error while compling the metaprogram in prolog

Question

I am trying to implement a meta-program in ECLiPSe Prolog, and here's the code that i have written -

:- dynamic go/1.
sol(true):- !.
sol((A,B)):- !, sol(A), sol(B).
sol(A):- clause(A, Body), sol(Body).
go(X):- X is 5. 

Now when I query with sol(go(X)). , I get the error accessing a procedure defined in another module in clause(X is 5, _292) and it aborts. I tried clearing all toplevel modules and reopening ECLiPSe and then running, but still the same error.

What could be the reason?

Thanks!

Answer 1

Predicate p/1 is using the built-in predicate (is)/2. Note that X is 5 is a syntactically more convenient way of writing is(X,5). But your meta-interpreter is only expecting user defined predicates and the control constructs (',')/2 and true/0. If you want to handle (is)/2 you have to introduce a separate clause for it.

sol(X is Y) :- !, X is Y.

Within ISO Prolog, the goal predicate_property(Goal,built_in) can be used to test if Goal is a built-in predicate. This works in many systems like B, GNU, SICStus, SWI, XSB, YAP. So you can write:

sol(Bip) :- predicate_property(Bip, built_in), !, Bip.

In ECLiPSe this built-in is not directly available. You have to load a library. The index of the manual suggests to use library swi or quintus. For some (unclear) reason it is not part of the ECLiPSe library iso, yet it is ISO. So state

:- use_module(library(swi)).

in your file (or at the toplevel) first.

If you want a meta-interpreter to cover the full Prolog language you will have to handle all control constructs explicitly. Here they are - as defined in the standard (7.8 Control constructs).

1. true/0
2. fail/0
3. call/1
4. !/0
5. (',')/2
6. (;)/2 - disjunction
7. (->)/2
8. (;)/2 - if-then-else
9. catch/3
10. throw/1

Please be aware that only a few of them can be handled by directly calling the goal. Most of them must be handled explicitly!