error handling in gprolog?

Question

In order to run a group of queries in a simple query, I have tagged these queries and use forall/2 to call them:

query_all :-
    forall(query(Q), (Q ->
        format('yes: ~w~n',[Q]) ;
        format('no : ~w~n',[Q]))).

so if I define something like query(true)., I'll be able to see yes: true from the output.

The problem here is that query( ... ) do not always exist, when prolog can't find anything that tagged query, forall/2 will fail and cause exception saying "error(existence_error(procedure,query/1),forall/2)"

I want to handle this exception, but not to break the whole control flow.

I know catch/3 would help me but I don't know how to use it, my code is:

catch(query_all, error(existence_error(procedure,_),_), recovery).

recovery :-
    format('error occurred.~n',[]).

but prolog says "native code procedure catch/3 cannot be redefined". Is there something I've missed?

Answer 1

You can either declare query/1 as dynamic in your code adding this line:

:-dynamic(query/1).

or use catch/3 as you suggested, however you don't have to redefine it but use it instead, e.g:

query_all :-
  catch(
    forall(query(Q), (Q ->
        format('yes: ~w~n',[Q]) ;
        format('no : ~w~n',[Q]))),
        error(existence_error(procedure, _), _), format('error occurred.~n', [])).