4.15.4.1 Instantiation Errors

An instantiation error occurs when a predicate or command is called with one of its input arguments insufficiently instantiated.

The SICStus_Error term associated with an instantiation error is

instantiation_error(Goal, ArgNo)

where ArgNo is a non-negative integer indicating which argument caused the problem. ArgNo=0 means that the problem could not be localized to a single argument.

Note that the ArgNoth argument of Goal might well be a non-variable: the error is in that argument. For example, the goal

X is Y+1

where Y is uninstantiated throws the exception

error(instantiation_error,
      instantiation_error(_A is _B+1,2))

because the second argument to is/2 contains a variable.