[mercury-users] Assert/Retract

[Daniel Elenius]

Ralph Becket wrote:
> Daniel Elenius, Thursday, 26 January 2006:
>   
>> We are developing a "policy reasoner". Policies would be Mercury modules 
>> (currently they are Prolog modules), and can be loaded on/unloaded from 
>> the reasoner at runtime. Hence the (perceived) need for assert/retract.
>>
>> Requests are sent to the reasoner in the form of lists of facts 
>> (currently Prolog lists of Prolog atoms). The reasoner does 
>> goal-directed search (normal Prolog query) over the loaded policies, 
>> with the list of atoms as a parameter, and answers yes or no to the 
>> requests.
>>     
>
> Hmm.  Can't you implement each policy as a separate Mercury predicate
> and then just check for each query that at least one policy accepts it?
> Which policies are currently in effect is handled by only calling those
> policy predicates.  That is, have something like
>
>   
No, because *new* policies can be loaded at runtime. New code has to be 
loaded into the mercury or prolog engine, and I don't want to have to 
reload all the policies whenever a new one comes in.

> check_some_policy(Policies, Query) :-
> 	member(N, Policies),
> 	check_policy(Policies, N, Query).
>
> check_policy(Policies, 1, Query) := check_policy_1(Policies, Query).
> check_policy(Policies, 2, Query) := check_policy_2(Policies, Query).
> ...
> check_policy(Policies, N, Query) := check_policy_N(Policies, Query).
>
> where check_policy_i/2 implements the corresponding policy checks.
>
> -- Ralph
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