[mercury-users] Assert/Retract

[Daniel Elenius]

Ralph Becket wrote:
> Daniel Elenius, Monday, 23 January 2006:
>   
>> Is it possible to assert/retract statements in Mercury? Mercury looks 
>> like a very good fit for a project I'm doing, but I don't think I can 
>> manage without those "meta" facilities. Also, can I create a list of 
>> terms (atoms)?
>>     
>
> Following Peter's response, you can achieve similar results using the
> impure `mutables' facility recently added to Mercury (see "Module-local
> mutable variables" in the Reference Manual).
>
> However, in our experience it is *virtually always* better to write a
> pure program than an impure one, even if that means passing around an
> extra "state" parameter to your predicates.  Debugging and maintaining
> impure code is invariably very painful.  Mutables are intended for
> situations where there genuinely is no pure alternative (e.g., for the
> efficient implementation of all-solutions predicates or for holding
> constraint stores for CLP).
>
> If you can give us an outline of your problem, we may be able to suggest
> a good (pure) Mercury idiom that does away with the need for
> assert/retract.
>
>   
Sure, that would be great!

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.

There is also some clp(r) going on, but that's a separate issue...

Daniel
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