[m-users.] How do you go about logging?
olegsivokon at gmail.com
Tue Apr 8 04:39:04 AEST 2014
Thank you for answers!
I'll keep on reading and advancing in small steps; it's ok :)
On Mon, Apr 7, 2014 at 8:05 AM, Paul Bone <paul at bone.id.au> wrote:
> On Sun, Apr 06, 2014 at 09:20:54AM +0300, 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.
>> % 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.
>> :- 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).
> it might be simplier to avoid module local mutables for now.
> Try making the trace goal conditional on either a runtime or compile time
> flag instead. For example:
> :- pred nonedivides(int::in, list(int)::out) is semidet.
> nonedivides(_, ).
> nonedivides(Test, [Factor | Factors]) :-
> trace [io(!IO), compile_time(flag("debug"))] (
> ( !.LoggingLevel > 1 ->
> io.write_string("Factor = ", !IO),
> io.write(Factor, !IO),
> Factor mod Test \= 0, nonedivides(Test, Factors).
> Then add --trace-flag debug to the mmc command line to enable this trace
> goal. One of the benifits of trace goals (rather than unchecked impurity)
> is that they 1) cannot bind variables 2) Can eaisly be turned off. Combined
> this means that they're less likely to affect the program's semantics,
> unlike conditional compilation in languages like C.
> Note that this clearer syntax also makes the control flow of your
> if-then-else goal clearer to the reader (future you).
> Also your 3 calls to write out the status can be replaced with a single
> io.format("Factor = %d\n", [i(Factor)], !IO)
> Paul Bone
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