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

[Julien Fischer]

Hi,

On Sun, 6 Apr 2014, Left Right wrote:

> Thanks for reply!
> Would you be so kind as to point me to the docs where I should read on
> what "trailed" means?

It's described in the ``Trailing'' section of the reference manual.
<http://www.mercurylang.org/information/doc-latest/mercury_ref/Trailing.html#Trailing>

Trailing is not relevant here other than the fact mutables are trailed
by default.

> 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.

No, ground here just means that a term has a definite value and doesn't
contain any variables.  In the example being discussed that means that
Factor must have a definite value by the time the call to io.write/3
is made -- that differs from C++ style constexprs where the value would
be required to be known at compile time.

> 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?

The mode system is discussed in chapter 4 of the reference manual.

> 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 :)

This is covered in chapter 3 of that book.  I think your experience
here may have been a bit misleading, probably due to the fact that
trace goals and mutables are _not_ "simple stuff".  The compiler could
also be a bit clearer about exactly what's wrong here.  (Incidentally,
if you remove the trace goal from the predicate, then the compiler is
quite clear about what's wrong.)

Cheers,
Julien.