Prolog throwing exception when predicate fails

Question

From my understanding, if I have a predicate in disjunctive normal form (like (A && B) || (C && D) --> pred) then I can do the followinging.

pred(parameters) :-
    A, B;
    C, D.

Is this correct? If so, look at the following code.

I have the following three predicates...

maze(X, Y, Maze, Path, Score) :-
  find_eggs_and_pika(X, Y, Maze, Path, Score, 0, 0, 0, Masterball_found),
  write(Score).

find_eggs_and_pika(X, Y, Maze, Path, Score, PrevDirection, Has_egg, Egg_steps, Masterball_found) :-

  is_masterball(X, Y, Maze, Masterball_found);

  ...more predicates

is_masterball(X, Y, Maze, Masterball_found) :-
  nth1(Y, Maze, Row),
  nth1(X, Row, mb),
  Masterball_found is 1,
  false.

When I use trace. I get the following:

{trace}
| ?- maze(1,1,[[ o,  e,  j,  p,  o], [ o,  j,  o,  o,  o], [ o,  j, mt,  j,  o], [ o,  o,  e, o, o],[ p,  o,  j, mb,  o]], Path, 0).
      1    1  Call: maze(1,1,[[o,e,j,p,o],[o,j,o,o,o],[o,j,mt,j,o],[o,o,e,o,...],[p,o,j,...]],_339,0) ?
      2    2  Call: find_eggs_and_pika(1,1,[[o,e,j,p,o],[o,j,o,o,o],[o,j,mt,j,o],[o,o,e,o,...],[p,o,j,...]],_339,0,0,0,0,_373) ?
      3    3  Call: is_masterball(1,1,[[o,e,j,p,o],[o,j,o,o,o],[o,j,mt,j,o],[o,o,e,o,...],[p,o,j,...]],_373) ?
      4    4  Call: nth1(1,[[o,e,j,p,o],[o,j,o,o,o],[o,j,mt,j,o],[o,o,e,o,...],[p,o,j,...]],_470) ?
      4    4  Exit: nth1(1,[[o,e,j,p,o],[o,j,o,o,o],[o,j,mt,j,o],[o,o,e,o,...],[p,o,j,...]],[o,e,j,p,o]) ?
      5    4  Call: nth1(1,[o,e,j,p,o],mb) ?
      5    4  Fail: nth1(1,[o,e,j,p,o],mb) ?
      3    3  Fail: is_masterball(1,1,[[o,e,j,p,o],[o,j,o,o,o],[o,j,mt,j,o],[o,o,e,o,...],[p,o,j,...]],_373) ?
      3    3  Call: '$call'(0,find_eggs_and_pika,9,true) ?
      3    3  Exception: '$call'(0,find_eggs_and_pika,9,true) ?
      2    2  Exception: find_eggs_and_pika(1,1,[[o,e,j,p,o],[o,j,o,o,o],[o,j,mt,j,o],[o,o,e,o,...],[p,o,j,...]],_339,0,0,0,0,_373) ?

is_masterball() fails (as it should in this example) but then it's like Prolog is trying to call find_eggs_and_pika() again? Instead of moving onto the ...more predicates portion.

ayushkum36 ayushkum36 · Accepted Answer · 2016-11-03T20:20:00

In your code you are using nth1(Y, Maze, Row) and nth1(X, Row, mb), I think you need to write "Mb" instead of "mb". You should know that in prolog the words starting with capital letters are treated as Variables.

Prolog throwing exception when predicate fails

2 Answers