CLHS: Issue THE-AMBIGUITY Writeup

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## Issue THE-AMBIGUITY Writeup

    Forum:        cleanup
    Issue:        THE-AMBIGUITY
    References:   THE (page 161)
    Category:     CLARIFICATION
    Edit history: 21-Oct-88, version 1 by Rees
                  11-Jan-89, version 2 by Masinter (fix typos)

    Problem description:

      CLtL does not explicitly say whether the type specifier in a THE
      form may be any type specifier or must be a type specifier suitable
      for discrimination.  Although THE is decsribed as a "declaration"
      form, some CL implementations have assumed that the specifier must
      be for discrimination, and disallow e.g.,

        (THE (FUNCTION (T T) CONS) #'CONS)

      We should either say that the implementations are right, or
      explicitly say that they are wrong, since this case is easily
      overlooked.

    Proposal (THE-AMBIGUITY:FOR-DECLARATION):

      Clarify that the type specifier in

    	(THE type exp)

      may be any valid type specifier.  In the case that exp returns one
      value and type is not a VALUES type specifier, (THE type exp) is
      equivalent to

    	(LET ((g exp))
    	  (DECLARE (TYPE type g))
    	  g)

      where "g" is a gensym.

    Proposal (THE-AMBIGUITY:FOR-DISCRIMINATION):

      Clarify that the type specifier in

    	(THE type exp)

      must be a valid type for discrimination, as for TYPEP, or it must
      be of the form (VALUES type*) where type* are all valid for discrimination.

    Current practice:

      The Symbolics Genera and VAX LISP V2.2 interpreters signal errors for

    	(THE (FUNCTION (T T) CONS) #'CONS),

      but this may not be intentional.  CLtL would seem to allow it.

    Test case:

      (THE (FUNCTION (T T) CONS) #'CONS),

      should return the CONS function under FOR-DECLARATION,
      and should be an error under FOR-DISCRIMINATION.

    Cost to implementors:

      Trivial cost for THE-AMBIGUITY:FOR-DISCRIMINATION; this is a compatible
      restriction.

      For THE-AMBIGUITY:FOR-DECLARATION, implementations that do not
      already allow arbitrary type specifiers but which want to check that
      the type in a THE is satisfied would have to create an internal
      version of TYPEP which could manage not to signal invalid-type-specifier
      errors in those situations where TYPEP would because the type is a
      declaration-only one.

    Cost to users:

      Users of implementations that support THE-AMBIGUITY:FOR-DECLARATION
      might have to remove or change some uses of THE in their code if the
      opposing alternative is adopted.

    Benefits:

      Either way, an ambiguity in the language specification would be clarified.

    Aesthetics:

      THE-AMBIGUITY:FOR-DECLARATION would seem to be more consistent with
      DECLARE and with the intent of THE, which is supposed to be a way to
      provide information for documentation and for the benefit of compilation.

    Discussion:

      Rees supports THE-AMBIGUITY:FOR-DECLARATION.

      Appropriate error situation terminology must be chosen for the
      situation that a THE declaration (or other declaration) is
      unsatisfied, but that must be done regardless of this proposal.

      This proposal would suggest that a function should be added to CL to
      do the checking that THE would want to do:

    	  (PROBABLY-TYPEP object type-spec)

      [terrible name of course] returns multiple values a la SUBTYPEP: T T
      if the object definitely has the type, NIL T if it definitely
      doesn't, and T NIL (or NIL NIL?) otherwise.  Assuming that an
      interpreted THE-expression actually checks types, you could almost
      define this function using the condition system and EVAL.  (Ugh!)
      Without PROBABLY-TYPEP, a meta-circular interpreter is more
      difficult to write.

      If a suitable name was found for this function, the additional
      functionality could be suggested as an independent proposal, since
      regardless of the outcome of this issue, the functionality is still
      useful for checking DECLARE's.

      Various implementation mechanisms were discussed for dealing
      with THE checking.

      Are there any remaining type specifiers beyond the list form
      of the FUNCTION type that differ between "declaration" and
      "discrimination"?

      "I support FOR-DECLARATION.  Lucid CL has the same bug in
      the interpreter as the others (a "bug" assuming FOR-DECLARATION).
      TYPEP is used to check the legality of the type specifier in THE."

      In considering possible ways in which the type-checking logic
      for THE and DECLARE might work, don't forget things like
    	(the (not (function (t t) integer)) 7),
      which you would want to signal an error.  I don't think this can be
      done with only TYPEP and conditions.

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