User-defined function on dict not found

General
1

I have a dictionary of dictionaries, like this:

type{name: person, args: [arg{name: name}, arg{name:dob}]}

Types and arguments have optional descriptions; if missing, the description is to be sourced from the name. I want to say that with a user-defined function:

D.description() := D.get(description) :- !.
D.description() := D.name.

When I use the function within the module where I defined it as with SomeType.description(), I get:

ERROR: -g main: eval_dict_function/4: Unknown procedure: type:description/2

Reverting to a normal predicate works just fine:

spec_description(Spec, Description) :-
  Description = Spec.get(description), !.
spec_description(Spec, Spec.name).

Perplexingly, the user-defined function works fine when I use it in a different module.

Side question: how come these user-defined functions don’t have to be published in the module header in order to be used in other modules? Are these definitions global?

Side question: what’s the best way to make a user-defined function work only with certain dictionary types? Do I need to put conditions on the dictionary tag name? D.description() := … :- D = type{_}?

Side note: the error message doesn’t have a file and line number like typical error messages do. That makes dictionary error debugging more cumbersome than it needs to be.

2

The function is looked up in the module with the same name as your tag (which also answers your second side question). See SWI-Prolog -- User defined functions on dicts
I created a module named mydict, and from the user module saw the below.

mydict.pl

:-module(mydict,[]).
D.description() := D.get('description',D.name).

And at the top level.

?- use_module(mydict).
true.

?- Z=mydict{description:'Hello',name:'N/A'},writeln(Z.description()).
Hello
Z = mydict{description:'Hello', name:'N/A'}.

?- module(mydict).
true.

mydict:  ?- Z=mydict{description:'Hello',name:'N/A'},writeln(Z.description()).
Hello
Z = mydict{description:'Hello', name:'N/A'}.

mydict:  ?- Z=mydict{name:'Ian'}.description().
Z = 'Ian'.
3

I’ve read something in the doc about there being a module for each dictionary. Then do I have to define description() once for type and once for arg, in separate modules?

And can I create the same module in multiple places? It seems so based on your example.

4

Also, does the very fact of defining a dictionary create a module? How does that interact with modules of the same name elsewhere in the project? Do we have to keep the module names separate across the board?

5

Your question is a bit ambiguous, but seems to me that modules are involved only when you’re defining your own code for a specialized dictionary.
FWIW, a quick test could use current_module/1:

...
For built-in help, use ?- help(Topic). or ?- apropos(Word).

?- setof(M,current_module(M),Ms).
Ms = [ansi_term, base32, link_xpce, pce_swi_hooks, predicate_options, predopts_analysis, prolog, prolog_clause, prolog_debug|...].

?- P=a_point{x:1,y:2}.
P = a_point{x:1, y:2}.

?- setof(M,current_module(M),Ms).
Ms = [ansi_term, base32, link_xpce, pce_swi_hooks, predicate_options, predopts_analysis, prolog, prolog_clause, prolog_debug|...].