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Here is a proposal for the core of an exception system, derived
from the Common Lisp condition system and from Kent Pitman's
Not-Quite-Proposal from last September. To be complete it
needs a specification of standard condition types and restart
tags and which Scheme procedures raise which conditions, as
well as procedures and macros encapsulating common idioms.
The core has three parts: conditions, handlers, and restarts.
A condition contains information about an exceptional situation.
A handler is a procedure that is invoked when a condition is
raised. A restart is a recovery procedure that can be invoked
by handlers, or anyone else for that matter.
This proposal addresses Kent Pitman's idea that the purpose of
an exception system is to mediate between separate programs
(which isn't suprising; he wrote the Common Lisp proposal on
which this is based). The specification of a procedure can
include the types of conditions it may raise and the restarts
that it may make available. Users of the procedure can then
make use of the both procedure's normal and exceptional
Conditions are records with multiple inheritance. Multiple
inheritance is necessary because recovery strategies for several
types of conditions might be appropriate for a particular exceptional
situation. Multiple inheritance is likely to make field access
slower than for simple records, but conditions are only used in
Implementation of all this is simple, especially if the condition
procedures do not need to be super fast. A condition could contain
an a-list of (<type> . #(slot1 slot2 ...)) for example.
Conditions are records with multiple inheritance.
(make-condition-type supertypes field-names)
<Supertypes> is a list of condition types, <field-names> is a list
of symbols. The supertypes of the returned condition type are the
<supertypes> along with the union of thier supertypes. An instance
of a condition type has values for the condition-type's fields and
for the fields of its supertypes. The <field-names> must be distinct
from those of the supertypes.
(condition-maker condition-type field-names)
Returns a procedure that constructs conditions of the given type.
<Field-names> must include all of the fields of <condition-type>
and its supertypes. The arguments to the resulting constructor
are the values for the condition's fields, in the order listed
Returns a predicate that returns #T when passed a condition of
type <condition-type> or one whose type is a subtypes of
(condition-accessor condition-type field-name)
Returns a function that, given a condition whose type is <condition-
type> or a subtype of <condition-type>, will return the value of the
specified field for that condition.
Signalling and handlers ----------------
Invoke the current handler on <condition>.
(with-handler handler thunk)
Calls <thunk> with no arguments, and, if no condition is signalled,
returns the values that <thunk> returns.
If a condition is signalled during the execution of <thunk> it is
passed to <handler>, which should be a procedure of one argument.
If <handler> declines to handle the condition it should return a
condition which will then be passed to the handler in place at the
time of the call to WITH-HANDLER. Signalling a condition does not
change the currently installed handler.
(make-restart-tag id) -> restart-tag
<Id> is used for printing the tag.
(call-with-restart restart-tag predicate restart-procedure
Invokes <thunk> in such a way that if a condition accepted by
<predicate> is signalled, a restart containing <restart-procedure>
will be visible. <Discloser> is passed an output port and explains
the restart in natural language. <Interactive-procedure> is passed
an output port and an input port, prompts the user, and procedes
using the result supplied. The <restart-procedure> and <interactive-
procedure> are invoked with the continuation of the call to
A suitable macro could be used to make calls to CALL-WITH-RESTART
(restart? restart) tests for a restart
(interactive-restart? restart) tests for an interactive restart
(find-restarts restart-tag condition) -> list of restarts
Returns any currently available restarts for <restart-tag> whose
predicate accepts <condition>.
(invoke-restart restart . arguments)
Calls <restart>'s restart procedure on <arguments>.
(invoke-restart-interactively restart input-port output-port)
Calls <restart>'s interactive procedure.
(disclose-restart restart port)
Calls <restart>'s discloser on <port>.
As an example of how restarts can be used, here is a definition
of function that adds one to integers. It assumes that there is
an `interactive' restart tag for restarts that require user
intervention and a condition type for wrong-type-arguments.
(define (add-one x)
(if (integer? x)
(+ x 1)
(call-with-restart interactive ; tag
wrong-type-argument? ; predicate
add-one ; restart procedure
(lambda (out) ; discloser
(display "Supply a new argument ADD-ONE and proceed"
(lambda (in out) ; interactive procedure
(display "New argument to ADD-ONE: " out)
(add-one (read in)))