3.6 Undefined Predicates

There is a difference between predicates that have no definition and predicates that have no clauses. The latter case is meaningful e.g. for dynamic predicates (see ref-lod-dcl) that clauses are being added to or removed from. There are good reasons for treating calls to undefined predicates as errors, as such calls easily arise from typing errors.

The system can optionally catch calls to predicates that have no definition. First, the user defined predicate user:unknown_predicate_handler/3 (see ref-ere) is called. If undefined or if the call fails, then the action is governed by the state of the unknown Prolog flag; see ref-lps-flg. Calls to predicates that have no clauses are not caught. See mpg-ref-unknown_predicate_handler. Two development system predicates are handy in this context:

| ?- unknown(X,error).
% Undefined predicates will raise an exception (error)
X = error

This sets the flag and prints a message about the new value.

| ?- debugging.
The debugger is switched off
Using leashing stopping at [call,exit,redo,fail,exception] ports
Undefined predicates will raise an exception (error)
There are no breakpoints

This prints a message about the current value, as well as information about the state of the debugger.