4.15.1 Overview

Whenever the Prolog system encounters a situation where it cannot continue execution, it throws an exception. For example, if a built-in predicate detects an argument of the wrong type, it throws a type_error exception. The manual page description of each built-in predicate lists the kinds of exceptions that can be thrown by that built-in predicate.

The default effect of throwing an exception is to terminate the current computation and then print an error message. After the error message, you are back at Prolog's top-level. For example, if the goal

     X is a/2

is executed somewhere in a program you get

     ! Type error in argument 2 of (is)/2
     ! expected evaluable, but found a/0
     ! goal:  _255 is a/2
     | ?-

Particular things to notice in this message are:

‘!’

This character indicates that this is an error message rather than a warning¹ or informational message.

‘Type Error’

This is the error class. Exceptions thrown by the Prolog system are called errors. Every error is categorized into one of a small number of classes. The classes are listed in ref-ere-err.

‘goal:’

The goal that caused the exception to be thrown.

Built-in predicates check their arguments, but predicates exported by library modules generally don't, although some do check their arguments to a lesser or greater extent.