SWI-Prolog - Fail to Assert

Question

I define an operator as follows:

:- op(500, xfx, =>).

When I try something like:

assert(a => b).

Prolog raises an error that says 'No permission to modify static_procedure (=>)/2'.

Any solution?

Answer 1

As a security, you have to warn SWI that you are going to modify a predicate at runtime:

:- dynamic (=>)/2.

put at the top of the file should do it.

Answer 2

You must have meant another symbol in place of (=>)/2. Probably (->)/2 which is a control construct that cannot be modified.

Welcome to SWI-Prolog (Multi-threaded, 32 bits, Version 6.1.3-116-gf1c7e06)
...
?- asserta((a -> b)).
ERROR: asserta/1: No permission to modify static procedure `(->)/2'
ERROR: Defined at /opt/gupu/pl-devel/lib/swipl-6.1.3/boot/init.pl:194
?- op(500, xfx, =>).
true.

?- asserta(a => b).
true.