:- chr_type type ---> body.

If the type term is a functor of arity zero (i.e. one having zero arguments), it names a monomorphic type. Otherwise, it names a polymorphic type; the arguments of the functor must be distinct type variables. The body term is defined as a sequence of constructor definitions separated by semi-colons.

Each constructor definition must be a functor whose arguments (if any) are types. Discriminated union definitions must be transparent: all type variables occurring in the body must also occur in the type.

Here are some examples of algebraic data type definitions:

:- chr_type color ---> red ; blue ; yellow ; green.

:- chr_type tree --->  empty ; leaf(int) ; branch(tree, tree).

:- chr_type list(T) ---> [] ; [T | list(T)].

:- chr_type pair(T1, T2) ---> (T1 - T2).

Each algebraic data type definition introduces a distinct type. Two algebraic data types that have the same bodies are considered to be distinct types (name equivalence).

Constructors may be overloaded among different types: there may be any number of constructors with a given name and arity, so long as they all have different types.

Aliases can be defined using ==. For example, if your program uses lists of lists of integers, you can define an alias as follows:

:- chr_type lli == list(list(int)).