This part of a simple Prolog program I'm working on is supposed to find a list of coordinates that are at most some distance away from a given point. My code works but also produces an error for each element in the given list.
Code:
nearby_places(Places, Position, Signal, NearbyPlaces) :-
findall(
X,
(
Places,
member(X, Places),
is_reachable(Position, X, Signal)
),
NearbyPlaces
).
is_reachable(Position, Target, Signal) :-
manhattan_distance(Position, Target, Distance),
Distance =< Signal * 3.
manhattan_distance(pos(X1, Y1), pos(X2, Y2), Distance) :-
Dx is X1 - X2,
Dy is Y1 - Y2,
ADx is abs(Dx),
ADy is abs(Dy),
Distance is ADx + ADy.
Test run:
?- nearby_places([pos(0,0), pos(1,1), pos(2, 50), pos(4, 9)], pos(0,0), 10, N).
ERROR: Type error: `file_path' expected, found `pos(0,0)'
ERROR: Type error: `file_path' expected, found `pos(1,1)'
ERROR: Type error: `file_path' expected, found `pos(2,50)'
ERROR: Type error: `file_path' expected, found `pos(4,9)'
N = [pos(0, 0), pos(1, 1), pos(4, 9)].
All the tests I've made produce the correct result but also these errors.
I haven't found anything even remotely helpful anywhere.
I am using SWI-Prolog 5.10.4 on Linux.
Thank you for your time!