Failing to use //div as xpath selector

Help!
1

?- prolog_flag(version,X).
X = ‘SWI-Prolog 8.0.3 (x86_64-linux): Jun 19 2019, 14:01:15’.

My code looks like this:

:- use_module(library(http/http_client)).
:- use_module(library(http/http_open)).
:- use_module(library(xpath)).

?- j2(P, //div, X),length(X,XL), show(X).
ERROR: Syntax error: Unbalanced operator
ERROR: j2(P, //di
ERROR: ** here **
ERROR: v, X),length(X,XL), show(X) .

If I wrap it in singles as an atom, it just fails:

?- j2(P, '//div', X),length(X,XL), show(X).
false.

I am sure I had this problem a very long time ago but I couldn’t find it on the forums… how on earth do I get to be able to specify a DIV tag!

I was having such fun until this came along!! I am sure it’s simple once you know what you are doing…

Thanks,
Sean.

2

I believe that the ‘//’ is an operator, which is why '//div' fails. Does '//'(div) work?

EDIT: not sure why it’s not working as //div though; xpath should be exporting that op…

3

div/2 is also an operator!

Let me try… yes, it does.

So…attempting to understand…using //div just like that is functor(args), is ‘//’(div) is writing it in functional form so that it doesn’t get parsed the “wrong” way because of operator precedence?

And, thanks James!

4

Oh right, div is also an arithmetic operator, of course!

I believe so; //'div' should work properly as well and is probably a better solution (because the problem isn’t about // as an operator, it’s that div is being treated as an operator, not an atom).

5

Yes, //‘div’ also works… but not sure why…so…hacky…I see that…

?- ‘//’(div) = //‘div’.
true.

so they are the same thing, the functor ‘//’ with the atom div as the sole argument.

6

Just a case of operator clash as @jamesnvc pointed out:

?- current_op(P, T, //).
P = 400,
T = fx ;
P = 400,
T = yfx.

?- current_op(P, T, div).
P = 400,
T = yfx.

As usual, you can solve the clash using parenthesis:

?- write(//(div)).
// (div)
true.
1 Like
7

Ah! Thanks Paulo.

8

If you use modules and have no use for div for arithmetic in that module you can also do this to locally kill the operator:

:- op(0, yfx, div).

Not sure this should be considered good or bad style. It certainly suffers from portability as some Prolog systems (still) have only global operators.

9

the op//3 and generally definiting operators is not something I have ever done so it’s all a bit mysterious to me…it’s something I keep meaning to study but somehow I have managed thus far without needed a custom operator.

Do you know of any good tutorials, I have found some but they tend to be terse and not very helpful…if I can understand I#d write my own…currently working in a custom little web server with Prolog (I didn’t get on with