." $Header: /na/franz/doc/RCS/ch10.n,v 1.1 83/01/31 07:08:20 jkf Exp $
.Lc Exception\ Handling 10
.sh 2 Errset\ and\ Error\ Handler\ Functions 10
allows the user to handle in a number of ways the errors
which arise during computation.
One way is through the use of the
If an error occurs during the evaluation of the
the locus of control will return to the errset which will
return nil (except in special cases, such as
The other method of error handling is through an error handler
When an error occurs, the error handler is called and
is given as an argument a description of the
error which just occurred.
The error handler may take one of the following actions:
it could take some drastic action like a
it could, assuming that the error is continuable,
to the function which noticed the error.
The error handler indicates that it wants to return a value from
the error by returning a list whose
is the value it wants to return.
it could decide not to handle the error and return a non-list to
.sh 2 "The Anatomy of an error"
Each error is described by a list of these items:
error type - This is a symbol which indicates the
general classification of the error.
This classification may determine which function handles this
unique id - This is a fixnum unique to this error.
continuable - If this is non-nil then this error is continuable.
There are some who feel that every error should be continuable
and the reason that some (in fact most) errors in
are not continuable is due to the laziness of the programmers.
message string - This is a symbol whose print name is a
message describing the error.
data - There may be from zero to three lisp values which help
describe this particular error.
For example, the unbound variable error contains one datum value,
the symbol whose value is unbound.
The list describing that error might look like:
(ER%misc 0 t |Unbound Variable:| foobar)
.sh 2 "Error handling algorithm"
This is the sequence of operations which is done when an
then this value is the name of an error handler function.
That function is called with a description of the error.
If that function returns (and of course it may choose not to)
and the value is a list and this error is continuable, then
of the list to the function which called the error.
Presumably the function will use this value to retry the operation.
On the other hand, if the error handler returns a non list, then
it has chosen not to handle this error, so we go on to step (2).
Something special happens before we call the
handler which does not happen in any of the
other cases we will describe below.
To help insure that we don't get infinitely recursive
Thus it is the responsibility of the
itself by storing its name in
Next the specific error handler for the type of error
which just occurred is called (if one exists) to see if
it wants to handle the error.
The names of the handlers for the specific types of errors are stored
as the values of the symbols whose names are the types.
For example the handler for miscellaneous errors is stored as the
has a value of nil, then there is no error
handler for this type of error.
Appendix B contains list of all error types.
The process of classifying the errors is not complete and thus most
errors are lumped into the \fBER%misc\fP category.
the error handler function may choose not to handle the error
by returning a non-list, and then we go to step (3).
Next a check is made to see if there is an
If so the second argument to the
If the second argument was not given or is non nil
then the error message associated with this error is printed.
Finally the stack is popped
has a value then it is the
name of an error handler which is called in a manner similar
If it chooses not to handle the error, we go to step (5).
At this point it has been determined that the user doesn't
want to handle this error.
Thus the error message is printed out and
is done to send the flow of control to the top-level.
To summarize the error handling system:
When an error occurs, you have two chances to handle it before
you have one more chance to handle the error before control
Every error handler works in the same way:
It is given a description of the error (as described in the
It may or may not return.
If it returns, then it returns
either a list or a non-list.
If it returns a list and the error is continuable, then
of the list is returned to the function which noticed the error.
Otherwise the error handler has decided not to handle the error
and we go on to something else.
There are two standard error handlers which will probably
handle the needs of most users.
One of these is the lisp coded function
which is the default value of
Thus when all other handlers have ignored an error,
It will print out the error message and
go into a read-eval-print loop.
The other standard error handler is
This handler is designed to be connected to
is useful if your program uses
are told to call an undefined function, an \fBER%undef\fP
The default handler for this error is
This function checks the property list of the undefined function for
If present, the value of that indicator should be the name of the file
which contains the definition of the undefined function.
will load the file and check if it has defined the function which caused
If it has, the error handler will return and the computation will continue
as if the error did not occur.
This provides a way for the user to tell the lisp system about the location
of commonly used functions.
The trace package sets up an autoload property to point to /usr/lib/lisp/trace.
.sh +0 Interrupt\ processing
The UNIX operating system provides one user interrupt character which
defaults to ^C.\*[\(dg\*]
\*[\(dg\*]Actually there are two but the lisp system does not allow you
to catch the QUIT interrupt.
The user may select a lisp function to run when an interrupt occurs.
Since this interrupt could occur at any time, and in particular could
occur at a time when the internal stack pointers were in an inconsistent
state, the processing of the interrupt may be delayed until a safe
When the first ^C is typed, the lisp system sets a flag that an interrupt
This flag is checked at safe places within the interpreter
If the lisp system doesn't respond to the first ^C, another ^C should
This will cause all of the transfer tables to be cleared forcing
all calls from compiled code to go through the
function where the interrupt flag will be checked.
If the lisp system still doesn't respond, a third ^C will cause
This interrupt will not necessarily be in a safe place so the
the lisp system as soon as possible.
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