[mercury-users] Request for comments on this code

[Nicolas Pelletier]

Peter Moulder <Peter.Moulder at infotech.monash.edu.au> writes:

> On Mon, Jul 03, 2006 at 02:57:53PM +0200, Nicolas Pelletier wrote:
>
>>         solutions((pred (S :: out)) is nondet :- queens(N, S), Q),
>
> Incidentally, queens/2 has just one mode, with its last argument being
> `out', so it's fine to write:
>
> 	  solutions(queens(N), Q),

I tried it but the compiler does not  seem to have  support for it yet
(0.13.0-beta-2006-07-05).

nicolas at reglisse:~/dev/dames$ mmc --make --verbose-error-messages queens
Making Mercury/int3s/queens.int3
Making Mercury/cs/queens.c
Uncaught Mercury exception:
Software Error: polymorphism.m: Sorry, not implemented: determinism inference for higher order predicate terms.
Stack dump not available in this grade.
** Error making `Mercury/cs/queens.c'.

>>         clock(R, ! IO),
>
[skipping explanation of issues in my use of the clock predicate]

Thank you for your explanation; I was not careful enough when using clock.

> You can force an ordering by using if-then-else: Mercury guarantees that
> the `if' goal is executed entirely before the `then' goal (modulo any
> backtracking).  We want R0 to be calculated before the solutions call,
> and the solutions call to be before R1, so we use two if-then-else's:
>
>   ( clock(R0, !IO) ->
>   	( solutions(queens(N), Q) ->
> 		clock(R1, !IO),
> 		...
> 	 ; false
> 	)
>    ; false
>   )

As  this code is in the  det mode predicate main,  I  cannot afford to
call false and used error instead. However, the  code is rejected (see
below): the (clock(R0, ! IO) -> part is taken to mean that the call to
clock may fail, and thus !.IO  on that line  does not get instantiated
with the right mode.

main(! IO) :-
    command_line_arguments(Args, ! IO),
    (Args = [StringNum], to_int(StringNum, N)
    ->
        (clock(R0, ! IO)
        ->
            (solutions((pred(S :: out) is nondet) :- queens(N, S), Q)
            ->
                clock(R1, ! IO),
                foldl(print_nl, Q, ! IO),
                length(Q, Nq),
                format("%u solutions: %u seconds",
                       [i(Nq), i((R1 - R0) / clocks_per_sec)], L),
                print_nl(L, ! IO)
            ;
                error("No solutions")
            )
        ;
            error("Cannot get current time")
        )
    ;
        error("Usage: queens <number_of_queens>")
    ).

I do not see a '&&' operator in the builtin operators section of the manual.

>>         flush_output(! IO)
>
> Shouldn't be necessary given that it's just before program exit --
> just as fflush(stdout) isn't necessary in `hello world' in C.

Okay, I'm removing it. Thank you for you various comments.

-- 
Nicolas
--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------