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

[Terrence Brannon]

Fergus Henderson writes:
 > On 22-Apr-2001, Terrence Brannon <princepawn at earthlink.net> wrote:
 > > :- type employee
 > >         --->    employee(
 > >                        name        :: string,
 > >                        age         :: int,
 > >                        department  :: string
 > >                 ).
 > ...
 > > what if a field is optional?
 > > Am I required to supply an empty value for this field?
 > 
 > The usual way of representing optional fields in Mercury
 > is to use the `maybe' type from the `std_util' module:
 > 
 > 	:- type maybe(T)
 > 		--->	yes(T)
 > 		;	no.
 > 
 > For example, if you want the `department' field to be
 > optional, you can write
 > 
 > 	:- type employee
 > 		--->    employee(
 > 			       name        :: string,
 > 			       age         :: int,
 > 			       department  :: maybe(string),
 > 			).
 > 
 > Then when constructing a new employee, you will supply either
 > `yes("some department name")' or `no' for the department
 > field, e.g.
 > 
 > 	:- func fred = employee.
 > 	fred = employee("Fred", 33, no).
 > 
 > 	:- func jerry = employee.
 > 	jerry = employee("Jerry", 42, yes("Marketing")).
 > 
 > Does that answer your question?

Yes. 
0- that is something I haven't seen before. each instance of employee
is a function. how do you access the fields of each instance? I saw
the field access section of the reference manual, but it doesn't show
how to access fields when the type instance is a function.

1- And what happens if you attempt to access it and it is not bound.

 > 
 > -- 
 > Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
 >                                     |  of excellence is a lethal habit"
 > WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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