Declaring a predicate dynamic in gprolog

Question

I have this code in Prolog:

dynamic(player_at/1).
player_at(house).
goto(X) :- retract(player_at(house)), assert(player_at(X)).

But I still get this error:

uncaught exception: error(permission_error(modify,static_procedure,player_at/1),retract/1)

when I execute goto(foo).

I've read the dynamic documentation, but I can't figure out how to use it, at least in gprolog. Am I missing something?

Fabien

Eventually you should get already an exception during the consult of your program. It should not allow a dynamic(_) fact. The ISO standard only says that dynamic is a directive, but in most Prologs it is also a built-in, and can thus not be asserted as a fact. – Northnortheast

Answer 1

Fix the first line by prepending :-:

:- dynamic(player_at/1).

Without :- the line would dreefine predicate dynamic/1, instead of executing the existing dynamic predicate.

Other prolog implementations (but not gprolog) support this as well:

:- dynamic player_at/1.

Compurgation

I tried that - I get syntax error: . or operator expected after expression. It doesn't like the dynamic without a parenthesis. – Fabien