Check if object already exists?

Question

i have :

:- use_module(library(logtalk)).
:- {buffer}.
:- initialization(main).

main :-
    create_object(env,[instantiates(buffer)],[],[]),

it works but every time I re-consult the file it barks an error because the object already exists.

Is there a way to check if the object already exists and skip recreating it ?

more code:

:- use_module(library(logtalk)).

:- consult(utils).

%% :- initialization((
%%     logtalk_load([buffer,env]).
%% )).
:- {buffer}.
:- initialization(main).

main :-
    %% create_object(env,[instantiates(buffer)],[],[]), 
    env::set(uid,0), env::set(name,"").

this worked :

:- initialization((
    logtalk_load([buffer])
)).

i.e. no dot and no "env"

Paulo Moura · Accepted Answer · 2021-01-19T00:14:45.033

1

You can use the current_object/1 predicate to check if an object exists. But from your code fragment is seems that you could simply define the env object in a source file. If you need env to be a dynamic object (why?), then use the dynamic/0 directive:

:- object(env,
    instantiates(buffer)).

    :- (dynamic)/0.

:- end_object

Btw, never use top-level abbreviations (such as {}/1) in source files; they are not part of the language. Write instead:

:- use_module(library(logtalk)).
:- initialization((
    logtalk_load([buffer, env])
)).

P.S. You're using the logtalk pack for SWI-Prolog. But this pack meant for deployment, not development, as it hides all files on the Logtalk distribution (including documentation) inside the hidden directory used for packs.

edited Jan 19 '21 at 00:14

answered Jan 18 '21 at 19:38

Paulo Moura

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i have other stuff in "main" – sten Jan 18 '21 at 19:57
Note that you can have multiple `initialization/1` directives in the same source file. – Paulo Moura Jan 18 '21 at 19:59
i had env in buffer.lgt but i thought it will be better in the source file. may be u are right i dont need it in the source cause it is not dynamic – sten Jan 18 '21 at 20:00
whats the benefit of not using pack ? – sten Jan 18 '21 at 20:01
{buffer} works the other doesnt.... i'm using env in main – sten Jan 18 '21 at 20:06
How are you using the `env` object in `main/0`? What errors do you get? Can you expand the question with those details? – Paulo Moura Jan 18 '21 at 20:12
There's nothing in your edited code that would trigger an error when using an `initialization /1` directive to load the files. The initialization goals are run in order. Thus, by the time you call `main/0`, the `buffer` and `env` source files are already loaded. If you're getting any error or warning with my suggestion, add it to your question to clarify. – Paulo Moura Jan 18 '21 at 23:19
i put env in buffer not as separate file ;) – sten Jan 18 '21 at 23:42
The dot as a typo indeed. Fixed. But no reason why having the objects in separate files would cause an error or warning. As far as I understand, `env` depends on `buffer`. so, as long as you load `buffer` before the `env`, you should get a clean compilation. – Paulo Moura Jan 19 '21 at 00:18

Check if object already exists?

1 Answers1