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## Issue ARGUMENT-MISMATCH-ERROR Writeup

    Issue:         ARGUMENT-MISMATCH-ERROR
    Forum:         Cleanup
    References:    ANSI CL spec (Aug 29, 1989 draft) pp.4-11,4-12,4-13,4-15,1-10
                   88-002R p.1-26 (appears also on p.4-20 of ANSI CL draft)
    Category:      CHANGE
    Edit history:  1-Feb-90, Version 1 by Moon
                   9-May-90, Version 2 by Moon
                    (address another inconsistency, found by Kim Barrett)
                   30-Oct-90, Version 3 by Pitman
                    (first attempt to formalize X3J13 amendments to v2)
                   01-Nov-90, Version 4 by Pitman (comments by Moon, Barrett)
    	       01-Nov-90, Version 5 by Pitman (more comments by Moon, Barrett)
                   24-Feb-91, Version 6 by Loosemore (propose more amendments)
                   14-Mar-91, Version 7 by Loosemore (change item 11)
    Status:        v2+amendments (reflected in v5) accepted by X3J13, June 1990

    Problem description:

      The draft ANSI Common Lisp specification is inconsistent about what kind
      of error handling occurs when a function is called with arguments that
      do not match its definition.

       1. For too few arguments, p.4-11 says "there must be at least n passed
       arguments," which probably (page 1-10) means "the consequences are
       undefined" if there are too few arguments.

       2. For too many arguments, p.4-12 says "an error of type error is
       signalled."

       3. For unrecognized keyword arguments, p.4-13 says "the consequences
       are undefined."  On the other hand, p.4-15 says "and error would be
       signalled."

       4. For keyword argument names that are not symbols, p.4-13 says "the
       consequences are undefined."

       5. For unrecognized keyword arguments supplied to generic functions,
       p.4-20 and 88-002R p.1-26 say "an error is signalled."

      This is Symbolics issue #5.

    Proposal (ARGUMENT-MISMATCH-ERROR:MUST-SIGNAL-WHEN-SAFE-OR-SYSTEM):

      Define that an error must be signalled in cases 1, 2, 3, and 5 of the
      argument mismatch situations in the problem description if the caller,
      the callee, and the point of functional evaluation all appear in a
      context where a `safe' optimization setting is in effect (i.e.,
      SAFETY=3).  In all other scenarios for these situations, the
      consequences are undefined.

      Case 4 is treated the same, except that when :allow-other-keys t or
      &allow-other-keys is involved, error detection is optional (i.e., the
      consequences are undefined if the keyword argument names are not symbols).

      If any of the caller, the callee, or the point of functional evaluation
      was not user code, and was instead supplied by the implementation as a
      pre-defined definition, or as automatically generated code (e.g., as
      in method combination), then it must be treated as safe unless some
      user code involved in the scenario is not safe.

      Clarify that a reference to the symbolic name of the function or to the
      contents of the symbol-function of a symbol does not count as a functional
      evaluation.  For the purposes of this definition, functional evaluation
      occurs either explicitly due to a use of the FUNCTION special form, or
      implicitly due to the use of a function name in the car of a normal
      functional form.

      The naive model which is intended is that the user can rely on error
      checking in these situations if he has taken all reasonable steps to
      ensure that the situations will be safe.

      The exact time that the error will be signalled is implementation-dependent,
      but will always be prior to the execution of the body of the function being
      called.

    Clarifications:

      The error might be signalled at compile time or at run time.  If the
      error is signalled at run time, it might be signalled by either the
      caller or the callee.

      The reason that this terminology is used, and not the normal, "should
      signal" terminology is because system code may be involved, and the
      user may not know in general whether system code was compiled `safe'
      or `unsafe'.  An implication of this definition is that all code
      compiled by the system will behave as if compiled safe unless some
      user code involved in the scenario is not. So, for example, if a user
      calls MAPCAR from safe code and passes a function which was compiled
      safe, the system is required to ensure that MAPCAR will make a safe
      call as well.

    Proposal (ARGUMENT-MISMATCH-ERROR:MORE-CLARIFICATIONS):

      Amend proposal MUST-SIGNAL-WHEN-SAFE-OR-SYSTEM as follows:

      (1) Clarify that if the callee is a generic function, the generic function
          definition (if it was defined explicitly), the method definitions for
          all applicable methods, and the method-combination definition must all
          be safe for the callee to be considered safe.

      (2) Clarify that for a form (COERCE <lambda-expression> 'FUNCTION),
          the value of the OPTIMIZE SAFETY quality in the global environment
          at the time the COERCE is executed applies to the resulting function.

      (3) Clarify that for a form (ENCLOSE <lambda-expression> <environment>),
          the value of the OPTIMIZE SAFETY quality in the <environment>
          argument applies to the resulting function.

      (4) Clarify that for GENERIC-FUNCTION, GENERIC-FLET, and GENERIC-LABELS
          forms, the value of the OPTIMIZE SAFETY quality in the environment
          in which the form appears applies to the resulting generic functions
          and method definitions.

      (5) Clarify that for a form (ENSURE-GENERIC-FUNCTION . arguments), the
          value of the OPTIMIZE SAFETY quality in the environment passed as
          the :environment keyword applies to the resulting generic function.

      (6) Clarify that for a form (COMPILE <name> <lambda-expression>), the
          value of the OPTIMIZE SAFETY quality in the global environment at
          the time the COMPILE is executed applies to the resulting
          compiled function.

      (7) Clarify that for a form (COMPILE <name>), if the original definition
          of the function was at a `safe' optimization setting, then the
          resulting compiled function must also be `safe'.

          Rationale: although many implementations do not save the information
          about the global environment for interpreted functions, this is
          consistent with the idea that safety is a lexical property that is
          captured at the time code promotion occurs.  Implementations that do
          not save the necessary information must treat interpreted functions
          as being always "safe".

      (8) Clarify that "automatically generated code" does not apply to the
          expansions of macros defined in the standard, which inherit safety
          from the environment in which the macro call appears.

          Rationale:  this term isn't very well defined.

      (9) Replace the words "supplied by the implementation as a pre-defined
          definition" with "supplied by the implementation and part of the
          Common Lisp standard".

          Rationale:  the original wording prohibits implementations from
          providing *any* functions that do no argument checking, even
          internal functions that users might accidentally get their hands on
          as well as officially supported extensions.  There's no good
          reason to prohibit this, since presumably users have to ask for
          these unsafe functions explicitly.

     (10) Clarify that a call to a method via CALL-NEXT-METHOD must be
          considered `safe' if the generic function, applicable methods,
          method-combination, and the definition and efunctuation of the
          CALL-NEXT-METHOD itself are all `safe'.

     (11) Specify that if CALL-NEXT-METHOD is called with arguments, the ordered
          set of applicable methods for the changed set of arguments for
          CALL-NEXT-METHOD must be the same as the ordered set of applicable
          methods for the original arguments to the generic function, or an error
          should be signaled.  [This is a change -- 88-002R p2-13 says this
          situation must always be detected.]

          Clarify that the comparison between the set of methods applicable to the
          new arguments and the set applicable to the original arguments is
          insensitive to order differences among methods with the same
          specializers.

     (12) Clarify that if CALL-NEXT-METHOD is called with arguments that specify
          a different ordered set of applicable methods and there is no next
          method available, the test for different methods and the associated
          error signalling (when present) takes precedence over calling
          NO-NEXT-METHOD.

    Examples:

     A. Given ...

        (declaim (optimize (safety 3)))

        (defun     foo (x)                  (print 'test-failed) x)
        (defun     bar (&key x)             (print 'test-failed) x)
        (defmethod baz ((a integer) &key x) (print 'test-failed) x)

        (defun funcall-it (x) (funcall x))

        Then every implementation must arrange for an error to be signalled
        no later than the body (i.e., before (print 'test-failed) is executed)
        if the following forms also occur in safe code:

         A.1:  (foo)
         A.2:  (foo 1 2)
         A.3:  (bar :y 1)
         A.4:  (bar 1 2)
         A.5:  (baz 1 :y 7)
         A.6:  (funcall #'foo)
         A.7:  (funcall 'foo)
         A.8:  (mapcar #'foo '(1 2) '(1 2))
         A.9:  (mapcar 'foo '(1 2) '(1 2))
         A.8:  (mapcar #'1+ '(1 2) '(1 2))
         A.9:  (mapcar '1+ '(1 2) '(1 2))
         A.10: (funcall-it #'foo)
         A.10: (funcall-it 'foo)
         A.11: (funcall-it #'1+)
         A.12: (funcall-it '1+)
         A.13: (let ((x (locally (declare (optimize (safety 0))) 'foo)))
                 (funcall-it x))
         A.14: (let ((x (locally (declare (optimize (safety 0))) '1+)))
                 (funcall-it x))
         A.15: (let ((x (locally (declare (optimize (safety 0)))
    		      (symbol-function 'foo))))
                 (funcall-it x))
         A.16: (let ((x (locally (declare (optimize (safety 0)))
    		      (symbol-function '1+))))
                 (funcall-it x))

     B. Here are some examples of situations that might signal an error, but
        are not required to signal an error.  In effect, the consequences are
        undefined in these cases, even if the surrounding code is declared safe:

              ;; Functional evaluation is not safe:
        B.1:  (let ((x (locally (declare (optimize (safety 2))) #'foo)))
                (funcall-it x))
        B.2:  (let ((x (locally (declare (optimize (safety 2))) #'1+)))
                (funcall-it x))

              ;; Point of call is not safe:
        B.3:  (let ((x #'foo))
                (locally (declare (optimize (safety 2)))
                  (funcall x)))
        B.4:  (let ((x #'1+))
                (locally (declare (optimize (safety 2)))
                  (funcall x)))

              ;; Callee is not safe:
              (locally (declare (optimize (safety 2)))
                (defun foo1 (x) x))
        B.5:  (foo1)
        B.6:  (mapcar #'foo1 '(1 2) '(1 2))
        B.6:  (funcall-it 'foo1)

    Rationale:

      It's important for the document to be consistent and always say the
      same thing about each individual error situation.  It also seems
      important for the five error situations to be treated equally.

      Further, it's important that programmers be able to debug their code
      conveniently in a safe environment.  Once they start tampering with
      safety, they may run immediately into situations that expose
      variations in how implementations deal with function calling, but
      where safety is uniformly requested by the code, they should be able
      to insulate themselves from such differences.

      An exception is made in the name of efficiency for case 4 when
      :ALLOW-OTHER-KEYS T or &ALLOW-OTHER-KEYS is used.

    Current practice:

      CLtL did not require this level of error checking, so it's entirely
      likely that there are implementations which do not conform.

      Symbolics Genera conforms to this proposal.

    Cost to Implementors:

      Some implementations already implement most or all of this, so their
      cost will be minimal.

      A few implementations may not implement this.  The cost will vary
      depending on the implementation. In some cases, it could take a fairly
      substantial amount of work to make these changes.

      An implementation not willing to make these changes might prefer to
      identify itself as implementing only a low-safety subset of the
      language, and simply refuse to compile code which was declared high
      safety.  This might be appropriate for certain delivery situations.

    Cost to Users:

      More robust code.

    Cost of non-adoption:

      The specification document will not even be self-consistent.

    Performance impact:

      None.

    Benefits:

      Language consistency.

    Aesthetics:

      This is an improvement over the inconsistent situation which preceded it.

    Discussion:

      The idea of making this merely "undefined" was discussed and rejected
      at the June 1990 meeting.  There was consensus that signalling an
      error was fine, but the main sticking points were saying under what
      situations the user could depend on such signalling, and at what time
      the signalling might be permitted to occur.  The following `amendment'
      to the previous proposal was proposed and adopted, with the intent
      that it be clarified later:

        "should signal"
               +
        [ efunctuation ]
        [ caller       ]  [ system or safe ]
        [ callee       ]
               +
        "and no later than the body of the callee"

      Versions 3-5 were an attempt to clarify what was voted upon.

      Version 6 is an attempt to fix some lingering problems in the
      writeup from version 5, mostly related to other situations in which
      function promotion can occur that were not dealt with explicitly
      in version 5.

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