bug#21526: 24.5; prolog-mode: broken indentation for if-then-else construct

[Stefan Monnier]

> Yes.  All issues I reported so far are deviations from Stefan Bruda's
> original version.  His version also highlights DCG heads with zero
> arguments correctly.

Hmm... I wonder how we lost this.  Has it always highlighted those, or
is it something newish (e.g. newer than say Emacs-23.3)?

>>> (c) Arguments of directives should be more indented.  For example, a
>>> directive whose arguments span several lines should look like this:
>>> 
>>> :- multifile
>>>        pred1,
>>>        pred2,
>>>        pred3.
>> 
>> I have no idea what "directives" are nor what the above "multifile"
>> means nor what is its syntax.  Does Bruda's code handle that?

> Yes. All issues I reported so far are deviations from Stefan Bruda's
> original version. His version also highlights DCG heads with zero
> arguments correctly.

> A directive starts with :- or ?- in a source file.

> (:-)/1 can be read just like a Prolog rule, only without head. So, after
> :-, I expect the same indentation like in a rule. A directive can
> include any regular Prolog goal.

I don't understand.  You say you want

    :- multifile
           pred1,
           pred2,
           pred3.

and you say that it is indented like in a normal rule, but in normal rule,
I expect neither

    foo(X):- multifile
                 pred1,
                 pred2,
                 pred3.
nor
    foo(X) :- multifile
        pred1,
        pred2,
        pred3.

AFAIK the problem is not with the ":- without a left-hand side", but
with how to parse the

       multifile
           pred1,
           pred2,
           pred3

Currently it's parsed as:

       (multifile
           pred1),
           pred2,
           pred3

whereas clearly you'd like

       multifile
           (pred1,
           pred2,
           pred3)

hence my questions: What means "subsequent lines"?
Anything until a "."?  What about a ";" or a ":-" or a "-->"?
IOW what should happen with something like

     :- multifile
           pred1;
           pred2 -->
           pred3.

Furthermore, it seems that this is linked to the (:-)/1 operator, so
what should happen if "multifile" appears elsewhere?  what other keyword
can appear where you have "multifile" and do they all use this
same syntax?

> Regarding comments, one more thing I noticed is that in a multiline
> comment like /* ... */, we actually expect a small indentation like:

> as in the default C mode, but currently get:
>
> /*
> Not indented.
> */

Indeed, we have some problem here (it seems TAB doesn't even do
anything).  Patch welcome.


        Stefan