CLHS: Issue EVAL-TOP-LEVEL Writeup

#

## Issue EVAL-TOP-LEVEL Writeup

    Forum:		Public Review
    Issue:		EVAL-TOP-LEVEL
    References:	Loosemore's public review comment #29 (X3J13/92-1329)
    		Section 3.2.3.1, "Processing of Top Level Forms"
    		EVAL, LOAD, COMPILE-FILE
    		Issue EVAL-WHEN-NON-TOP-LEVEL
    Category:	CLARIFICATION, CHANGE
    Edit history:	12 Dec 90, Version 1 by Tim Moore
    		14 Dec 90, Version 2 by Moore w/ comments from Loosemore
    		08 Mar 91, Version 3 by Moore, discussion from rwk and JonL
    		06 Feb 93, Version 4 by Loosemore
    		  (new proposal, update writeup)
    Status:		Proposal LIKE-COMPILER (version 3) failed 4-9 at
    		  the March 1991 meeting.
    		Proposal LOAD-LIKE-COMPILE-FILE passed 5-2 at the
    		  October 1993 meeting.

    Problem description:

      Section 3.2.3.1, "Processing of Top Level Forms" specifies the
      processing of top-level forms in the file compiler in great detail.
      Unfortunately, there is no such specification of the evaluator or
      loader's top level behavior.  If the evaluator or loader does not
      process top-level forms in the same way that the compiler does, there
      can be a surprising semantic gap between the compiler and interpreter.
      It may be impossible to evaluate forms that macroexpand into more than
      one top-level form.  For example:

        (progn
          (defsetf foo setf-foo)
          (defun bar (u v)
            (setf (foo u) v)))

      If the DEFSETF form is not evaluated before the function definition is
      processed, the SETF form will expand incorrectly into:

        (funcall (function (setf foo)) v u)

      instead of:

        (setf-foo u v)

      The net result is that code which has been carefully structured to
      compile correctly under the order-of-processing constraints given in
      section 3.2.3.1 may fail to work when loaded interpretively.

      At least two implementations -- UCL and WCL -- have unwittingly
      stumbled into this trap by implementing evaluators that perform
      implicit compilation before executing any code.

    Proposal (EVAL-TOP-LEVEL:LOAD-LIKE-COMPILE-FILE):

      Replace the fourth paragraph of the description section of the
      dictionary entry for LOAD with:

        \funref{load} sequentially executes each \term{form} it encounters
        in the file named by \param{filespec}.  If the file is a source
        file and the implementation chooses to perform
        \term{implicit compilation}, \funref{load} must recognize
        \term{top level forms} as described in {\secref\TopLevelForms} and
        arrange for each \term{top level form} to be executed before
        beginning \term{implicit compilation} of the next.  (Note, however,
        that processing of \specref{eval-when} forms by \funref{load} is
        controlled by the \kwd{execute} situation.)

    Rationale:

      Everybody seems to agree that it's a bug for implementations *not*
      to be able to load files containing top-level forms such as that
      given in the problem description section.  Having the standard say
      something explicit about it will hopefully prevent implementors
      from overlooking this source of bugs in the future.

    Current practice:

      UCL and WCL have had problems with this in the past.  It's not known
      whether or how they have been fixed.

    Cost to implementors:

      In those implementations that do implicit compilation, minor hacks
      to make LOAD or EVAL to do stuff similar to the top-level COMPILE-FILE
      processing.  None in other implementations.

    Cost to users:

      None.

    Aesthetics:

      Neutral.

    Editorial impact:

      Inserting the text included above.

      (It would probably be better to reorganize this material into a separate
      section describing program structure rather than talking about the
      behavior of LOAD and COMPILE-FILE, but that's too much work!)

    Discussion:

      Sandra Loosemore says:

        I submitted this as a public review comment after I discovered that
        WCL was "broken" while trying to port some code last fall.  I had
        completely forgotten that the exact same problem had previously
        been presented as a cleanup issue to the committee until Kim Barrett
        mentioned it while we were sorting through the comments after the
        end of the public review period.

      Version 3 of this issue contained a different proposal, LIKE-COMPILER,
      which would have constrained EVAL rather than LOAD.  That proposal was
      defeated at the March 1991 meeting, with people who voted against the
      proposal arguing that

        (eval x)

      should have completely equivalent semantics to

        (funcall (compile nil `(lambda () ,x)))

      The LOAD-LIKE-COMPILE-FILE proposal in version 4 avoids this problem
      by constraining LOAD only.  However, it may be implemented either by
      tweaking LOAD specifically or EVAL in general to process forms in
      the required order.

Copyright 1996-2001, Xanalys Inc. All rights reserved.