4.15.4 Error Classes

Exceptions raised by the Prolog system are called errors. The set of exception classes used by the system has been kept small. Here is a complete list:

Instantiation Error
An input argument is insufficiently instantiated.
Type Error
An input argument is of the wrong type.
Domain Error
An input argument is illegal but of the right type.
Evaluation Error
An incorrect arithmetic expression was evaluated.
Representation Error
A computed value cannot be represented.
Existence Error
Something does not exist.
Permission Error
Specified operation is not permitted.
Context Error
Specified operation is not permitted in this context.
Consistency Error
Two otherwise correct values are inconsistent with each other.
Syntax Error
Error in reading a term.
Resource Error
Some resource limit has been exceeded.
System Error
An error detected by the operating system.

The format of the exception raised by the built-in predicates is:

     error(ISO_Error, SICStus_Error)

where ISO_Error is the error term prescribed by the ISO Prolog standard, while SICStus_Error is the part defined by the standard to be implementation dependent. In the case of SICStus Prolog, this is the SICStus error term, which normally contains additional information, such as the goal and the argument number causing the error. Arguments are numbered from 1 upwards.

The list below itemizes the error terms, showing the ISO_Error and SICStus_Error form of each one, in that order. Note that the SICStus and ISO error terms do not always belong to the same error class, and that the context and consistency error classes are extensions to the ISO Prolog standard.

The goal part of the error term may optionally have the form $@(Callable,PC) where PC is an internal encoding of the line of code containing the culprit goal or one of its ancestors. To decompose an annotated goal AGoal into a Goal proper and a SourceInfo descriptor term, indicating the source position of the goal, use:

     ?- goal_source_info(AGoal, Goal, SourceInfo).

The reference page gives details about the SourceInfo format.

Goal was called with insufficiently instantiated variables.
Goal was called with the wrong type of argument(s). TypeName is the expected type and Culprit what was actually found.
Goal was called with argument(s) of the right type but with illegal value(s). Domain is the expected domain and Culprit what was actually found.
Something does not exist as indicated by the arguments. If the unknown Prolog flag is set to error, this error is raised with ArgNo set to 0 when an undefined predicate is called.
The Operation is not permitted on Culprit of the ObjectType.
The CommandType is not permitted in ContextType.
A syntax error was found when reading a term with read/[1,2] or assembling a number from its characters with number_chars/2 or number_codes/2. In the former case this error is raised only if the syntax_errors flag is set to error.
An incorrect arithmetic expression was evaluated.
A representation error occurs when the program tries to compute some well-defined value that cannot be represented, such as a compound term with arity > 255.
A consistency error occurs when two otherwise valid values or operations have been specified that are inconsistent with each other.
A resource error occurs when SICStus Prolog has insufficient resources to complete execution. The only value for ResourceType that is currently in use is memory.
An error occurred while dealing with the operating system.

Most exception terms include a copy of the Goal that raised the exception.

In general, built-in predicates that cause side-effects, such as the opening of a stream or asserting a clause into the Prolog database, attempt to do all error checking before the side-effect is performed. Unless otherwise indicated in the documentation for a particular predicate or error class, it should be assumed that goals that raise exceptions have not performed any side-effect.

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