[m-users.] How do you go about logging?

[Left Right]

Thanks for reply!
Would you be so kind as to point me to the docs where I should read on
what "trailed" means?
Other then that: do I understand it correctly that "ground" in this
context means something roughly equivalent to C++ constexpr? I.e.
something that doesn't require calling functions to be computed. If
so, is there a way for the programmer to promise to the compiler that
this is the kind of variable it is? And if this is not the way it
works, then where could I read about this mechanism?
I've so far only red the book.pdf downloadable from the documentation
section of the site, but I see that it's far from being enough for
even simple stuff :)

Best,

Oleg

On Sun, Apr 6, 2014 at 9:38 AM, Julien Fischer <jfischer at opturion.com> wrote:
>
> Hi,
>
>
> On Sun, 6 Apr 2014, Left Right wrote:
>
>> Thanks for reply! But wouldn't you mind elaborating a bit? Here's what
>> I've tried, with the error message that resulted:
>>
>> % e3.m:011: Error: trailed mutable in non-trailing grade.
>
>
> Mutables are trailed by default, so if you want an untrailed mutable,
> as here, it needs to be declared with the ``untrailed'' attribute, for
> example:
>
>     :- mutable(logging_level, int, 0, ground, [untrailed]).
>
>
>> % e3.m:018: In clause for `nonedivides(in, out)':
>> % e3.m:018:   in argument 2 of call to predicate `io.write_string'/3:
>> % e3.m:018:   unique-mode error: the called procedure would clobber
>> its argument,
>> % e3.m:018:   but variable `STATE_VARIABLE_IO_9' is still live.
>> % e3.m:020:   The goal could not be reordered, because it was followed by
>> an
>> % e3.m:020:   impure goal.
>> % e3.m:016:   This is the location of the impure goal.
>
>
> Ugh!  That's all very mysterious.  What the compiler is trying to
> complain about there is the fact the variable ``Factor'' has inst free
> in the call to io.write/3.  Since the first argument of io.write/3 has
> mode ``in'', it needs to ground.
>
> I think the second mode in the declaration of nonedevices/2 is wrong,
> it should be ``in'' not ``out''.
>
>
>>
>> :- mutable(logging_level, int, 0, ground, []).
>>
>> :- pred nonedivides(int::in, list(int)::out) is semidet.
>> nonedivides(_, []).
>> nonedivides(Test, [Factor | Factors]):-
>>    trace [io(!IO), state(logging_level, !LoggingLevel)]
>>    (!.LoggingLevel > 1 ->
>>     io.write_string("Factor = ", !IO),
>>     io.write(Factor, !IO),
>>     io.nl(!IO) ; true),
>>    Factor mod Test \= 0, nonedivides(Test, Factors).
>
>
> Cheers,
> Julien.