Using Attribute Variables for custom type checking (at runtime)

Help!
1

Consider the assoc library.

One aspect of it is that it has an “initializer” for an empty assoc: empty_assoc (?Assoc)

What if i were to wrap this predicate to add an attribute variable indicating the intended type of the assoc – say, its of type ‘a->b’

new_teacher_student_mapping(Assoc) :-
    empty_assoc(Assoc)
   add_type('teacher->student', Assoc).     % adds the type symbol as attribute variable to Assoc

get_students(Assoc, Students) :-
     must_be_of_type('teacher->student', Assoc),    % raise exception, if wrong type
    assoc_to_values(Assoc, Students).

Btw, this can be done without attribute variables. For example, by returning a pair instead of a vanilla assoc, and then checking for that pair.

How do these two approaches compare – e.g. performance wise.

All predicates that subsequently expect an assoc as argument could have a type check, by retrieving the assoc and checking for the expected type.

Is this something that could be done with attribute variables?

thanks,

Dan

2

The easiest way to add this kind of information to Prolog terms is to wrap them inside a compound.

new_teacher_student_mapping('teacher->student'(Assoc)) :-
    empty_assoc(Assoc).

You can now use syntactic unification in the head of the predicate that uses this “kind” of mapping. It will by default fail if you give it a different kind of a mapping, but I don’t want to go into that topic again.

3

Thanks Boris.

Sometimes, the simplest solution doesn’t occur to me :slight_smile:

Although, i think using compounds as arguments might make it harder to remove type information and testing via the term expansion approach I am now using – its a bit too built in as compound – i think.

Edit:

Also, i wonder if having compound terms passed around has a negative effect on indexing … if i recall correctly, O’Keefe in his Crafts book wasn’t too fond of using compound terms to type arguments.

Dan