CLHS: Issue LOCALLY-TOP-LEVEL Writeup

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## Issue LOCALLY-TOP-LEVEL Writeup

    Issue:         LOCALLY-TOP-LEVEL

    References:    None

    Related issues: EVAL-WHEN-NON-TOP-LEVEL, DECLARATION-SCOPE

    Category:      CLARIFICATION / ADDITION

    Edit history:  Version 1,  9-Mar-89, by Moon
                   Version 2, 16-Mar-89, by Moon, fix referenced proposal name

    Problem description:

      It is desirable to be able to wrap LOCALLY around one or more
      top-level forms and have them continue to be treated as top-level
      forms.  Three examples of how this is useful:

       - to put an OPTIMIZE or INLINE declaration into force around
         several related forms.

       - to put declarations into force around DEFCLASS, or any other
         top-level form that lacks a syntax for embedded declarations.

       - DECLARATION-SCOPE:NO-HOISTING, which passed in January,
         removed the ability to use a DECLARE at the head of the body of a
         DEFUN or DEFMACRO to make a declaration that applies to the entire
         form, including the lambda-list.  We are supposed to use LOCALLY
         instead, but forms in the body of LOCALLY are not top-level,
         and that changes the semantics of DEFMACRO.

      Issue EVAL-WHEN-NON-TOP-LEVEL could not define LOCALLY to treat
      its body as top-level forms, because only a special form can do
      that and LOCALLY is a macro.

    Proposal (LOCALLY-TOP-LEVEL:SPECIAL-FORM):

      Change LOCALLY from a macro to a special form, and change the
      definition of compiler processing (in EVAL-WHEN-NON-TOP-LEVEL)
      so that when a LOCALLY form appears at top level the forms in
      its body are processed at top level.

    Examples:

      (locally (declare (optimize (safety 3) (space 3) (speed 0)))
        (defmacro frob (&environment e x y &optional (z (foo x y)))
          (mumble x y z e)))

      Without this proposal, this would have to be written

      (defmacro frob (&environment e x y &optional (z (locally
                                                        (declare
                                                          (optimize
                                                            (safety 3)
                                                            (space 3)
                                                            (speed 0)))
                                                        (foo x y))))
        (locally (declare (optimize (safety 3) (space 3) (speed 0)))
          (mumble x y z e)))

    Rationale:

      Wrapping LOCALLY around a form should not change its semantics except
      as specified by the declarations, hence the body of a top-level
      LOCALLY should be top-level.

      A macro cannot have a top-level body unless it expands into a special
      form that has a top-level body; otherwise the macro invocation and
      the macro expansion would not have identical semantics as top-level
      forms.  There is no available special form for LOCALLY to macroexpand
      into (CLtL doesn't say, but presumably the intent was to expand into
      a LET with an empty binding list).

    Current practice:

      The Zetalisp equivalent of LOCALLY worked to surround top-level forms,
      because it was a macro that expanded into COMPILER-LET (stashing the
      declarations in a special variable the compiler would look at).  This
      is of course the wrong way to do declarations, but it shows that the
      idea was that you could wrap declarations around a bunch of top-level
      forms.

      Symbolics Genera 7.4.0 does not implement the proposal (but it does
      not implement DECLARATION-SCOPE:NO-HOISTING either).  I did
      not survey any other implementations.

    Cost to Implementors:

      A half dozen lines of code in the compiler and a smaller amount
      in the interpreter and any program-analyzing programs.

    Cost to Users:

      None.

    Cost of non-adoption:

      See the horrible example above.

    Performance impact:

      None.

    Benefits:

      More consistent language.

    Esthetics:

      Improved.

    Discussion:

      None.

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