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Subject: Re: [mercury-users] determinism suprises
From: Fergus Henderson (fjh@cs.mu.OZ.AU)
Date: Sat Dec 09 2000 - 20:20:36 EST
On 07-Dec-2000, William Lee Irwin III <wli@holomorphy.com> wrote:
> Another "catch" I found was the following:
>
> :- pred color(squareindex, color).
> :- mode color(in, out) is det.
> :- mode color(in, in) is semidet.
> :- mode color(out, in) is multi.
> color({File, Rank}, Color) :-
> file_parity(File, FileParity),
> rank_parity(Rank, RankParity),
> add_parity(FileParity, RankParity, Residue),
> parity_to_color(Residue, Color).
>
> Now, file_parity, rank_parity, and parity_to_color have all the
> necessary modes and determinisms to allow all of color's modes and
> determinisms given the appropriate ordering of conjunctions, but I
> still found that the reordering was not done for each mode:
>
> board.m:018: In `color(out, in)':
> board.m:018: error: determinism declaration not satisfied.
> board.m:018: Declared `multi', inferred `nondet'.
> board.m:167: call to `parity_to_color(in, in)' can fail.
>
> So I tried to reorder the conjunction as follows:
>
> color({File, Rank}, Color) :-
> parity_to_color(Residue, Color),
> add_parity(FileParity, RankParity, Residue),
> file_parity(File, FileParity),
> rank_parity(Rank, RankParity).
>
> but this runs into the following error:
> board.m:016: In `color(in, out)':
> board.m:016: error: determinism declaration not satisfied.
> board.m:016: Declared `det', inferred `nondet'.
> board.m:164: call to `parity_to_color(out, out)' can succeed more than once.
> board.m:165: call to `add_parity(out, out, in)' can succeed more than once.
> board.m:166: call to `file_parity(in, in)' can fail.
> board.m:167: call to `rank_parity(in, in)' can fail.
and then in a subsequence mail
> :- pred add_parity(parity, parity, parity).
> :- mode add_parity(in, in, out) is det.
> :- mode add_parity(in, out, in) is det.
> :- mode add_parity(out, in, in) is det.
> :- mode add_parity(in, in, in) is semidet.
> :- mode add_parity(out, out, in) is multi.
> :- mode add_parity(out, in, out) is multi.
> :- mode add_parity(in, out, out) is multi.
> :- mode add_parity(out, out, out) is multi.
> add_parity(even, even, even).
> add_parity(even, odd, odd).
> add_parity(odd, even, odd).
> add_parity(odd, odd, even).
The problem here is essentially that you have declared *too many* modes
for the called predicates `add_parity', `file_parity', and `rank_parity'.
In general, you should not declare modes for predicates in which they
produce values for output arguments that are unconstrained, as is the
case for all the modes of `add_parity' that have determinism `multi'.
Instead, you should use the "constrain and then generate" paradigm.
You should omit declarations for those modes, and instead use an
explicit generator predicate, e.g. `gen_parity', to generate
unconstrained values.
:- pred gen_parity(parity).
:- mode gen_parity(in) is det.
:- mode gen_parity(out) is multi.
gen_parity(even).
gen_parity(odd).
Code which needs to generate values should first call all the
predicates which impose constraints, and then finally call a generator
to generate values for values which remain unconstrained:
color({File, Rank}, Color) :-
% first apply the constraints
file_parity(File, FilePar),
rank_parity(Rank, RankPar),
add_parity(FilePar, RankPar, Res),
parity_to_color(Res, Color),
% then generate any values which haven't
% been determined by the constraints
gen_parity(FilePar).
I enclose below a patch to your example code which uses this idiom and
which the Mercury compiler will happily accept.
--- old/pathology.m Sat Dec 9 19:56:40 2000
+++ ./pathology.m Sat Dec 9 20:12:01 2000
@@ -18,7 +18,6 @@
:- mode rank_parity(in, in) is semidet.
:- mode rank_parity(in, out) is det.
:- mode rank_parity(out, in) is multi.
-:- mode rank_parity(out, out) is multi.
rank_parity(first, odd). rank_parity(second, even). rank_parity(third, odd).
rank_parity(fourth, even). rank_parity(fifth, odd). rank_parity(sixth, even).
rank_parity(seventh, odd). rank_parity(eighth, even).
@@ -27,7 +26,6 @@
:- mode file_parity(in, in) is semidet.
:- mode file_parity(in, out) is det.
:- mode file_parity(out, in) is multi.
-:- mode file_parity(out, out) is multi.
file_parity(a,odd). file_parity(b,even). file_parity(c,odd).
file_parity(d,even). file_parity(e,odd).
file_parity(f,even). file_parity(g,odd). file_parity(h,even).
@@ -36,7 +34,6 @@
:- mode parity_to_color(in, in) is semidet.
:- mode parity_to_color(in, out) is det.
:- mode parity_to_color(out, in) is det.
-:- mode parity_to_color(out, out) is multi.
parity_to_color(even, black).
parity_to_color(odd, white).
@@ -45,26 +42,25 @@
:- mode add_parity(in, out, in) is det.
:- mode add_parity(out, in, in) is det.
:- mode add_parity(in, in, in) is semidet.
-:- mode add_parity(out, out, in) is multi.
-:- mode add_parity(out, in, out) is multi.
-:- mode add_parity(in, out, out) is multi.
-:- mode add_parity(out, out, out) is multi.
add_parity(even, even, even).
add_parity(even, odd, odd).
add_parity(odd, even, odd).
add_parity(odd, odd, even).
-% What this really should have been was:
-% color({File, Rank}, Color) :-
-% parity_to_color(Res, Color),
-% add_parity(FilePar, RankPar, Res),
-% file_parity(File, FilePar),
-% rank_parity(Rank, RankPar).
-% but its determinism could not be correctly inferred by the compiler.
color({File, Rank}, Color) :-
+ % first apply the constraints
file_parity(File, FilePar),
rank_parity(Rank, RankPar),
add_parity(FilePar, RankPar, Res),
- parity_to_color(Res, Color).
+ parity_to_color(Res, Color),
+ % then generate any values which haven't
+ % been determined by the constraints
+ gen_parity(FilePar).
+
+:- pred gen_parity(parity).
+:- mode gen_parity(in) is det.
+:- mode gen_parity(out) is multi.
+gen_parity(even).
+gen_parity(odd).
:- end_module pathology.
--
Fergus Henderson <fjh@cs.mu.oz.au> | "I have always known that the pursuit
| of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.
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