[m-users.] Problem with DCG producing false negative

[Volker Wysk]

Am Montag, dem 17.10.2022 um 22:51 +1100 schrieb Mark Brown:
> On Mon, Oct 17, 2022 at 10:20 PM Volker Wysk <post at volker-wysk.de> wrote:
> > 
> > Hmm... I think I misunderstood what you're trying to do with seq/3. You
> > probably *want* it to be multi and not det. Is that so?
> 
> That was my take on it. I assume seq(A, B, C) is intended to mean that
> A ++ B = C (i.e., it is append/3 with the last two arguments swapped).

That's A ++ C = B, yes.

> But that reinforces my point that people need to know the programmer's
> intention.

Agreed. Therefore I always comment my code elaborately.

Bye
Volker

> 
> Mark
> 
> > 
> > Am Montag, dem 17.10.2022 um 12:54 +0200 schrieb Volker Wysk:
> > > Hi. Please post your source code, as complete as necessary. Mark the lines
> > > which are named in compile error messages, so we can see what's going on.
> > > 
> > > Regarding your determinism errors, you have this:
> > > 
> > > :- pred seq(list(T),list(T),list(T)).
> > > :- mode seq(out,in,out) is multi.
> > > seq([])     --> [].
> > > seq([E|Es]) --> [E], seq(Es).
> > > 
> > > This isn't deterministic, because the first rule will always succeed. The []
> > > pattern doesn't mean the end of the input, but a pattern that will always
> > > succeed. That seq should be written like this:
> > > 
> > > :- pred seq(list(T),list(T),list(T)).
> > > :- mode seq(out,in,out) is det.
> > > seq(L) -->
> > >    ( if   [E]
> > >      then seq(Es),
> > >           { L = [E|Es] }
> > >      else { L = [] }
> > >    ).
> > > 
> > > With this deterministic seq, the tls rule will possibly also be
> > > deterministic. Try it out.
> > > 
> > > Have fun,
> > > Volker
> > > 
> > > 
> > > Am Montag, dem 17.10.2022 um 15:56 +0530 schrieb Razetime:
> > > > hm, it makes sense that the dcg rule should be semidet or det. These
> > > > are the problems I had when trying to change the rules.
> > > > 
> > > > 1)
> > > > a07.m:011: In `tls'(out, out, in, out):
> > > > a07.m:011:   error: determinism declaration not satisfied.
> > > > a07.m:011:   Declared `semidet', inferred `nondet'.
> > > > a07.m:011:   The reasons for the difference are the following.
> > > > a07.m:012:   Disjunction has multiple clauses with solutions.
> > > > a07.m:013:   Call to `alib.seq'(out, in, out) can succeed more than once.
> > > > a07.m:014:   Call to `alib.seq'(out, in, out) can succeed more than once.
> > > > 
> > > > 2) after changing alib.seq to semidet (changing to det also gives the
> > > > same error)
> > > > a07.m:011: In `tls'(out, out, in, out):
> > > > a07.m:011:   error: determinism declaration not satisfied.
> > > > a07.m:011:   Declared `semidet', inferred `nondet'.
> > > > a07.m:011:   The reason for the difference is the following.
> > > > a07.m:012:   Disjunction has multiple clauses with solutions.
> > > > 
> > > > I assume that `any_true(abba,[X0|X])' is causing this but i do not
> > > > know how to fix it. For now I am separating the logic from the DCG.
> > > > 
> > > > On 10/17/22, Volker Wysk <post at volker-wysk.de> wrote:
> > > > > Hi
> > > > > 
> > > > > I don't fully understand what you're doing here. But you should try to make
> > > > > the DCG rules det or semidet. Then you can let the compiler spot your
> > > > > errors, when it doesn't agree with the determinism category which you have
> > > > > declared.
> > > > > 
> > > > > Hope that helps,
> > > > > Volker
> > > > > 
> > > > > 
> > > > > Am Montag, dem 17.10.2022 um 00:14 +0530 schrieb Razetime:
> > > > > > I have created the following predicate tls to match lines that satisfy
> > > > > > the conditions given in Advent of Code Day 7:
> > > > > > https://adventofcode.com/2016/day/7 (Spoilers)
> > > > > > 
> > > > > > For some reason during testing ,the DCG fails when given
> > > > > > L="fslvgbiibdkhchajyb[zpbhqrokrbfuqrowop]gqqzoqvfsdfcjcdurrs[xhqfcfytbbekivnvod]jxjwuxivnyhppvfhaol[evfnrmrjnnhychtpv]emiyjcjsnojxexs",
> > > > > > tls(X,Y,to_char_list(L),[])
> > > > > > and I am not sure why it is giving that result. What is done wrong here?
> > > > > > 
> > > > > > :- pred tls(list(cl)::out,list(cl)::out,cl::in,cl::out) is nondet.
> > > > > > tls([X],[])        --> {alp(X)},seq(X).
> > > > > > tls([X0|X],[Y0|Y]) --> {alp(X0),alp(Y0),\+abba(Y0)},
> > > > > > 
> > > > > > seq(X0),['['],seq(Y0),[']'],tls(X,Y),{any_true(abba,[X0|X])}.
> > > > > > 
> > > > > > :- pred seq(list(T),list(T),list(T)).
> > > > > > :- mode seq(out,in,out) is multi.
> > > > > > seq([])     --> [].
> > > > > > seq([E|Es]) --> [E], seq(Es).
> > > > > > 
> > > > > > :- pred alp(cl::in) is semidet.
> > > > > > alp(L):-all_true(is_alpha,L).
> > > > > > 
> > > > > > :- pred abba(cl::in) is semidet.
> > > > > > abba([A,B,B,A|L]):-A\=B.
> > > > > > abba([_,A,B,C|L]):-abba([A,B,C|L]).
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