RfD: Escaped Strings S\"
19 July 2007, Stephen Pelc
20070719 Modified ambiguous condition
Added ambiguous conditions to definition of S\"
Added test cases
Corrected Reference Implementation
20070712 Redrafted non-normative portions.
20060822 Updated solution section.
20060821 First draft.
Rationale
=========
Problem
-------
The word S" 6.1.2165 is the primary word for generating strings.
In more complex applications, it suffers from several deficiencies:
1) the S" string can only contain printable characters,
2) the S" string cannot contain the '"' character,
3) the S" string cannot be used with wide characters as discussed
in the Forth 200x internationalisation and XCHAR proposals.
Current practice
----------------
At least SwiftForth, gForth and VFX Forth support S\" with very
similar operations. S\" behaves like S", but uses the '\' character
as an escape character for the entry of characters that cannot be
used with S".
This technique is widespread in languages other than Forth.
It has benefit in areas such as
1) construction of multiline strings for display by operating
system services,
2) construction of HTTP headers,
3) generation of GSM modem and Telnet control strings.
The majority of current Forth systems contain code, either in the
kernel or in application code, that assumes char=byte=au. To avoid
breaking existing code, we have to live with this practice.
The following list describes what is currently available in the
surveyed Forth systems that support escaped strings.
\a BEL (alert, ASCII 7)
\b BS (backspace, ASCII 8)
\e ESC (not in C99, ASCII 27)
\f FF (form feed, ASCII 12)
\l LF (ASCII 10)
\m CR/LF pair (ASCII 13, 10) - for HTML etc.
\n newline - CRLF for Windows/DOS, LF for Unices
\q double-quote (ASCII 34)
\r CR (ASCII 13)
\t HT (tab, ASCII 9)
\v VT (ASCII 11)
\z NUL (ASCII 0)
\" "
\[0-7]+ Octal numerical character value, finishes at the
first non-octal character
\x[0-9a-f]+ Hex numerical character value, finishes at the
first non-hex character
\\ backslash itself
\ before any other character represents that character
Considerations
--------------
We are trying to integrate several issues:
1) no/least code breakage
2) minimal standards changes
3) variable width character sets
4) small system functionality
Item 1) is about the common char=byte=au assumption.
Item 2) includes the use of COUNT to step through memory and the
impact of char in the file word sets.
Item 3) has to rationalise a fixed width serial/comms channel
with 1..4 byte characters, e.g. UTF-8
Item 4) should enable 16 bit systems to handle UTF-8 and UTF-32.
The basis of the current approach is to use the terminology of
primitive characters and extended characters. A primitive character
(called a pchar here) is a fixed-width unit handled by EMIT and
friends as well as C@, C! and friends. A pchar corresponds to the
current ANS definition of a character. Characters that may be
wider than a pchar are called "extended characters" or xchars.
The xchars are an integer multiple of pchars. An xchar consists
of one or more primitive characters and represents the encoding
for a "display unit". A string is represented by caddr/len
in terms of primitive characters.
The consequences of this are:
1) No existing code is broken.
2) Most systems have only one keyboard and only one screen/display
unit, but may have several additional comms channels. The
impact of a keyboard driver having to convert Chinese or Russian
characters into a (say) UTF-8 sequence is minimal compared to
handling the key stroke sequences. Similarly on display.
3) Comms channels and files work as expected.
4) 16-bit embedded systems can handle all character widths as they
are described as strings.
5) No conflict arises with the XCHARs proposal.
Multiple encodings can be handled if they share a common primitive
character size - nearly all encodings are described in terms of
octets, e.g. TCP/IP, UTF-8, UTF-16, UTF-32, ...
Approach
--------
This proposal does not require systems to handle xchars, and does
not disenfranchise those that do.
S\" is used like S" but treats the '\' character specially. One
or more characters after the '\' indicate what is substituted.
The following three of these cause parsing and readability
problems. As far as I know, requiring characters to come in
8 bit units will not upset any systems. Systems with characters
less than 7 bits are non-compliant, and I know of no 7 bit CPUs.
All current systems use character units of 8 bits or more.
Of observed current practice, the following two are problematic.
\[0-7]+ Octal numerical character value, finishes at the
first non-octal character
\x[0-9a-f]+ Hex numerical character value, finishes at the
first non-hex character
Why do we need two representations, both of variable length?
This proposal selects the hexadecimal representation, requiring
two hex digits. A consequence of this is that xchars must be
represented as a sequence of pchars. Although initially seen as a
problem by some people, it avoids at least the following problems:
1) Endian issues when transmitting an xchar, e.g. big-endian host
to little-endian comms channel
2) Issues when an xchar is larger than a cell, e.g. UTF-32 on
a 16 bit system.
3) Does not have problems in distinguishing the end of the
number from a following character such as '0' or 'A'.
At least one system (Gforth) already supports UTF-8 as its native
character set, and one system (JaxForth) used UTF-16. These systems
are not affected.
\ before any other character represents that character
This is an unnecessary general case, and so is not mandated. By
making it an ambiguous condition, we do not disenfranchise
existing implementations, and leave the way open for future
extensions.
Proposal
========
6.2.xxxx S\"
s-slash-quote CORE EXT
Interpretation:
Interpretation semantics for this word are undefined.
Compilation: ( "ccc<quote>" -- )
Parse ccc delimited by " (double-quote), using the translation
rules below. Append the run-time semantics given below to the
current definition.
Translation rules:
Characters are processed one at a time and appended to the
compiled string. If the character is a '\' character it is
processed by parsing and substituting one or more characters
as follows:
\a BEL (alert, ASCII 7)
\b BS (backspace, ASCII 8)
\e ESC (not in C99, ASCII 27)
\f FF (form feed, ASCII 12)
\l LF (ASCII 10)
\m CR/LF pair (ASCII 13, 10)
\n implementation dependent newline, e.g. CR/LF, LF, or LF/CR.
\q double-quote (ASCII 34)
\r CR (ASCII 13)
\t HT (tab, ASCII 9)
\v VT (ASCII 11)
\z NUL (ASCII 0)
\" "
\xAB A and B are Hexadecimal numerical characters. The resulting
character is the conversion of these two characters. An
ambiguous conditions exists if \x is not followed by two
hexadecimal characters.
\\ backslash itself
\ An ambiguous condition exists if a \ is placed before any
character, other than those defined in 6.2.xxx s\".
Run-time: ( -- c-addr u )
Return c-addr and u describing a string consisting of the translation
of the characters ccc. A program shall not alter the returned string.
See: 3.4.1 Parsing, 6.2.0855 C" , 11.6.1.2165 S" , A.6.1.2165 S"
Labelling
=========
Ambiguous conditions occur:
If \x is not followed by two hexadecimal characters.
If a \ is placed before any character, other than those defined
in 6.2.xxx s\".
Reference Implementation
========================
Taken from the VFX Forth source tree and modified to remove most
implementation dependencies. Assumes the use of the # and $ numeric
prefixes to indicate decimal and hexadecimal respectively.
Another implementation (with some deviations) can be found at
http://b2.complang.tuwien.ac.at/cgi-bin/viewcvs.cgi/*checkout*/gforth...
decimal
: PLACE \ c-addr1 u c-addr2 --
\ *G Copy the string described by c-addr1 u to a counted string at
\ ** the memory address described by c-addr2.
2dup 2>r \ write count last
1 chars + swap move
2r> c! \ to avoid in-place problems
;
: $, \ caddr len --
\ *G Lay the string into the dictionary at *\fo{HERE}, reserve
\ ** space for it and *\fo{ALIGN} the dictionary.
dup >r
here place
r> 1 chars + allot
align
;
: addchar \ char string --
\ *G Add the character to the end of the counted string.
tuck count + c!
1 swap c+!
;
: append \ c-addr u $dest --
\ *G Add the string described by C-ADDR U to the counted string at
\ ** $DEST. The strings must not overlap.
>r
tuck r@ count + swap cmove \ add source to end
r> c+! \ add length to count
;
: extract2H \ caddr len -- caddr' len' u
\ *G Extract a two-digit hex number in the given base from the
\ ** start of the* string, returning the remaining string
\ ** and the converted number.
base @ >r hex
0 0 2over drop 2 >number 2drop drop
>r 2 /string r>
r> base !
;
create EscapeTable \ -- addr
\ *G Table of translations for \a..\z.
7 c, \ \a
8 c, \ \b
char c c, \ \c
char d c, \ \d
#27 c, \ \e
#12 c, \ \f
char g c, \ \g
char h c, \ \h
char i c, \ \i
char j c, \ \j
char k c, \ \k
#10 c, \ \l
char m c, \ \m
#10 c, \ \n (Unices only)
char o c, \ \o
char p c, \ \p
char " c, \ \q
#13 c, \ \r
char s c, \ \s
9 c, \ \t
char u c, \ \u
#11 c, \ \v
char w c, \ \w
char x c, \ \x
char y c, \ \y
0 c, \ \z
create CRLF$ \ -- addr ; CR/LF as counted string
2 c, #13 c, #10 c,
internal
: addEscape \ caddr len dest -- caddr' len'
\ *G Add an escape sequence
...
