## 10.4 Exceptions

The module `Exception` provides procedures to construct exceptions and raise them.

Special Exceptions

Any value may be raised as exception, although commonly only records are used. Some of these serve special purposes: error exceptions are records with label `error`. These are raised when a programming error occurs; it is not recommended to catch these. System exceptions are records with label `system`. These are raised when an unforeseeable runtime condition occurs; a file operations library might raise system exceptions when a file cannot be opened. It is recommended to always handle such exceptions. Failure exceptions are records with label `failure`; these are raised when a tell operation fails.

Dispatch Fields

Both error and system exceptions have a dispatch field. This is the subtree at feature `1` of the exception record. This is usually a record further describing the exact condition that occurred.

Debug Information

If an exception is a record and has a feature `debug`, then the implementation may replace the corresponding subtree by implementation-dependent debugging information. This is to be printed out in the case of uncaught exceptions.

All procedures in the Base Environment only ever raise special-purpose exceptions as described above.

`error`

  > `{Exception.error` `X``` `?Y``}`

  returns an error exception record with dispatch field `X`.

`system`

  > `{Exception.system` `X``` `?Y``}`

  returns a system exception record with dispatch field `X`.

`failure`

  > `{Exception.failure` `X``` `?Y``}`

  returns a failure exception. The value `X` may give a hint on why failure occurred; implementations may store this inside the constructed exception's `debug` field.

`Raise`

  > `{Exception.'raise'` `X``}`

  raises `X` as an exception.

`raiseError`

  > `{Exception.raiseError` `X``}`

  wraps `X` into an error exception and raises this. This procedure can be defined as follows:

  > `proc {Exception.raiseError X}
  > {Exception.'raise' {Exception.error X}}
  > end`