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Source for wiki ExceptionHandlingCowan version 3











= Design =

When a problem situation is detected either by the implementation or by a
user program, a representation of that situation called a ''condition''
(which can be any Scheme object) is constructed, and the situation
represented by the condition is announced by an action called ''raising
the condition''.  This action allows a dynamically established ''handler''
an opportunity to resolve the problem; the current handler is the value of
the parameter `condition-handler`.  At any given time, only one handler
is active.  The current handler is stored in a SRFI 39 parameter.

When a condition is raised, the active handler is called with one
argument, a condition which represents the situation.  The
handler function will execute in the dynamic environment of the call to
`raise`, except that the value of ''condition-handler'' is re­bound
to match the dynamic handler state that was current at the point that the
handler function was established as the active handler.  This means that
handlers are not expected to handle errors in themselves. In order to have
a handler handle its own errors, the handler may re-­establish itself as
a handler within its own body by rebinding the current handler to itself.

If a handler returns normally, the dynamically enclosing handler is
invoked in its place, and so on until the root handler is executed.
Exiting to non-handler control is performed by calling a captured
continuation.  For example, a handler may return to the point of raise
by invoking a continuation stored inside the condition.  Alternatively,
it may return to the point where it was made active by invoking a
continuation bound to a variable lexically visible to the handler.
In any case, the appropriate current handler will be restored as part of the
dynamic environment.

There is a system-specific procedure known as the ''root condition
handler'' that is the initial condition handler.  It generally invokes
a top-level exit continuation for the whole program, or the current
thread (if some concept of threading exists), possibly displaying
useful debugging information to some interested parties in some
implementation-specific way.  Implementations are permitted to provide
an interactive debugger that lets the programmer perform actions other
than invoking the top-level exit continuation, perhaps invoking retries
or other arbitrary continuations.

= Procedures =

  * `(raise `''object''`)`

This raises the condition ''object''.  It never returns normally.

  * `(error `''string''` . ` ''objects''`)`

This constructs a condition using ''string'' and the optional ''objects''
and raises it.  It is compatible with SRFI 23.

  * `(root-condition-handler)`

This returns the root condition handler.

  * `(condition-handler `[''new-handler'']`)`

This is a parameter representing the current condition handler.
Its initial value is the root condition handler.  Invoking it with no
arguments returns the handler; invoking it with one argument sets the
handler.  Passing it to the `parameterize` syntax form causes it to be
dynamically rebound to a new handler.

= Thanks =

Thanks to Taylor Campbell, Alaric Snell-Pym as the author of
ErrorsSnellPym, and Kent Pitman as the author of the ISLisp specification,
from all of whom I have derived inspiration.  They bear no responsibility
for infelicities in this proposal.


2010-07-04 19:49:15