(I apologize if I'm rehashing an already closed topic.  I'm late to the
party,
and have not read every message in the archives.)

I am very, very happy to see this much attention lavished on Unicode
data representation.  However, I strongly regret the extended string
lexical syntax in this proposal, and so would like to propose an
alternative solution.

RECITATIONS

1. CL compatibility
R5RS lexical strings are very close to CL lexical strings, though the
wording of th Scheme spec leaves an out for extended string syntaxes.
I would love to preserve consistency with CL here.

2. Simplicity
In a CL string's external syntax, a backslash ("single escape
character") does not change the meaning of the next character; rather,
it removes any meta-meaning, forcing the next character as (READ) to
be taken literally.  This makes string literal parsing and
construction very simple: in any place where you're assembling a
literal string and for some reason don't know for certain what the
next character you're appending is, you can prepend a backslash and
thus ensure safety.  And any parsing likewise doesn't need to take
into account differences in various implementations' backslash
escapes.

3. Archaic C escape sequences
Do we need literal string codes for alarm, tab, linefeed, vertical
tab?  How many Scheme programs nowadays drive lineprinters with data
embedded in static strings?

4. Non-extensibility of extended lexical syntax
Escape sequences cannot be altered by the program, since they take
effect at read time: by the time that the program receives the program
text, the string has already been lexed, with escapes interpreted and
meta information discarded.  Having a bevy of magic sequences
certainly suggests that a client could change or at least amend to
those sequences ("Hey! backslash-q is free - let's use it to flag
internationalization data" "Hey! this implementation doesn't support
baclslash-a - let's add that"), perhaps adding compile-time or run-time
information (e.g. field width specifiers in C printf).  CL solves this
by moving everything into the library.

5. Redundancy with character lexical syntax
Finally, we have two separate ways to denote non-program-plaintext
characters: one in character lexical syntax, and one in string syntax.
Reducing this would be good for both implementation and user.

PROPOSAL

SRFI 75 could adopt a two-layered literal string syntax.

The basic string literal syntax would be that of CL, or R5RS without
the "behavior is unspecified" escape clause.  A string datum is a
sequence of characters read from the program text, delimited by double
quotes.  A backslash forces the next character in the sequence to be
taken literally.  Since the only magic characters in this syntax are
double-quote and backslash, backslash only changes their behavior.

The second layer would be the extended string syntax, introduced by
sharp-doublequote- leftparen and closed by rightparen.  Elements
within the extended string delimiters are read like any token: they
may include simple strings, or characters (including the extended
syntax of SRFI 75), which become a single element in the string.  The
elements in an extended string are concatenated in sequence, yielding
a single string literal.  Including an element of other than simple
string or character yields implementation-specific rsults.

Examples:

#"("this") -> "this"
#"("this" "that") -> "thisthat"
#"("this" #\a) -> "thisa"
#"("this" #\space "that") -> "this that"
#"("G" #\u0246 "del") -> "Goedel"
#"("This is "
    "the symphony "
    "that Schubert wrote, but never finished")
  -> "This is the symphony that Schubert wrote, but never finished"
#"("One fish." #\newline "Two fish." #newline "Red fish." #\newline
"Blue fish.")
  -> "One fish.
Two fish.
Red fish.
Blue fish."

Two obvious extensions to extended strings are symbols, which are
converted to strings per standard, and numbers, which represent a
single Unicode codepoint and are subject to the same limitations as
characters.

#"("this" symbol) -> "thissymbol"
#"("G" 246 "del") -> "Goedel"
#"("G" #xF6 "del") -> "Goedel"

COUNTERARGUMENTS

Per my recitations above
Q1. Why carry on about CL compatibility, then?  There's no such
extended string syntax in CL
A1. Yes, but the part that looks like CL now behaves very much like
CL.  Dual users can carry their expectations about backslash and
doublequote back and forth between the two.

Q2. Is that any simpler than backslashes?
A2. It is for the purpose of emitting string literals in the
double-quote syntax, which presumably will subsequently be consumed by
another program.  Parsing-wise, any system that can read a vector can
now read an extended string.

Q3.  My popular and well-liked Scheme system already supports
C-style string escapes.
A3. [looks at feet, shuffles uncomfortably]

Q4. This isn't extensible at runtime, either.
A4. Right.  But since it doesn't embed characters inside literally
typed strings, it doesn't /look/ extensible.  Any extensibility will
have to take place through a real character in the string, such as the
tilde used by FORMAT.

And others
Qa.  Man, #"("G" #\u0246 "del") is ugly.  If my dog was that ugly....
Aa.  Uglier than "G\u0246del"?  Neither is pretty.  One's easier to
parse and IMO read, one's easier to type.  Extended gets better if you allow
integers to represent codepoints ala #"("G" 246 "del").
Qb.  Doesn't the WRITE procedure become more complicated in the
presence of extended strings?  Now the runtime must preprocess any written
string to see if it requires the extended syntax before writing it.
Ab.  That is correct.  Backslash syntax has the advantage of pushing this
decision down to the per-character level.

Anyway.

For your consideration,
Ben