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Source for wiki ExceptionHandlingCowan version 8

author

cowan

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198.185.18.207

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ExceptionHandlingCowan

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0

text

= Design =

When a problem situation, or ''exception'', 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 exception is
announced by an action called ''signaling
the condition''.  This action allows a dynamically established ''condition handler''
an opportunity to resolve the problem.  At any given time, only one handler
is active.

When a condition is signaled, 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
`signal`, except that the current condition handler
becomes the ''enclosing handler'', which is the handler that was current at the point that the
current handler function was established as the active handler.  This means that
a handler is not expected to handle conditions signaled within its dynamic extent.

If a handler returns normally, its values are discarded, and the condition is passed
to its enclosing handler until there are no more handlers.
Escaping from handler control is performed by invoking a captured
continuation.  For example, a handler may return to the point where it was
signaled by invoking a continuation stored inside the condition.  Alternatively,
it may return to the point where it was bound by invoking a
continuation in a variable that is lexically visible to the handler.
In any case, the appropriate current handler will be restored as part of the
dynamic environment.

The outermost condition handler is implementation-defined.  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 may 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.

See StandardConditionPredicatesCowan for standard predicates and accessors useful
with implementation-defined conditions.

= Procedures =

 * `(current-condition-handler)`

Returns the current condition handler.

 * `(with-condition-handler `''handler''` `''thunk''`)`

Applies ''thunk'' within a dynamic environment in
which the current condition handler is ``handler.``  The supplied handler is executed
in the dynamic environment of the call to `signal` that signals the condition,
except that the condition handler which was in effect when `with-condition-handler` was called is
reinstated for the dynamic extent of the handler, so if it signals any conditions without
explicitly changing to a different dynamic environment through applying continuations,
they will be passed to the previous handler, not the same one.

  * `(signal `''object''`)`

Signals the condition ''object''.  It never returns normally.

  * `(error-in `''who''` `''string''`. ` ''irritants''`)`

Constructs a condition and signals it.
The condition specifies a problem with an object identified by the string or symbol ''who''
that is described by ''string''.
The optional ''irritants'' are other objects relevant to the exception.
The predicates `who-condition`, `message-condition`
and `irritants-condition` return `#t` on this condition.
This procedure is compatible with R6RS `error`.

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

Constructs a condition and signals it.
The condition specifies a problem that is described by ''string''.
The optional ''irritants'' are other objects relevant to the exception.
The predicates `message-condition`
and `irritants-condition` return `#t` on this condition.
This procedure is compatible with R6RS `error`.

= Thanks =

Thanks to Taylor Campbell, Alaric Snell-Pym as the author of
ErrorsSnellPym, Kent Pitman as the author of the ISLisp specification,
and the editors of R6RS,
from all of whom I have derived inspiration and stolen descriptions.  They bear no responsibility
for infelicities in this proposal.

time

2010-07-09 20:12:24

version

8