[mercury-users] Optional fields in a datatype - how to represent?

[Terrence Brannon]

Under modes, the manual does state:

 To describe states of instantiation, we use information provided by
 the type system. Types can be viewed as regular trees with two kinds
 of nodes: or-nodes representing types and and-nodes representing
 constructors. The children of an or-node are the constructors that
 can be used to construct terms of that type; the children of an
 and-node are the types of the arguments of the constructors. We
 attach mode information to the or-nodes of type trees. 


My interpretation of this statement is: "in a given Mercury program,
one either brings in datatypes with a use_module/import statement or
defines them inline. The entire set of such datatypes when applied an
or operator represents the set of possible datatypes within a
program... this is what is referred to as an or-node in the manual
text. What and-node is (and what bothers me) is a 'child' of one of
the terms or'ed together in the previous operation. The branches of an
and-node are the fields of a particular datatype. E.g.:

:- type employee
        --->    employee(
                       name        :: string,
                       age         :: int,
                       department  :: string
                ).

contributes the or-node employee(name,age,department) to the or
operation and it's and-node is name, age and department."

Now, assuming my re-intepretation (and expansion by example) of the
manual is correct, then my question is: what if a field is optional?
Am I required to supply an empty value for this field?

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