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, then 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, then 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 warning⁹ 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 do not, although some do check their arguments to a lesser or greater extent.