Formal comment #125 (enhancement)

Quasiquote should support unquotation within bytes object
Reported by:	Felix Klock

Component:	bytes
Version:	5.91

It seems natural and useful to support unquotation for the elements of
a bytes object.

For example: 

(let ((a 0) (b 1) (cs '(2 3))) 

`#vu8(,a ,b ,@cs)) ==> #vu8(0 1 2 3) 

This is relevant to the section on quasiquotation (which is unnumbered
in R{5.91}RS but immediately follows section 9.19) on pages 52-53.

To support this: 

1.) The grammar for quasiquotation would need to be extended with a
template for bytes, something like the following:

<bytes qq template D> -> #vu8(<qq template or splice D>*) 

2.) The report would need to specify whether spliced expressions can
evaluate to either an octet or a byte at whim, or if the expressions
must always evaluate to octets, or always to bytes.

This corresponds to deciding whether unquote within a #vu8(...) can
expand unconditionally to bytes-u8-set! (or bytes-s8-set!), or if some
new procedure will need to be introduced that handles both octets and
bytes as arguments.

3.) The report would need to specify the behavior when an unquoted
expression evaluates to a value that cannot be stored in a bytes
object; one choice would be to raise a &contract exception.

One contentious aspect of this suggestion is that the bytes objects
support a variety of -set! and -ref procedures, but quasiquote would
presumably only accept unquoting of expressions that evaluate to
single bytes (or, in the case of unquote-splicing, lists of bytes).

RESPONSE:

The comment was withdrawn as per

http://lists.r6rs.org/pipermail/r6rs-discuss/2006-November/000928.html