ERROR: Type error: `file_path' expected <p>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.</p> <p><strong>Code:</strong> </p> <pre><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 =&lt; 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. </code></pre> <p><strong>Test run</strong>:</p> <pre><code>?- 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)]. </code></pre> <p>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.</p> <p>Thank you for your time!</p>