Prolog parsing is running out of stack -

Prolog parsing is running out of stack -

i have code

s(w) :- append(w1,w2,w), np(w1), vp(w2). vp(w) :- append(w1,w2,w), v(w1), np(w2). np(w) :- ( append(w1,w2,w), pn(w1), ph(w2) ; append(w1,w2,w), det(w1), n(w2) ). pn([hans]). ph([]). v([beobachtet]). n([mann]). n([fernrohr]). p([mit]). det([den]). det([dem]).

when cast np(x). vp(x). or pp(x) 1 possible parse , out of stack error. when cast s(x). don't parse. know because there infinite loop running, cant point out @ point gets looped. figured might happen because of using same names variables, changed them individual ones , didn't alter anything.

any 1 got hint?

thanks in advance!

reason out of stack

let's @ next line code:

vp(w) :- append(w1,w2,w), v(w1), np(w2).

running append(w1, w2, w) in isolation gives following:

?- append(w1, w2, w). w1 = [], w2 = w ; w1 = [_g1108], w = [_g1108|w2] ; w1 = [_g1108, _g1114], w = [_g1108, _g1114|w2] ; w1 = [_g1108, _g1114, _g1120], w = [_g1108, _g1114, _g1120|w2] .

as can see, w1 list of increasing length. length 1 give solution (since v(w1)). after first instantiation, w1 gets longer , longer , longer , ..., v(w1) not succeed lists of longer length.

dcg grammar

in prolog can utilize dcg notation create grammar. grammar follows:

s --> np, vp. np --> pn. np --> det, n. vp --> v, np. det --> [den]. det --> [dem]. n --> [mann]. n --> [fernrohr]. pn --> [hans]. v --> [beobachtet]. example of use ?- phrase(s, s). s = [hans, beobachtet, hans] ; s = [hans, beobachtet, den, mann] ; s = [hans, beobachtet, den, fernrohr] ; s = [hans, beobachtet, dem, mann] ; s = [hans, beobachtet, dem, fernrohr] ; s = [den, mann, beobachtet, hans] ; s = [den, mann, beobachtet, den, mann] ; s = [den, mann, beobachtet, den, fernrohr] ; s = [den, mann, beobachtet, dem, mann] ; s = [den, mann, beobachtet, dem, fernrohr] ; s = [den, fernrohr, beobachtet, hans] ; s = [den, fernrohr, beobachtet, den, mann] ; s = [den, fernrohr, beobachtet, den, fernrohr] ; s = [den, fernrohr, beobachtet, dem, mann] ; s = [den, fernrohr, beobachtet, dem, fernrohr] ; s = [dem, mann, beobachtet, hans] ; s = [dem, mann, beobachtet, den, mann] ; s = [dem, mann, beobachtet, den, fernrohr] ; s = [dem, mann, beobachtet, dem, mann] ; s = [dem, mann, beobachtet, dem, fernrohr] ; s = [dem, fernrohr, beobachtet, hans] ; s = [dem, fernrohr, beobachtet, den, mann] ; s = [dem, fernrohr, beobachtet, den, fernrohr] ; s = [dem, fernrohr, beobachtet, dem, mann] ; s = [dem, fernrohr, beobachtet, dem, fernrohr].

parsing prolog dcg failure-slice non-termination