I am trying to run this prolog code in DrRacket: http://www.anselm.edu/homepage/mmalita/culpro/graf1.html
#lang datalog
arc(a,b).
arc(b,c).
arc(a,c).
arc(a,d).
arc(b,e).
arc(e,f).
arc(b,f).
arc(f,g).
pathall(X,X,[]).
pathall(X,Y,[X,Z|L]):- arc(X,Z),pathall(Z,Y,L). % error on this line;
pathall(a,g)?
However, it is giving following error:
read: expected a `]' to close `['
I suspect '|' symbol is not being read as head-tail separator of the list. Additionally, [] is also giving error (if subsequent line is removed):
#%app: missing procedure expression;
probably originally (), which is an illegal empty application in: (#%app)
How can these be corrected so that the code works and searches for paths between a and g ?
The Datalog module in DrRacket is not an implementation of Prolog, and the syntax that you have used is not allowed (see the manual for the syntax allowed).
In particular terms cannot be data structures like lists ([]). To run a program like that of above you need a Prolog interpreter with data structures.
What you can do is define for instance a predicate path, like in the example that you have linked:
path(X,Y):- arc(X,Y).
path(X,Y):- arc(X,Z),path(Z,Y).
and, for instance, ask if a path exists or not, as in:
path(a,g)?
or print all the paths to a certain node with
path(X,g)?
etc.