Exceptions

In addition to success and failure, a program may give an exception that is explicitly thrown by a call of throw/1, is raised by a built-in, or is caused by the user typing ^C. An exception that is raised by a built-in is a one-argument structure, where the functor shows the type of the exception, and the argument shows the source of the exception.^5.1

The following lists some of the exceptions:

• divide_by_zero(Goal): Goal divides a number by zero.
• file_not_found(Goal): Goal tries to open a file that does not exist.
• illegal_arguments(Goal): Goal has an illegal argument.
• number_expected(Goal): Goal evaluates an invalid expression.
• out_of_range(Goal): Goal tries to access an element of a structure or an array by using an index that is out of range.

The exception that is caused by the typing of ^C is an atom named interrupt.

An exception that is not caught by a program will be handled by the system. The system reports the type and the source of the exception, and aborts execution of the query. For example, for the query a=:=1, the system will report:

   ***  error(type_error(evaluable,a/0),=:=/2)

where evaluable is the type, and =:=/2 is the source.