[m-users.] How to user filter_map with a nondet function

[Volker Wysk]

Hi

When calling a predicate, the modes must match one of the mode lines of the
predicate. Since filter_map has only one mode/mode line, the passed
predicate must match that. This means that the passed predicate must be
semidet, not cc_nondet.

You might be able to copy the source code of the filter_map predicate,
rename it to, say, filter_map_1 and add a second mode line, with the
determinism category cc_nondet of the higher order argument.

However, I don't understand what a cc_nondet predicate would do. I've never
seen it be used that way. Of course, you can just try it out. 

Cheers,
Volker


Am Samstag, dem 19.11.2022 um 22:58 +0530 schrieb Razetime:
> :- module a10.
> 
> :- interface.
> :- import_module io.
> :- pred main(io::di,io::uo) is det.
> 
> :- implementation.
> :- import_module alib.
> :- import_module list,string,int,char.
> 
> :- pred go(int::out,int::out,cl::in,cl::out) is nondet.
> go(X,Y) --> l("value "),num(X),l(" goes to bot "),num(Y).
> 
> :- pred give(int::out,int::out,int::out,cl::in,cl::out) is nondet.
> give(X,Y,Z) --> l("bot "),num(X),l(" gives low to bot "),num(Y),l("
> and high to "),num(Z).
> 
> main(!IO):-
>   rlinesc("inp/10",L0,!IO),
>   filter_map((pred(X::in,{Y,Z}::out) is cc_nondet:-
>     go(Y,Z,X,[])
>   ),L0,As,Mo),
>   print_line(As,!IO)
>   .
> 
> alib is here: https://github.com/razetime/aoc/blob/main/16/alib.m
> 
> I'm trying to use the DCG `go` with filter_map to parse lines which
> satisfy the predicate. I get the following error:
> 
> Making Mercury/int3s/a10.int3
> Making Mercury/ints/a10.int
> Making Mercury/cs/a10.c
> For more information, recompile with `-E'.
> a10.m:019: In clause for `main(di, uo)':
> a10.m:019:   in argument 1 of call to predicate `list.filter_map'/4:
> a10.m:019:   mode error: variable `V_14' has instantiatedness
> a10.m:019:     /* unique */ (pred(in, out) is cc_nondet),
> a10.m:019:   expected instantiatedness was `(pred(in, out) is semidet)'.
> a10.m:019: In clause for predicate `main'/2:
> a10.m:019:   warning: variable `Mo' occurs only once in this scope.
> ** Error making `Mercury/cs/a10.c'.
> 
> I do understand that I can change num(X) in alib.m, but I would like
> to know if i can make the determinism check succeed in other ways.
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