Status: Passed, as amended, Jun89 X3J13
References: 5.1.2 Variables (CLtL pp55-56),
Slots (88-002R, p1-10)
Edit history: 29-Nov-88, Version 1 by Pitman
3-Jul-89, Version 2 by Masinter (as per Jun89X3J13)
CLtL does not specify what happens if you attempt to call a named function
which is in fact undefined. In most implementations, it would be devastating to
actually jump to code which you had not verified to be a function, so this error
should be easily caught -- yet, CLtL does not guarantee that an error will be
signalled even in the safest, least fast OPTIMIZE settings.
CLtL (p56) specifies that "it is an error to refer to a variable that is unbound."
CLOS (p1-10) specifies that "when an unbound slot is read, the generic function
SLOT-UNBOUND is invoked. The system-supplied primary method for SLOT-UNBOUND
signals an error."
CLOS and CLtL are not in agreement on their treatment of unbound variables.
CLtL is very weak in that it guarantees no support for reliably detecting
and signalling an error when the error situation occurs, even in the safest,
least fast OPTIMIZE setting.
CLOS is very strong in that it forces detection of the error in all
situations -- even in the fastest, least safe OPTIMIZE setting.
The disparate positions for treatment of variables and slots should be
reconciled, either by finding a compromise or by justifying the apparent
inconsistency. The final story should explain function references as well.
Define that reading an undefined function or an unbound variable
must be detected in the highest safety setting,
but the effect is undefined in any other safety setting. That is,
- Reading an undefined function should signal an error.
- Reading an an unbound variable should signal an error.
By ``reading an undefined function'' in the above, we mean to
include both references to the function using the FUNCTION
special form, such as F in (FUNCTION F) and references to the
function in a call, such as F in (F X).
For the case of INLINE functions (in implementations where they are
supported), it is permissible to consider that performing the inlining
constitutes the read, so that an FBOUNDP check need not be done at
execution time. Put another way, the effect of FMAKUNBOUND of a function
on potentially inlined references to that function is undefined.
Specify that the type of error signalled when an undefined function
is detected is UNDEFINED-FUNCTION, and that the NAME slot of the
UNDEFINED-FUNCTION condition is initialized to the name of the
Specify that the type of error signalled when a unbound variable
is detected is UNBOUND-VARIABLE, and that the NAME slot of the
UNBOUND-VARIABLE condition is initialized to the name of the
Introduce a new condition type, UNBOUND-SLOT, which inherits from
CELL-ERROR. This new type has an additional slot, INSTANCE, which
can be initialized using the :INSTANCE keyword to MAKE-CONDITION.
Introdue a new function UNBOUND-SLOT-INSTANCE to access INSTANCE slot.
Specify that the type of error signalled by the default primary
method for the SLOT-UNBOUND generic function is UNBOUND-SLOT,
and that the NAME slot of the UNBOUND-SLOT condition is initialized
to the name of the offending variable, and that the INSTANCE slot
of the UNBOUND-SLOT condition is initialized to the offending instance.
(PROCLAIM '(OPTIMIZE (SAFETY 3) (SPEED 0)))
(DEFUN FOO () X)
>>Error: The variable X is not bound.
This makes it easier to treat slots like variables.
This makes it possible to better rely on an unbound variable error being
signalled when one has occurred.
This makes it possible to compile out useless error checking in CLOS
code where the code is debugged and the checking is redundant.
For the case of undefined functions, blindly jumping to an undefined
function is an incredibly dangerous thing to do. Every implementation
should guarantee at least some way to get error checking of undefined
Until recently, Symbolics Cloe did not ever signal an error on unbound
variable, even in the safest case. The excuse was that this was a CLtL
didn't require it, but it was sometimes an impediment to debugging.
Some benchmarks for Symbolics Cloe (which currently only claims to
implement New Flavors, not CLOS) could be faster if checking for unbound
instance variables could be optimized away.
Symbolics Genera doesn't care about safety issues in variable access
because the check can be done by microcode.
Cost to Implementors:
This change does not force a change to any current implementation, except
those which do not yet signal unbound variable or undefined function errors
even in the safest setting.
Cost to Users:
This checking might slow down some code which is written for the safest
setting yet does not need this check.
Any implementation-specific code which depended on these references not
signalling would be broken. Such code was not portable, of course.
Any CLOS code which depends on SLOT-UNBOUND being called even in low safety
settings would be broken. The amount of such code at this point is likely
to be little or none. If such cases did exist, local or global changes to
safety settings would correct the problem (at some cost in speed).
Cost of Non-Adoption:
Some important error checking would not occur.
Some important optimizations could not be done.
The language would seem internally less consistent.
The costs of non-adoption would be avoided.
This would regularize things a little.
Pitman thinks this would be a good idea.