4.10.3.2 Throwing exceptions from applications and libraries

User predicates are free to choose the structure of their exception terms (i.e., they can define their own conventions) but should adhere to the ISO standard if possible, in particular for libraries.

Notably, exceptions of the shape error(Formal,Context) are recognised by the development tools and therefore expressing unexpected situations using these exceptions improves the debugging experience.

In SWI-Prolog, the second argument of the exception term, i.e., the Context argument, is generally of the form context(Location, Message), where:

• Location
  describes the execution context in which the exception occurred. While the Location argument may be specified as a predicate indicator (Name/Arity), it is typically filled by the library(prolog_stack) library. This library recognises uncaught errors or errors caught by catch_with_backtrace/3 and fills the Location argument with a backtrace.

• Message
  provides an additional description of the error or can be left as a fresh variable if there is nothing appropriate to fill in.

ISO standard exceptions can be thrown via the predicates exported from library(error). Termwise, these predicates look exactly like the Formal of the ISO standard error term they throw:

• instantiation_error/1 (the argument is not used: ISO specifies no argument)
• uninstantiation_error/1
• type_error/2
• domain_error/2
• existence_error/2
• existence_error/3 (a SWI-Prolog extension that is not ISO)
• permission_error/3
• representation_error/1
• resource_error/1
• syntax_error/1