#19 Interaction of tabling with input/output predicates

[Carlos Viegas Damásio]

Dear All,

I have some doubts regarding the interaction of tabling and predicates with side-effects.
I've got unexpected results (at least for me...).

I wanted to fill up a table with the contents of a file, indexed by its position.

I've tried the following code:

:- import file_open/3 from file_io.
:- import file_close/1 from file_io.
:- import file_getbuf/4 from file_io.
:- import file_seek/4 from file_io.

:- table stream/3.

open_stream( FileName, FileDes ) :-
file_open( FileName, 0, FileDes ).

close_stream( FileDes ) :-
file_close( FileDes ).

stream( FileDes, N, Byte ) :-
% file_seek( FileDes, N, 0, _ ),
file_getbuf( FileDes, 1, Byte, Read ), Read = 1.

Here is an example:

[configuration loaded]
[sysinitrc loaded]
[packaging loaded]

XSB Version 2.0 (Gouden Carolus) of June 27, 1999
[x86-pc-windows; mode: optimal; engine: chat; scheduling: batched]
| ?- [stream0].
[Compiling .\stream0]
[stream0 compiled, cpu time used: 0.9300 seconds]
[stream0 loaded]

yes
| ?- open_stream('lixo.P',F).

F = 3;

no
| ?- stream(3,0,N).

N = c;

no
| ?- stream(3,1,N).

N = o;

no
| ?- stream(3,2,N).

N = n;

no
| ?- stream(3,0,N).

N = n;

no

The first result has been recomputed!!! (If I put the file_seek everything works fine, but XSB executes again the same code).

| ?- close_stream(3).

yes
| ?- stream(3,0,N).

N = n;

no
| ?- stream(3,1,N).

N = n;

no
| ?- stream(3,2,N).

N = n;

no

Then, instead of performing file input I tried with read predicate:

:- table stream/3.

open_stream( FileName, 0 ) :-
see( FileName ).

close_stream( 0 ) :-
seen.

stream( F, N, Byte ) :-
read( Byte ).

| ?- open_stream('lixo.P',F).

F = 0;

no

| ?- stream(0,0,N).

N = conn(inv4,i,inv1);

no
| ?- stream(0,1,N).

N = conn(and2,i1,inv1);

no
| ?- stream(0,2,N).

N = conn(and3,i2,inv1);

no
| ?- stream(0,0,N).

N = conn(inv4,i,inv1);

no
| ?- stream(0,2,N).

N = conn(and3,i2,inv1);

no

Now, it works as I expected! But if I use get0 instead of read the same problem occurs!
I also tried a version with assert and retract, and it also works.

:- table stream/3.

:- dynamic ctr/1.

open_stream( FileName, 0 ) :-
assert( ctr(0) ).

close_stream( 0 ) :-
retract( ctr(_) ).

stream( 0, N, Byte ) :-
retract(ctr(Byte)),
NewByte is Byte + 1,
assert( ctr(NewByte) ).

| ?- open_stream('lixo.P',F).

F = 0;

no
| ?- stream(0,0,N).

N = 0;

no
| ?- stream(0,1,N).

N = 1;

no
| ?- stream(0,2,N).

N = 2;

no
| ?- stream(0,0,N).

N = 0;

| ?- stream(0,2,N).

N = 2;

no
| ?- stream(0,1,N).

N = 1;

no
| ?- stream(0,3,N).

N = 3;

no

[Luis Castro]

Logged In: YES
user_id=6694

I could not reproduce your error in the current system. Does
it still happen to you?