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Prolog Debugging Hints

Instantiation errors

ERROR: Arguments are not sufficiently instantiated

Instantiation errors are very common and have many causes.

  • Ignoring singleton variable warnings
  • Using predicates that expect ground parameters before the variables are ground. For example: Num1 is Num + 1 requires Num to be ground before it can calculate Num1.
  • Queries do not belong in your .pl file! A query such as thing(X,Y). will unify at the command line. But if you add it to your .pl file you will get an instantiation error.

No permission to modify or cannot redefine errors

ERROR: No permission to modify static procedure '(,)/2'

or

ERROR: Full stop in clause-body?  Cannot redefine ,/2

This is typically the result of accidentally adding a . in the middle of a rule or failing to terminate a rule with ..

Undefined procedure

'<meta-call>'/1: Undefined procedure: myrule/5

You don’t have a procedure called myrule with 5 parameters. If myrule exists, then you likely called it with the wrong number of parameters.

Operator priority clash

Syntax error: Operator priority clash

One cause of this error is a missing body of a rule, for example, in the following code note how there is no body in the first rule (the base case):

do_something([],[]) :-
do_something([H|T],[T1]) :- H = 2, !, do_something(T, T1).
do_something([H|T],[T1]) :- do_something(T, T1).

Strange string output

X = [72, 101, 108, 108, 111, 44, 32, 119, 111|...].

SWI-Prolog treats double quoted strings as a list of character codes. (This behavior is configurable. The solution is to use single quoted strings which SWI-Prolog treats as an atom.

?- same("Hello, world!",X).
X = [72, 101, 108, 108, 111, 44, 32, 119, 111|...].
?- same('Hello, world!',X).
X = 'Hello, world!'.