[FEEDBACK] Message Format Unquoted Literals

[Mark Davis]

Summary

Consider relaxing constraints on literals, after v45

Background

Right now, unquoted literals are fairly narrowly constrained by
message.abnf
; here are the relevant lines:

unquoted = name / number-literal

; number-literal matches JSON number
(https://www.rfc-editor.org/rfc/rfc8259#section-6)

number-literal = \["-"\] (%x30 / (%x31-39 \*DIGIT)) \["." 1\*DIGIT\]
\[%i"e" \["-" / "+"\] 1\*DIGIT\]

; name matches https://www.w3.org/TR/REC-xml-names/#NT-NCName

name = name-start \*name-char

name-start = ALPHA / "\_"

/ %xC0-D6 / %xD8-F6 / %xF8-2FF

/ %x370-37D / %x37F-1FFF / %x200C-200D

/ %x2070-218F / %x2C00-2FEF / %x3001-D7FF

/ %xF900-FDCF / %xFDF0-FFFC / %x10000-EFFFF

name-char = name-start / DIGIT / "-" / "."

/ %xB7 / %x300-36F / %x203F-2040

Reason for reconsidering

However, for functions outside of the standard registry, this forces
many natural literals to use quotes. Here is an example from a function
that would handle MF1’s choice format:

\[0,1) {{{\$count} is zero or fraction}}

The natural literals to use would be intervals, which use [,(,),]
characters for ranges (the choice format would require some recasting
because it depends on ordering of variants. It currently uses >.) So
that would require

\|\[0,1)\| {{{\$count} is zero or fraction}}

Many Unicode symbols are included by XML’s NT-NCName (about 6,000
currently), while many are excluded (about 2,600 currently). But these
are literals, not identifiers, which is what name is
intended for. By expanding beyond identifier usage, it allows functions
to avoid requiring quoting in many cases. It also allows us to dispense
with the special formulation for number-literal.

The literals for number, date, etc could be specified elsewhere, but
wouldn’t have to be in the ABNF.

That would allow for various registries to have more sophisticated
literal without requiring quoting, and without privileging the
structured literals that we know about now.

Requirements

So, what restrictions on characters for a broadened definition of
unquoted literals would be required by a revised ABNF?

1. No ‘}’, because it would make .local $x = {literal} fail.

2. No ‘|’, because an initial one would conflict with quoting, and it is best to just forbid it anywhere in an unquoted literal to prevent confusion.

3. No ‘{’. Not strictly required, but for clarity wherever used.

4. None of the big blocks of ‘strange’ code points that XML forbids: controls, surrogates, private-use, noncharacters.

   1. These are all immutable (Unicode Character Encoding Stability).

   2. This also disallows the noncharacters that XML didn’t know about yet, before the noncharacter property was made immutable.

5. No whitespace, since variant uses that for separators between keys.

   1. This could be done by just disallowing the “s” production characters, but that could be very confusing. {a b} looks too much like two items (the space is an A0 NO-BREAK SPACE). So it should be broadened to the Unicode Whitespace characters.

   2. Unicode Whitespace is not guaranteed immutable, but has not changed for over a decade. Anyway, we would derive the code points as of now, so everything would be stable into the future.

6. (Any others?)

Not coincidentally, 2-3 are the characters in the reserved-escape
production.

Detailed Proposal

This would result in the following change:

OLD

unquoted = name / number-literal

; number-literal matches JSON number
(https://www.rfc-editor.org/rfc/rfc8259#section-6)

number-literal = \["-"\] (%x30 / (%x31-39 \*DIGIT)) \["." 1\*DIGIT\]
\[%i"e" \["-" / "+"\]

// The characters include the following (though name-char and
number-literal additions are positional):

// name-start is \[\\: A-Z \_ a-z \x{C0}-\x{D6} \x{D8}-\x{F6}
\x{F8}-\x{2FF} \x{370}-\x{37D} \x{37F}-\x{1FFF} \x{200C}-\x{200D}
\x{2070}-\x{218F} \x{2C00}-\x{2FEF} \x{3001}-\x{D7FF} \x{F900}-\x{FDCF}
\x{FDF0}-\x{FFFD} \x{10000}-\x{EFFFF}\]

// name-char adds \[\\- . 0-9 \x{B7} \x{0300}-\x{036F}
\x{203F}-\x{2040}\]

// number-literal adds \[+ e\]

NEW

Unquoted = literal-char+

// Then down in ; Restrictions on characters in various contexts

literal-char = _all but following list; simpler to leave in this format
until after feedback._

Needed to avoid syntax conflicts

U+007B LEFT CURLY BRACKET
U+007C VERTICAL LINE
U+007D RIGHT CURLY BRACKET

Whitespace

U+0020 SPACE
U+00A0 NO-BREAK SPACE
U+1680 OGHAM SPACE MARK
U+2000 - U+200A EN QUAD .. HAIR SPACE
U+2028 LINE SEPARATOR
U+2029 PARAGRAPH SEPARATOR
U+202F NARROW NO-BREAK SPACE
U+205F MEDIUM MATHEMATICAL SPACE
U+3000 IDEOGRAPHIC SPACE

Controls

U+0000 - U+001F
U+007F - U+009F

Surrogates

U+D800 - U+DFFF

Private Use

U+E000 - U+F8FF U+F0000 - U+FFFFD U+100000 - U+10FFFD

Noncharacters

U+FDD0 - U+FFFE U+FFFF U+1FFFE U+1FFFF U+2FFFE U+2FFFF U+3FFFE U+3FFFF
U+4FFFE U+4FFFF U+5FFFE U+5FFFF U+6FFFE U+6FFFF U+7FFFE U+7FFFF U+8FFFE
U+8FFFF U+9FFFE U+9FFFF U+AFFFE U+AFFFF U+BFFFE U+BFFFF U+CFFFE U+CFFFF
U+DFFFE U+DFFFF U+EFFFE U+EFFFF U+FFFFF U+FFFFE U+FFFFF U+10FFFE
U+10FFFF