bimbart@kulcs.uucp (Bart Demoen) (09/12/88)
In article <MATSC.88Sep7095859@skarabe.sics.se> Mats Carlson argues for
another behavior of bagof/setof
I am very glad with it: BIMprolog has always implemented bagof/setof with the
behavior Mats proposes (*) and I think it is very reasonable - although slowing
down some benchmarks
I still have a question:
database:
p(1,a) .
p(_,b) .
p(2,c) .
query:
?- setof(Y,p(X,Y),L) .
answers:
SICSTUSprolog:
X = 1 , L = [a,b]
X = 2 , L = [b,c]
BIMprolog:
X = 1 , L = [a]
X = _0 , L = [b]
X = 2 , L = [c]
CProlog:
X = 1 , L = [a,b]
X = 2 , L = [c]
my question is: which is more reasonable and/or useful and/or satisfying ?
(*) BIMprolog gives the answers in the order
X = _49, L = [a,b] ;
X = f(_141), L = [a,b] ;
X = f(_165,_141), L = [a,b] ;
X = f(_141,_141), L = [a,b] ;
(i.e. the last 2 solutions in a different order)
Mats, was that a misprint in your mail ?lee@munnari.oz (Lee Naish) (09/14/88)
In article <1436@kulcs.kulcs.uucp> bimbart@kulcs.UUCP (Bart Demoen) writes: > p(1,a) . > p(_,b) . > p(2,c) . > ?- setof(Y,p(X,Y),L) . > > SICSTUSprolog: > X = 1 , L = [a,b] > X = 2 , L = [b,c] "Logically", there are three answers (the last one containing two constraints on X): X = 1 , L = [a,b]; X = 2 , L = [b,c]; X ~= 1, X ~= 2, L = [b]. Therefore, I think Sicstus gives the most "logical" set of solutions. In other, less complex cases, setof generally returns what a logical all solutions predicate returns except for answers which must contain constraints. It seems that Sicstus is more consistent than other systems. Does anyone know if it is completely consistent? Perhaps there is another "logical" way of explaining the behaviour of other systrems. (NU-Prolog setof behaves the same as C-Prolog, and we hope most Edinburgh systems, for compatability). lee lee
ok@quintus.uucp (Richard A. O'Keefe) (09/14/88)
In article <1436@kulcs.kulcs.uucp> bimbart@kulcs.UUCP (Bart Demoen) writes: >database: > p(1, a). p(_, b). p(2, c). >query: > ?- setof(Y,p(X,Y),L) . >answers: > SICSTUSprolog: > X = 1, L = [a,b] ; > X = 2, L = [b,c] > > BIMprolog: > > X = 1, L = [a] ; > X = _0, L = [b] ; > X = 2, L = [c] > > C Prolog: > > X = 1, L = [a,b] ; > X = 2, L = [c] > >My question is: which is more reasonable and/or useful and/or satisfying? Quintus Prolog: X = _, L = [b] ; X = 1, L = [a] ; X = 2, L = [c] Note that the bindings of X are enumerated in standard order (variables precede numbers). Within each binding for X, the elements of L are in standard order. I am able to say that C Prolog's behaviour is a bug; that particular result is unintended. There are three ground solutions: X = 1 -> L = [a,b] ; X = 2 -> L = [b,c] ; ground(X), X ~= 1, X ~= 2 -> L = [b] SICStus Prolog gets two of them exactly right, but misses the third case entirely. BIMprolog and Quintus Prolog notice that there are three cases, and their results cover p. What this example illustrates is that to handle "all solutions" logically you need - coroutining (to wait until X is sufficiently instantiated), or - constraints (to obtain the solution X~=1, X~=2, L=[b]) As things stand, setof(Template, Generator, Set) is only defined when all the solutions of Generator found are ground.
matsc@sics.se (Mats Carlsson) (09/14/88)
Bart Demoen writes: >I still have a question: >database: p(1,a). p(_,b). p(2,c). >query: ?- setof(Y,p(X,Y),L). >answers: >... As of version 0.6 #6, SICStus Prolog answers: X = _49, L = [b] ; X = 1, L = [a] ; X = 2, L = [c] ; >(*) BIMprolog gives the answers in the order >... >(i.e. the last 2 solutions in a different order) >Mats, was that a misprint in your mail ? No. I consider the order immaterial. It depends on exactly how you go about renaming variables.
jax@well.UUCP (Jack J. Woehr) (09/15/88)
Uh, Rich, ole buddy, will you tell me when Quintus starts hiring so I too can get a job that lets me spend all day and all night posting to the net? :-) ********** jax@well programs(jax) :- jax@chariot uses_forth(vesta_technology), JAX on GEnie plays_with_Prolog(jax,when_boss_ain't_looking).