exception/2

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(exceptions): Exception classification
- Packages

Availability::- use_module(library(exceptions)).(can be autoloaded)

[det]exception(:Type, --Ball)

[semidet]exception(:Type, +Ball)

If Ball is unbound, adds a delayed goal that tests the error belongs to Type when Ball is instantiated (by catch/3). Else succeed is error is of the specified Type.

Note that the delayed goal is added using freeze/2 and therefore the stepwise instantiation of Ball does not work, e.g. exception(file_error, error(Formal,_)) immediately fails.

Error types may be defined or extended (e.g., by libraries) by adding clauses to the multifile predicates error_term/2 and exception_term/2. Modules may (re-)define local error types using the exception_type/2 directive.