Hello, we used Hugs' Prolog Demo as a base for a project in Programing
Languages, and found out that the cut implementation is wrong.
Here it is the portion of code in the latest release:
----------------------------------------------------------------------------------
prove :: Database -> [Term] -> [Subst]
prove db gl = solve 1 nullSubst gl []
where
solve :: Int -> Subst -> [Term] -> Stack -> [Subst]
solve n s [] ow = s : backtrack n ow
solve n s (g:gs) ow
| g==theCut = solve n s gs (cut ow)
| otherwise = choose n s gs (alts db n (app s g)) ow
choose ...
backtrack ...
--- Special definitions for the cut predicate:
theCut ...
cut :: Stack -> Stack
cut (top:(s,gl,_):ss) = top:(s,gl,[]):ss
cut ss = ss
-----------------------------------------------------------------------------------
And here it is our version, that we think is correct:
----------------------------------------------------------------------------------
prove :: Database -> [Term] -> [Subst]
prove db gl = solve 1 nullSubst gl []
where
solve :: Int -> Subst -> [Term] -> Stack -> [Subst]
solve n s [] ow = s : backtrack n ow
solve n s (g:gs) ow
| g == theCut = solve s gs (cut2 (1 + numberOC gs)
ow) -- Changed
| otherwise = choose n s gs (alts db n (app s g)) ow
choose ...
backtrack ...
--- Special definitions for the cut predicate (modified):
theCut ...
numberOC = length . filter (==theCut) -- Number of Cuts
cut2 :: Int -> Stack -> Stack
cut2 n [] = []
cut2 n ((s,gs,_):ow)
| (numberOC gs) >= n = (s,gs,[]): cut2 n ow
| (numberOC gs) < n = (s,gs,[]): ow
-----------------------------------------------------------------------------------
Sergio Urinovsky
[EMAIL PROTECTED]
Nicolas Wolovick
[EMAIL PROTECTED]
Undergraduate cs students at FaMAF - UNC
Cordoba - Argentina
