issue COMPILER-LET-CONFUSION

[sandra%defun@cs.utah.edu (Sandra J Loosemore)]

In an earlier message on this topic, you wrote:

> Suppose that instead of referencing the value as a special, you had 
> an operator COMPILER-SYMBOL-VALUE to get its value. [That could be a 
> function, macro, or special form.] I wouldn't have a problem calling
> a function to get this data because I still get to use the same basic
> `shape' of code. Also, uses of COMPILER-LET are rare enough that a
> bit of extra syntax is not overwhelming.

At the Hawaii meeting, Kim Barrett showed me a clever example of how
you can already get this effect using SYMBOL-MACROLET. 

Where one would now write something like:

    (compiler-let ((*foo*  (new-value)))
        ... (some-macro) ...)

    (defmacro some-macro ()
        ... *foo* ...)

one could instead use SYMBOL-MACROLET:

    (symbol-macrolet ((*foo*  (new-value)))
        ... (some-macro) ...)

    (defmacro some-macro (&environment env)
        ... (symbol-macro-value '*foo* env) ...)

    (defun symbol-macro-value (symbol env &optional default)
        (multiple-value-bind (expansion macro-p) (macroexpand-1 symbol env)
	    (if macro-p expansion default)))

How does this strike you?  I really like this approach, because it is
entirely lexical and avoids all the problems relating to the special
binding of COMPILER-LET.  There will be some conversion necessary for
user code, but as you noted in your earlier message, the basic "shape"
of the code remains the same.

-Sandra
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