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Re: quasiquotation semantics

From: KenD@apple.com (Ken Dickey)
Subject: Re: quasiquotation semantics
Date: Mon, 7 Aug 1995 11:55:19 -0700

> At 12:33 PM 95/08/7 -0400, Marc Feeley wrote:
> >Suppose I write the following quasiquote expression:  `(a ,X b ,Y)
> >
> >Is it equivalent to:
> >  (list 'a X 'b Y)
> >or
> >  (cons 'a (cons X (cons 'b (cons Y '()))))
> In implementations I am familiar with, the second, explicit cons, form is
> always used.
> Another interpretation is (roughly):
>  (let ( (temp (cons 'a (cons #f (cons 'b (cons #f '())))) ) )
>    (setf (cadr temp) X)
>    (setf (caddr temp Y)
>    temp
>  )
> where temp is shared amoung all invocations of the quasiquoted form, but I
> don't know of any implementations which do this.
> If we standardize something we should use the most common implementation
> (perhaps JARs?).
> Cheers,
> -Ken

I'm not sure I understand your worries.  Probably you are referring to 
the fact that the explicitely-CONSed list will be _partially_
reconstructed if we jump into the middle of the expression, while the
LISTed form will reconstruct the _entire_ expression.

This is indeed bad, because in Scheme we can observe the difference
(EQ?).  But I wouldn't go so far and blame quasiquote, call/cc, cons,
or list for that; I blame the very concept of EQ?-ness of cons cells.
If that wouldn't be in the language, then this particular problem
wouldn't exist, because it would be impossible to observe the

Scheme has *way* too many effects, nearly _everything_ has some sort
of effect, which inhibits a great deal of very useful optimizations
without being very useful in itself.  It is a shame that

	((lambda (x y) (cons x (cons y '()))) X Y)

cannot be beta-reduced to

	(cons X (cons Y '()))

only because EQ? could observe the difference between the two.

However, all this is only one part of the problem.

If X and Y in `(,X ,Y) can be arbitrary expressions with effects, then 
the result is already undefined, since Scheme doesn't specify the
order of argument evaluation.

	(list X Y)


	(cons X (cons Y '()))

suffer from the same problem here.


PS: Ken's alternative `solution' above is incorrect (IMO).  Not only
does it try to solve a problem which has its roots in the
pervasiveness of effects by introducing even more effects, it also
breaks the semantics of quasiquote. The latter isn't very well
formalized (witness the current thread), but the Scheme report does
say somewhere that quasiquote is "equivalent" to some sequence of
CONS, LIST, APPEND, VECTOR etc. calls.  No data mutation here.
In Ken's proposed solution you could use call/cc to modify a
quasiquote-constructed data item by jumping into the middle of its