[mercury-users] Extending a structure data type in Mercury

[Gregory D. Weber]

I want to "extend" a structure type by adding fields, like this:

:- type prule ---> prule(key, class, ftable, conds).
:- type cn2rule == prule.
:- type icnrule ---> icnrule(key, class, ftable, conds, info).

But want icnrule to be considered a subtype of prule,
also as a subtype of cn2rule, so that procedures developed for prules 
and cn2rules will also work for icnrules.

I've found a solution, but I'm wondering if there's a better way.
I add an extra field of variable type to the prule type,
and then instantiate it in different ways to make different kinds of rules:

:- type prule(I) ---> prule(key, class, ftable, conds, I).

:- type cn2rule_info == {}. % empty tuple
:- type cn2rule == prule(cn2rule_info).

:- type icnrule_info == {ftable}.
:- type icnrule == prule(icnrule_info).

One complication of this approach is that, to use a cn2rule as an
icnrule, I have to replace an {} with an {ftable}, and vice-versa.

-- 
Gregory D. Weber             http://mypage.iu.edu/~gdweber/
                             Telephone (765) 973-8420
Associate Professor of Computer Science; Chair, Informatics Committee
Indiana University East
2325 Chester Boulevard, Richmond, Indiana 47374-1289, U.S.A.
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