instantiated by URI values like
urn:isbn:0-486-27557-4, and so on.
Literals are one of the primitive types in tasl. Literals are configured with a datatype.
A value of a literal type is a Unicode string, whose possible values are further constrained by the datatype. In general, tasl itself doesn't use the datatype to enforce any constraints on values; it's the responsibility of data producers and consumers to use datatypes to coordinate value formats.
A datatype is a URI term used to configure a literal type. The datatype is used to indicate the format and meaning of the values of type.
In tasl, literal types for some common datatypes from the XSD namespace are defined as global variables, like
A product type. Intuitively, a product type represents a combination of its constituent types (called components). A value of a product type is tuple with one value for each of its components.
A product type is composed of zero or more entries called components. A component has a URI key and a type, and is written as
(key) -> (type). Component keys within a product must be unique.
A unit type is an empty product type, ie a product with no components.
A sum type. Intuitively, a coproduct type represents a choice between its constituent types (called options). A value of a coproduct type is a tuple
(key, value) where
key is one of the type's option keys, and
value is a value for that option's type.
"Sum type", "variant", "discriminated union", and "tagged union" are all common names for coproducts; we prefer "coproduct" because it avoids confusion with untagged union types, emphasizes its duality with products, and is unambiguous to say out loud (unlike "sum type", which can be misunderstood as "some type").
A coproduct type is composed of zero or more entries called options. An option has a URI key and a type, and is written as
(key) <- (type). Option keys within a coproduct must be unique.
Reference types point to a specific class in the schema; a value of a reference type is a pointer to an individual element of the referenced class.
Reference types are analogous to foreign key constraints in a relational database.
A pointer is a value of a reference type. There's no real difference between these phrasings:
... we just find the last one to be clearer.