Issue SYMBOL-MACROLET-SEMANTICS Writeup

Status: Version 5 passed Jan 89 X3J13

	Version 6 passed Mar 89 X3J13


Issue:		SYMBOL-MACROLET-SEMANTICS

References:	SYMBOL-MACROLET (88-002R page 2-81)



Related Issues: SYMBOL-MACROLET-DECLARE

Category:	CHANGE

Edit history:	29-July-88, Version 1 by Piazza

		21-September-88, Version 2 by Piazza

		22-September-88, Version 3 by Piazza 

		22-September-88, Version 4 by Piazza

		30-Nov-88, Version 5 by Masinter

		14-Mar-89, Version 6 by Steele


Problem Description:


    The SYMBOL-MACROLET construct, introduced with CLOS in X3J13 document

    88-002R, profoundly alters the interpretation of symbols appearing as

    forms in a Common Lisp program--what previously was necessarily a variable

    might now be a symbol macro instead.  Macros which appear in the body of a

    SYMBOL-MACROLET form are currently unable to determine whether a symbol

    form is a variable or a symbol macro, and, if the latter, what the

    expansion of the symbol macro is.  Consequently, complex macros (such as

    SETF or PUSH) which depend on the form of their argument(s), are unable to

    produce their desired results in some cases, as in the following example:


	    (let ((a (make-array 5))

		  (i 0))

	      (symbol-macrolet ((place  (aref a (incf i))))

	        (push x place))

	      i)		==> 2


    In addition, it would be both natural and nice to be able to write


  (with-slots (rho theta) point

    (declare (single-float rho theta))

    ...computation...)


    as well as DECLARE within SYMBOL-MACROLET forms.


Proposal (SYMBOL-MACROLET-SEMANTICS:SPECIAL-FORM):


    Change the definition of SYMBOL-MACROLET to specify that it is a special

    form, which affects the evaluation environment for symbols.  Enhance

    MACROEXPAND and MACROEXPAND-1 so that they can expand a symbol macro.

    Modify SETF et al to use the new MACROEXPAND and MACROEXPAND-1 to examine

    even symbol subforms.  Specify that the expansion of a symbol macro IS

    subject to further macro expansion in the same lexical environment as the

    symbol macro invocation, exactly analogous to normal macros. Clarify that

    within the body of a SYMBOL-MACROLET, SETQ of a symbol defined as

    a symbol macro will be treated as if it were a SETF.


    Furthermore PSETQ of a symbol defined as a symbol macro will

    behave as if it were a PSETF, and MULTIPLE-VALUE-SETQ will behave

    as if SETQ were used on each variable to be set.


    When MACROEXPAND or MACROEXPAND-1 sees a symbol macro, it calls

    the value of *MACROEXPAND-HOOK* in the same manner as for an

    ordinary macro.  The three values given to the hook function

    in this case will be an expansion function, a form (in this case

    the symbol naming the symbol macro), and an environment.  The

    only guaranteed property of the expansion function is that when

    it is applied to the form and the environment it will return the

    correct expansion of the symbol macro.  (In particular, nothing

    it said in this specification whether the expansion is conceptually

    stored in the expansion function, the environment, or both.)


Rationale:


    The potential for interaction between macros is exactly why &environment

    arguments were originally added to macros.  Changing SYMBOL-MACROLET to be

    a special form, which communicates through the &environment arguments to

    macros with MACROEXPAND and MACROEXPAND-1, would allow PUSH and SETF

    (among others) to work with SYMBOL-MACROLET in the same way they work with

    MACROLET.


    This change cannot (reasonably) support the currently specified semantics

    that the expansion text is "outside" the scope of the symbol macro.  For

    indeed, when the symbol macro is expanded, (a copy of) the expansion is

    then within the scope of the SYMBOL-MACROLET, and should then be subject

    to further scrutiny.  The issue of "infinite expansion" of symbol macros is

    no more dangerous than that of normal macros.


Current Practice:


    Portable Common Loops provides a code-walking implementation of

    SYMBOL-MACROLET as specified in 88-002R.  Symbolics Cloe has both a

    code-walking version of a SYMBOL-MACROLET macro and compiler support for

    a SYMBOL-MACROLET special form.


Cost to Implementors:


    If SYMBOL-MACROLET is modified to be a special form, compilers and

    interpreters will have to change, as well as MACROEXPAND, MACROEXPAND-1,

    PUSH, INCF, DECF, and others.


Cost to Users:


    If SYMBOL-MACROLET is converted to a special form, code-walking programs

    will have to be modified to handle SYMBOL-MACROLET correctly.  Those same

    programs would have to be modified to handle the other special forms

    specified in CLOS, anyway.


Cost of Non-Adoption:


    SYMBOL-MACROLET will retain its confusing semantics, leading to bugs when

    it interacts with complex macros and forms which produce side-effects.


    Implementations which support ONCE-ONLY will break.  For that matter, any

    mechanism which examines code and assumes that "variables" have no side

    effects will break.


Benefits:


    SYMBOL-MACROLET-SEMANTICS:SPECIAL-FORM avoids the hairiest problems

    surrounding interaction of macros (like SETF) and side effects, and makes

    SYMBOL-MACROLET consistent with MACROLET.


Aesthetics:


    If SYMBOL-MACROLET is made to be a special form, aesthetics are improved

    by making symbol macros consistent with normal macros.


Discussion:


    A case could be made for adding a new function, SYMBOL-MACRO-FUNCTION, as

    a dual of MACRO-FUNCTION.  However, symbol macros are simpler than normal

    macros: a symbol macro is associated with a single expansion form, rather

    than an arbitrary function which computes the expansion.  For this reason,

    the augmented MACROEXPAND-1 proposed here can also fill the role of

    SYMBOL-MACRO-FUNCTION: the second value of (macroexpand-1 sym env) will be

    T if and only if sym is a symbol macro, while the first value gives the

    expansion of sym, if it has one.


    Rather than extending the existing MACROEXPAND and MACROEXPAND-1

   functions, new functions could be introduced to expand symbol macros. 

   However, there seems to be no particular reason to do this.




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