:- 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)).