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

[Fergus Henderson]

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?

-- 
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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