pli is demo Prolog intepreter written in Haskell.

• Both facts fact(a). and rules rule(X) :- subgoal1(X), subgoal2(X). are supported.
• All predicates must be written in form functor(...) - even the unification: =(X, p(a)).
• Cuts ! are supported.
• Lists are written as .(a, .(b, .(c, []))) (element a, b and c).
• Natural numbers are written as s(s(0)) (this is 2).
• Unknown predicates are treated as false (beware of typos!).

To run the program (cabal must be installed):

cabal run pli -- INPUT_FILE

where INPUT_FILE is the path to the prolog program file.

Examples to try:

• examples/lib.pl (contains few essential predicates)
  • =(Xs, .(a,.(b,.(c,.(d,[])))) ), length(Xs, Length). (get length of the list [a,b,c,d])
  • =(Xs, .(a,.(b,.(c,.(d,[])))) ), reverse(Xs, Rev). (reverse the list [a,b,c,d])
  • length(Xs, s(s(0))). (generate a list of length 2)
  • length(Xs, Len). ([Enter] or ;+[Enter] to generate next result; .+[Enter] to stop)
  • length(Xs, Len), =(Len, s(s(s(_))) ), =(Xs, .(first, _) ). (list of at least 3 elements, the first of which is first)
  • reverse(Xs, Rev), =(Xs, .(first,_) ), =(Rev, .(last,_) ). (reverse of a list that starts with first and ends with last)
• examples/rle.pl (Run Length Encoding)
  • rle( .(a,.(a,.(a,.(b,.(b,[]))))) , RLE). (encode [a,a,a,b,b])