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(librep.info)Errors

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Errors
......

   Errors are a type of non-local exit; when a form can not be evaluated
for some reason an error is normally "signalled". If an error-handler
has been installed for that type of error, control is passed to the
handler for that error, and evaluation continues. If there is no
suitable handler, control is passed back to the innermost input loop
and a suitable error message is printed.

 - Function: signal error-symbol data
     Signals that an error has happened. ERROR-SYMBOL is a symbol
     classifying the type of error, it should have a property
     `error-message' (a string) which is the error message to be
     printed.

     DATA is a list of objects which are relevant to the error -- they
     will be made available to any error-handler or printed with the
     error message otherwise.

          (signal 'void-value '(some-symbol))
              error--> Value as variable is void: some-symbol

 - Variable: debug-on-error
     This variable is consulted by the function `signal'. If its value
     is either `t' or a list containing the ERROR-SYMBOL to `signal' as
     one of its elements, the Lisp debugger is entered.  When the
     debugger exits the error is signalled as normal.

 - Variable: backtrace-on-error
     Similar to `debug-on-error', but if an error is matched, the
     current backtrace is printed to the standard error stream, and
     control continues.

   When you expect an error to occur and need to be able to regain
control afterwards the `condition-case' special form may be used.

 - Macro: condition-case symbol body-form error-handlers...
     `condition-case' evaluates the BODY-FORM with the ERROR-HANDLERS
     in place. If an error occurs and one of the handles matches the
     error, then it is evaluated with the value of SYMBOL set to the
     error information.

     Each of the ERROR-HANDLERS is a list whose car is a symbol
     defining the type of error which this handler catches. The cdr of
     the list is a list of forms to be evaluated in a `progn' if the
     handler is invoked.

     While the forms of the error handler are being evaluated the
     variable SYMBOL is bound to the value `(ERROR-SYMBOL .  DATA)'
     (these were the arguments to the `signal' form which caused the
     error). If SYMBOL is the symbol `nil' (or the empty list `()'),
     then the error information is not available to the handler.

     The special value, the symbol `error', in the car of one of the
     ERROR-HANDLERS will catch _all_ types of errors.

          (condition-case data
              (signal 'file-error '("File not found" "/tmp/foo"))
            (file-error
             data)
            (error
             (setq x z)))         ;Default handler
              => (file-error "File not found" "/tmp/foo")