The Error Listener |

The Error Listener receives notification of all warnings and errors (whether recoverable or not) generated when compiling the stylesheet, or produced by the `XM_XSLT_TRANSFORMER`.

XSLT defines some errors as recoverable (and specifies optional recovery options), and others as non-recoverable. Conditions not defined by XSLT are raised by the library as warnings.

## Features

The following features may be of interest to the Eiffel programmer:

`recovery_policy: INTEGER / set_recovery_policy (a_recovery_policy: like recovery_policy)`
  The recovery policy is an instruction to the `error` routine as to how to handle recoverable errors. If it evaluates to `Recover_silently`, then `error` is supposed to set `recovered` to `True`, and not issue any message (although it may choose to log the error).

  If it evaluates to `Recover_with_warnings`, then it should additionally inform the user of the recovered condition, provided `recoverable_error_threshold` has not been exceeded (although it is not obliged to honour this threshold).

  If, however, it evaluates to `Do_not_recover`, then it should inform the user of the error condition, and set `recovered` to `False`.

`warning_threshold: INTEGER / set_warning_threshold (a_warning_threshold: like warning_threshold)`
  This threshold is available to request the `warning` routine not to issue an informatory message once this number of warning messages have been issued.
`recoverable_error_threshold: INTEGER / set_recoverable_error_threshold (a_recoverable_error_threshold: like recoverable_error_threshold)`
  This threshold is available to request the `error` routine not to issue an informatory message once this number of recoverable errors have been seen.
`warning (a_message: STRING; a_locator: XM_XPATH_LOCATOR)`
  This routine is called by the library to warn of problematic conditions which are not defined as errors by XSLT. It should inform the user (in some unspecified way) about the condition, provided that `warning_threshold` has not been exceeded (but there is no obligation to maintain a count of warning messages, so an error listener could ignore this threshold).
`error (a_error: XM_XPATH_ERROR_VALUE)`
  This routine is called by the library to inform the user (in some unspecified way) about a recoverable error, provided that `recoverable_error_threshold` has not been exceeded (but there is no obligation to maintain a count of error messages, so an error listener could ignore this threshold) and perhaps recover from the error (both actions depend upon the current setting of `recovery_policy`).

  This routine must set `recovered` to `True` if it recovers from an error, so that the transformer can take the defined recovery action.

  If the error listener exposes the error code (and its namespace uri) in any way, then it must honour the `error_change_stack`. See `XM_XSLT_DEFAULT_ERROR_LISTENER` for how to do this. It may be necessary to copy the error location information.

`fatal_error (a_error: XM_XPATH_ERROR_VALUE)`
  This routine is called by the library to inform the user (in some unspecified way) about a non-recoverable error.

  If the error listener exposes the error code (and its namespace uri) in any way, then it must honour the `error_change_stack`. See `XM_XSLT_DEFAULT_ERROR_LISTENER` for how to do this. It may be necessary to copy the error location information.

`treat_warnings_as_recoverable_errors`
  This routine may be called by the programmer on an error listener, to request the `warning` routine to behave identically to the `error` routine.

## The default error listener: `XM_XSLT_DEFAULT_ERROR_LISTENER`

This is the chief concrete error listener provided by the library. It maintains error counts. The recovery policy is passed as an argument to the creation routine, as also is a `UT_ERROR_HANDLER` to receive the warning and error messages.

## A testing error listener: `XM_XSLT_TESTING_ERROR_LISTENER`

This error listener supresses all output, but records which error message codes it has seen. It was written for use in the XSLT test suite, so as to avoid confusing the user by displaying error messages which are intentionally being produced.