I've just verified that the implementations of unquote and unquote-splicing
in Guile do not conform to R6RS. Perhaps this is intentional, but it would
be a shame because the change in R6RS is certainly one for the better.
Scheme's quasiquotation is defined very well (it's even used as a reference
in Common Lisp's backquote definition) but ... it did have a major drawback.

unquote (and unquote-splicing as well) cannot be applied to multiple
expressions. This messes things up a little, especially when you're dealing
with nested quasiquotations. Anyway, you can't do things like this (like you
can in CL and R6RS):

(let ((name 'foo))
  `((unquote name name name)))
                => (foo foo foo)

But the latter example is nothing. You can get around it in a second. There
are however MUCH worse-case scenarios which, quite possibly, cannot be
worked around (or at most with ugly solutions).
The current rules for evaluation are (from R6RS):

If an (unquote <expression> ...) form appears inside a <qq template>,
however, the <expression>s are evaluated (“unquoted”) and their results are
inserted into the structure instead of the unquote form.

If an (unquote-splicing <expression> ...) form appears inside a <qq
template>, then the <expression>s must evaluate to lists; the opening and
closing parentheses of the lists are then “stripped away” and the elements
of the lists are inserted in place of the unquote-splicing form.

Any unquote-splicing or multi-operand unquote form must appear only within a
list or vector <qq template>.

That just about says it all. Alan Bawden discussed the previous flaw in
Scheme. Now it's cured. Hope to see the new rules in Guile too.

Rocco Rossi

E-Mail: rocco.ro...@gmail.com
Profilo: http://www.google.com/profiles/rocco.rossi

"Sono nato senza conoscere nulla e ho avuto un po' di tempo per cambiare qua
e là questa mia condizione." (Richard P. Feynman)

Reply via email to