CLHS: Issue COMPILER-LET-CONFUSION Writeup

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## Issue COMPILER-LET-CONFUSION Writeup

    Issue:		COMPILER-LET-CONFUSION
    Forum:	        Compiler
    References:	CLtL p. 112
    		Issue DEFINE-OPTIMIZER
    Category:	CHANGE
    Edit History:   V1, 27 Sep 1988, Sandra Loosemore (initial version)
                    V2, 04 Oct 1988, Sandra Loosemore (add another example)
    		V3, 31 Oct 1988, Sandra Loosemore (only proposal ELIMINATE)
    	        V4, 08 Jan 1989, Kent M. Pitman (new alternative)
    		V5, 09 Jan 1989, Sandra Loosemore (discussion)
    		V6, 08 Mar 1989, Sandra Loosemore (general updating)
    		V7, 13 Mar 1989, Sandra Loosemore (fix bug from V6)
    		V8, 23 Mar 1989, Sandra Loosemore (fix another bug, add
    					to discussion)
    Status:		Proposal ELIMINATE passed March 89.

    Problem Description:

     The description of the COMPILER-LET special form in CLtL is confusing
     to many people.  There are no examples provided to make it clear how it
     is supposed to be used. The only description which is offered is overly
     concrete, which has led to confusion about the intent of COMPILER-LET,
     and about its implementability.

     The intent of COMPILER-LET was to permit information to be communicated
     between macros by use of dynamic variables at macroexpansion time.
     It was not necessary to the intended uses of COMPILER-LET that such
     variables ever be bound at execution time.

     Unfortunately, probably because some implementations did not primitively
     support COMPILER-LET at the time CLtL was written, an exception was
     permitted to make COMPILER-LET `more or less work' in interpreters:
     the COMPILER-LET variables were permitted to be bound at execution time.
     The problem was further compounded by the fact that CLtL presented this
     exception as part of COMPILER-LET's contract rather than as an
     implementation note, and by the fact that no examples of actually using
     COMPILER-LET correctly are provided.

     One particular case where problems have resulted is a situation like
       (compiler-let ((*v* 1))
         #'(lambda () (m)))
     where M is a macro that refers to *V*.  In some implementations, M is
     not macroexpanded until the dynamic extent of the *V* binding has
     ended.

     Subtle bugs can be introduced because of the different handling of the
     variable bindings in the interpreter and the compiler.  In compiled
     code, the bindings are only lexically visible during the expansion of
     macros at compile time, while in interpreted code the bindings have
     dynamic scope and may also be seen during ordinary evaluation if
     evaluation and macroexpansion happen "in parallel".

     Further compatibility problems can result from the value forms being
     evaluated in a null lexical environment in the compiler and the ordinary
     lexical environment in the interpreter.

    Background and Analysis:

     It should be clear up front that COMPILER-LET is not computationally
     essential. Most (if not all) uses of it can be rewritten using MACROLET
     or SYMBOL-MACROLET.

     A typical use of COMPILER-LET might be:

      (defvar *local-type-declarations* '())

      (defmacro local-type-declare (declarations &body forms)
        `(compiler-let ((*local-type-declarations*
    		      (append ',declarations *local-type-declarations*)))
           ,@forms))

      (defmacro typed-var (var)
        (let ((type (assoc var *local-type-declarations*)))
          (if type `(the ,(cadr type) ,var) var)))

      (defun f (x y)
        (local-type-declare ((x fixnum) (y float))
          (+ (typed-var x) (typed-var y))))

     The same thing could be accomplished using MACROLET:

      (defmacro local-type-declare (declarations &body forms)
        (local-type-declare-aux declarations forms))

      (defmacro typed-var (var) var)

      (eval-when (eval compile load)
        (defun local-type-declare-aux (declarations forms)
          `(macrolet ((typed-var (var)
    		    (let ((type  (assoc var ',declarations)))
    		      (if type `(the ,(cadr type) ,var) var)))
    		  (local-type-declare (new-declarations &body new-forms)
    		    (local-type-declare-aux
    		      (append new-declarations ',declarations)
    		      new-forms)))
    	 ,@forms)))

     A further alternative would be to use SYMBOL-MACROLET (this particular
     implementation assumes that issue DEFINING-MACROS-NON-TOP-LEVEL passes):

      (let ((temp  (gensym)))
        (defmacro local-type-declare (declarations &body forms &environment env)
          `(symbol-macrolet ((,temp  ',(append declarations
    					  (symbol-macro-value temp env))))
             ,@forms))
        (defmacro typed-var (var &environment env)
          (let ((type  (assoc var (symbol-macro-value temp env))))
    	(if type `(the ,(cadr type) ,var) var)))
        )

      (defun symbol-macro-value (symbol env &optional default)
        (multiple-value-bind (expansion macro-p) (macroexpand symbol env)
          (if macro-p (eval expansion) default)))

     Opinion is divided as to which is more understandable.  Some
     people find the COMPILER-LET idiom more understandable (assuming that
     it can be made to work consistently in compiled and interpreted
     code), while others find it just as natural to use MACROLET or
     SYMBOL-MACROLET.

     The issues are these:

      - Is it possible to implement COMPILER-LET in a usefully consistent
        way in all implementations?

      - Are the benefits of providing a useful and compatible implementation
        of COMPILER-LET worth any associated cost?

     Two proposals are presented below:

      - Option REPAIR argues that COMPILER-LET provides interesting
        functionality that can be implemented in a manner that is usefully
        consistent across implementations, and that the associated cost
        is low enough for it to be worthwhile to do so.

      - Option ELIMINATE argues that COMPILER-LET complicates the language
        and that providing this construct is not worth the associated
        implementation cost.

    Proposal (COMPILER-LET-CONFUSION:REPAIR):

      Strike the existing definition of COMPILER-LET. Redefine it as follows:

      COMPILER-LET						  [Special form]

        COMPILER-LET is similar to LET, but it always makes special
        bindings and makes those bindings visible only during
        macroexpansion of forms in the body, not during the runtime
        execution of those forms.

        If proposal DEFINE-OPTIMIZER:NEW-FACILITY is accepted, then
        optimizer functions are also executed in a dynamic environment
        in which COMPILER-LET bindings are visible.

        The intent is that some macros might macroexpand into calls to
        COMPILER-LET in which the body would the contain references to
        macros which access the variables in the COMPILER-LET.

        The initial value forms of the bindings, if any, are always
        evaluated in a null lexical context, regardless of whether the
        COMPILER-LET expression is being interpreted or compiled.

        The initial value forms of the bindings, if any, are evaluated in
        a dynamic context where the bindings of any lexically enclosing
        COMPILER-LET are visible, and where dynamic execution-time
        environment may or may not be visible.

        Implementation Note: Permitting the execution-time dynamic
        environment to be visible when initializing COMPILER-LET variables
        is a concession to some interpreters which may have to do this in
        order to keep the cost down. Where feasible, implementors should
        try not to make the runtime environment visible.

      Rationale:

        This gives a consistent description of COMPILER-LET which separates
        issues of intent from those of implementation in a way that makes it
        possible for portable code to make serious use of it, and which does
        not force gratuitous incompatibilities between interpreters and
        compilers.

        This description of COMPILER-LET can be implemented without undue
        cost by all implementations. See "Cost to Implementors" for details.

      Cost to Implementors:

        Modest, but nontrivial in some implementations.

        In compiled code, and in interpreters doing a one-time semantic
        prepass, it should be fairly easy for COMPILER-LET to cause the
        variables to get bound (using PROGV) during semantic analysis.

        In interpreters which do not do a semantic-prepass, it is necessary
        to fully macroexpand the body. Assuming the presence of a
        SYSTEM::MACROEXPAND-ALL primitive, the definition of COMPILER-LET
        could look like:
          (DEFMACRO COMPILER-LET (BINDINGS &BODY FORMS &ENVIRONMENT ENV)
            (SETQ BINDINGS ;; Assure no non-atom bindings
    	      (MAPCAR #'(LAMBDA (BINDING)
    		          (IF (ATOM BINDING) (LIST BINDING) BINDING))
    		      BINDINGS))
            (PROGV (MAPCAR #'CAR BINDINGS)
    	       (MAPCAR #'(LAMBDA (BINDING) (EVAL (CADR BINDING))) BINDINGS)
    	  (SYSTEM::MACROEXPAND-ALL `(PROGN ,@FORMS) ENV)))
        This reduces the problem of writing a program capable of doing a
        full macroexpansion. Many systems already have such a facility.
        Pitman wrote such a facility in Cloe Runtime in order support
        SYMBOL-MACROLET (before it was christened a special form); it was
        about 750 lines of relatively straightforward, well-commented code.

      Cost to Users:

        Code currently depending on this feature is either non-existent or
        already not portable (due to wide variation in implementation
        strategy for COMPILER-LET).

        Most users will probably be happy for any interpretation which offers
        them a future shot at portability.

        Some users have indicated they dislike interpreters which do a semantic
        prepass, because they like to be able to dynamically redefine macros
        while debugging.

    Proposal (COMPILER-LET-CONFUSION:ELIMINATE):

      Remove COMPILER-LET from the language.

      Rationale:

        Some people think that having one less special form would simplify the
        language.  The revised COMPILER-LET semantics, which require
        COMPILER-LET to make special bindings which are only visible during
        expansion of macros which appear lexically within its body, are
        not shared by any other feature in the language, and require a
        fairly complex implementation technique.  There are other
        constructs which are strictly lexical that can be readily used
        to solve the same kinds of problems that COMPILER-LET is intended to
        be used for.

      Cost to Implementors:

        Minimal.  Implementations could continue to support COMPILER-LET as
        an extension.

      Cost to Users:

        Code currently depending on this feature is either non-existent or
        already not portable (due to wide variation in implementation
        strategy for COMPILER-LET).

        People who use COMPILER-LET would have to rewrite their programs to use
        some other construct.  As discussed above, most uses of COMPILER-LET
        for communication between macros can be handled using MACROLET or
        SYMBOL-MACROLET, though some perspicuity may be lost in the process.

    Current Practice:

     Some implementations have implemented the description in CLtL.
     Users of those implementations (quite reasonably) can't figure how to
     use COMPILER-LET and so don't use it much.

     Some implementations whose interpreters include a preprocessor to
     expand all macros have already implemented something similar to proposal
     COMPILER-LET-CONFUSION:REPAIR.  Users of such implementations
     probably use COMPILER-LET somewhat more often since it has an
     intelligible behavior, but their code is not portable since it relies
     on behaviors which are either contrary to or not guaranteed by CLtL.

    Benefits:

     Either way, a potential area of incompatibility between compiled and
     interpreted code would be eliminated.

     Either way, a potential area of portability trouble would be very
     drastically reduced (in the case of the REPAIR option) or eliminated
     (in the case of the ELIMINATE option).

    Discussion:

     Pitman strongly favors COMPILER-LET-CONFUSION:REPAIR.  He argues
     against the idea of using MACROLET instead of COMPILER-LET, saying:

      This is a little misleading because it's like saying you can
      do without LET given that you have FLET. You can, but you lose some things
      in the process:

      Just as rewriting a LET using FLET might slow your computation, so too
      a rewrite of COMPILER-LET using MACROLET might slow things down. However,
      compilation speed is generally not weighted as heavily as execution speed
      by many people, so the loss of speed here may not be as important.

      Just as rewriting a LET using FLET might obscure the simplicity of your
      intent, so too rewriting COMPILER-LET using MACROLET might obscure your
      intent. You'd probably get used to recognizing idioms if you used it often
      enough. Certainly this would be true if you didn't have LET. However,
      COMPILER-LET is used less often, so not having it would mean that the
      code you wrote instead would be much harder to read because people
      wouldn't have the necessary familiarity with the idioms involved and so
      wouldn't always understand them.

     Sandra Loosemore responds:

      The argument that using MACROLET is more inefficient than COMPILER-LET
      is questionable.  Both of the suggested implementation techniques for
      COMPILER-LET involve considerable overhead.

      If COMPILER-LET were not part of the language, people wouldn't think in
      terms of rewriting COMPILER-LETs as MACROLETs; instead, they'd think of
      how to use MACROLET in the first place to solve their problems.  This
      is what people who now use implementations with broken COMPILER-LETs
      already do.  Since MACROLET is now used much more frequently than
      COMPILER-LET, that argues that people are much more familiar with
      MACROLET idioms than COMPILER-LET idioms.

      Also, note that the intent of the revised COMPILER-LET definition is
      to make the binding only lexically visible within the body.  Using
      special binding for this purpose is troublesome.  Both the MACROLET
      and SYMBOL-MACROLET solutions are completely lexical and avoid all
      the problems associated with special binding.

    Glenn Burke thinks it needs to be emphasized that the code-walker
    mentioned in the REPAIR proposal does not need to be portable.  He
    says:

      The present wording makes it sound like a piece of cake to do with
      portable code, when the reality is that a good fraction of CL cleanup
      effort has involved the lack of capability of producing such a beast.
      Without one or more of a number of proposals being accepted, a fully
      correct portable code walker cannot be built, in my belief.

      I object to the flippant attitude of just "presupposing" this
      "trivial" function which "we know how to do".

    We have considered a number of other options on this issue, including
    a variety of proposals that would redefine COMPILER-LET to store
    information about the variable bindings in the lexical environment, and
    either having MACROEXPAND-1 make the special bindings or provide a
    function which could be used to access them directly.

    Kent Pitman says:
      People have suggested that if it comes to making an incompatible change
      on this one, it's probably better to just remove the feature and let
      people continue to provide it compatibly where they think it's useful.
      Even though I think the COMPILER-SYMBOL-VALUE thing is technically
      doable, I find myself swayed by arguments that it's not the correct
      avenue for us to pursue at this time.

    David Moon says:
      I think COMPILER-LET-CONFUSION:REPAIR should be split into four proposals.
      First, one that gives the general framework for repairing but doesn't
      say anything about how the interpreter implements COMPILER-LET, other
      than to say that an additional proposal is needed to cover that.  Then
      three proposals for how the interpreter implements COMPILER-LET:
       (1) The interpreter always does a "semantic pre-pass" like the compiler.
       (2) The interpreter expands all macros inside COMPILER-LET before
           evaluating any of the code inside COMPILER-LET.
       (3) COMPILER-LET passes the variable bindings to MACROEXPAND-1
           through the lexical environment, and the time when the interpreter
           expands macros is not changed.

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