Re: No permission to modify static procedure

On 19/01/2014, at 15:05, Lumj <[email protected]> wrote:

> Is it possible to write a module in which there's a predicate write/1?
> When I try to do this, I got the 'No permission to modify static procedure' error. What I intend to do is to have a write/1 in my module and the system:write/1 remain where it is.
> Module file attached:
> :-module(m,[write/1]).
> write(x).
> Am I missing anything?

Not really. This behavior in not exclusive of the SWI-Prolog module system. But you can redefine backend Prolog compiler predicates using Logtalk objects easily:

:- object(o).

	:- public(write/1).
	write(Term) :-
		...

:- end_object.

If you want to call the built-in write/1 predicate from within this object, you still have access to it by writing {write(Term)}. The {}/1 control construct allows you to bypass the Logtalk compiler. You can then use your own definition of write/1 from other objects easily. For example:

:- object(other).

	:- uses(o, [write/1]).

	foo(Term) :-
		...,
		write(Term),
		...

:- end_object.

Turning on the optimize flag and compiling the object "o" before the object "other" gives you static binding and thus the call o::write/1 will have the same performance of a local call without any messages sending overhead.

A practical example where redefining built-in predicates is useful is:

https://github.com/LogtalkDotOrg/logtalk3/blob/master/library/comparingp.lgt
https://github.com/LogtalkDotOrg/logtalk3/blob/master/examples/sicstus/sorting.lgt

Cheers,

Paulo

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Paulo Moura
Logtalk developer

Email: <mailto:[email protected]>
Web:   <http://logtalk.org/>
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