N00b question : Capturing new information and saving part of the database

Help!
1

Hi everyone,

I’m just starting a small (but real) application to try to get my head around Prolog (and logic programming in general).

In the application I’m developing, I want part of the database to be fixed. It’s generated by a separate program and just imported into Prolog.

Then the idea is that user adds to this database, giving some extra information, by “filling in the blanks” through a web-form.

The question is how I handle this.

Should I be using asserts to capture the extra information? And is there a way of saving just the extra data asserted in the run-session?

For the data which is generated elsewhere, that is the official version. I don’t want to do a listing. from my application and dump everything, because that external generated data might be more up-to-date.

At the same time, I want to capture the extra data that the user has filled in, as a kind of extra annotation of the knowledge available.

How do I typically handle this kind of requirement?

2

Hi interstar and welcome to the discourse group.

What you need is library(persistency), which has a pretty good example in the docs I’ve linked to.

Best of luck learning Prolog, any questions feel free to ask.

3

Hi Paul,

thanks for this.

I’m still pretty new at this so not fully understanding that code example.

Should I assume that, assert_user_role(U,R). is an automatically generated wrapper for something like assert(user_role(U,R)). ?

I’m assuming that inside this module, facts are bound to being in a user_db namespace. But only the user_role predicate is actually being persisted?

Presumably because of the line beginning “:- persistent user_role…”?

What is the File argument passed to attach_user ? A filename or a File object I have to make elsewhere?

What if I have some facts which use the same predicate which come from elsewhere, and I want to just append a few more here? Can I have some user_role statements elsewhere or are they only in scope in this module?

cheers

Phil

4

Should I assume that, assert_user_role(U,R). is an automatically generated wrapper for something like assert(user_role(U,R)). ?

When you call assert_user_role/2 it will assert user_role/2 into working memory. It will also update the persistent file, “sensibly”, you can modify how this works through the options for dbsync/2 like dbsync(gc(always)).

I’m assuming that inside this module, facts are bound to being in a user_db namespace. But only the user_role predicate is actually being persisted?

Within the module all predicates are within the module namespace unless explicitly exported, which in the example is all of them except user_role/2. There’s nothing stopping user_role/2 from also being exported except conventional design wisdom. And yes, only user_role/2 is persisted in the database file, the other predicates are persisted in the module and not updated.

Presumably because of the line beginning “:- persistent user_role…”?

Yes, this declares your predicates to be persisted in the database file.

What is the File argument passed to attach_user ? A filename or a File object I have to make elsewhere?

It can be just a filename, in which case it will be made in the local directory. It can also be a full path to a file, so it can be anywhere you choose.

What if I have some facts which use the same predicate which come from elsewhere, and I want to just append a few more here? Can I have some user_role statements elsewhere or are they only in scope in this module?

If you don’t use modules you can, but it’s wiser to use modules and create a shared interface:

user_role(A, B) :-
    always_user_role(A, B).
user_role(A, B) :-
    current_user_role(A, B).
5

Thanks Paul

I think this is enough for me to go and have a play and see if I can do what I want.

Many thanks again.

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