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?

Whittington

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.

Impunity

Nit: Above is invalid syntax (try GNU to see this). It should be (=>)/2 – Dioptase

@false: oh well, I hadn't any interpreter available to check that. Thanks, I'll edit :) – Impunity

It's always like that: a predicate indicator has to have brackets if it has a corresponding operator definition. So it is (;)/2 and (',')/2. The quotes serve only to delimit a token - like 'a b' they are needed independently of operator declarations. – Dioptase

@false: Ok! I encountered the notation in doc but didn't know if it was specific to SWI or the norm, thanks for those clarifications :) – Impunity

Sooner or later you will need the standard - see iso-prolog how to get it cheaply. – Dioptase

Yup I try to refer to it as soon as I have a doubt or have a choice between standard and non standard stuff. Though sometimes I just miss the fact that I'm using non standard stuff, so your clarifications help a lot in those cases :) – Impunity

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.

Dioptase

Right but I think it works if you are using Prolog interactively only! It does not work if you compile the file!! – Whittington

Which Prolog system do you refer to? SWI, YAP, B, GNU are all the same. – Dioptase

It's in the title (SWI-Prolog) – Whittington

But apart from that: It is a good idea to use dynamic declarations. – Dioptase