I am trying to write a grammar for the English language in Prolog, with some basic rules, such as:
s --> np, vp.
np --> pn.
np --> det, noun.
pn --> [vincent].
pn --> [mia].
det --> [a].
I do understand how these work and how to query them using the phrase statement, but when it comes to rules like:
noun(X, woman(X)) --> [woman].
iv(Y, snort(Y)) --> [snorts].
I'm lost. What do these mean? Why is variable X repeated?
noun(X, woman(X)) --> [woman].
is the same as
noun(X, woman(X), A, B) :- A = [woman | B].
This of course leaves the X variable uninstantiated. Perhaps you intended
noun(X, woman(X)) --> [woman, X].
which is equivalent to
noun(X, woman(X), A, B) :- A = [woman, X | B].
The repeated logical variables, as usual, indicate the sameness in unification:
4 ?- noun(X,Y,[woman,1],Z).
X = 1,
Y = woman(1),
Z = [].
5 ?- phrase(noun(X,Y),[woman,1]).
X = 1,
Y = woman(1).