CLHS: Issue UNDEFINED-VARIABLES-AND-FUNCTIONS Writeup

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## Issue UNDEFINED-VARIABLES-AND-FUNCTIONS Writeup

    Status: Passed, as amended, Jun89 X3J13
    Issue:        UNDEFINED-VARIABLES-AND-FUNCTIONS
    References:   5.1.2 Variables (CLtL pp55-56),
    	      Slots (88-002R, p1-10)
    Category:     CHANGE
    Edit history: 29-Nov-88, Version 1 by Pitman
    		3-Jul-89, Version 2 by Masinter (as per Jun89X3J13)
    Problem Description:

      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.

    Proposal (UNDEFINED-VARIABLES-AND-FUNCTIONS:COMPROMISE):

      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
      offending function.

      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
      offending variable.

      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.

    Test Case:

      (PROCLAIM '(OPTIMIZE (SAFETY 3) (SPEED 0)))
      (DEFUN FOO () X)
      (FOO)
      >>Error: The variable X is not bound.
      ...

    Rationale:

      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
      functions.

    Current Practice:

      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.

    Benefits:

      The costs of non-adoption would be avoided.

    Aesthetics:

      This would regularize things a little.

    Discussion:

      Pitman thinks this would be a good idea.

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