/usr/lib/swipl/library/exceptions.pl
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 exception(:Type, --Ball) is det
exception(:Type, +Ball) is semidet
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.