[HARLEQUIN][Common Lisp HyperSpec (TM)] [Previous][Up][Next]


Status: Version 5 passed Jan 89 X3J13

Version 6 passed Mar 89 X3J13


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


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))


as well as DECLARE within SYMBOL-MACROLET forms.


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.)


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


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.



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



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

by making symbol macros consistent with normal macros.


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.

----- End Forwarded Messages -----

[Starting Points][Contents][Index][Symbols][Glossary][Issues]
Copyright 1996, The Harlequin Group Limited. All Rights Reserved.