Exceptions are used to escape from method calls under unusual circumstances. For example, a robust numerical application may wish to provide an alternate means of solving a problem under unusual circumstances such as ill conditioning. Exceptions bypass the ordinary way of returning from methods and may be used to skip over multiple callers until a suitable handler is found.
Exceptions may be thought of as implicit alternate return values for all methods. Exceptions can be significantly slower than ordinary routine calls, so they should be avoided except for truly exceptional (unexpected) cases.
Example 10-1. Example:
protect ... when E then ... when $F then ... else ... end |
protect_statement ==>
protect statement_list { when type_specifier then statement_list } [ else statement_list ] end |
Sather uses exceptions to signal and recover from exceptional situations. Exceptions may be explicitly raised by a program (See raise statements) or generated by the system. Each exception is represented by an exception object whose type is used to select a handler from a protect statement. Execution of a protect statement begins with the statement list following the 'protect' keyword. These statements are executed to completion unless an exception is raised which is not caught by some nested protect.
When there is an uncaught exception in a protect statement, the system finds the first type specifier listed in the 'when' lists which is a supertype of the exception object type. The statement list following this specifier is executed and then control passes to the statement following the protect statement. An exception expression (See initial expressions) may be used to access the exception object in these handler statements. If none of the specified types are supertypes, then the statements in an 'else' clause are executed if it is present. If it is not present, the same exception object is raised to the next most recently entered protect statement which is still in progress. It is a fatal error to raise an exception which is not handled by some protect statement. protect statements may only contain iterator calls if they also contain the surrounding loop statement. protect statements without an else clause must have at least one when.