I propose that we restrict the literal syntax and disallow unquoted non-numeric literals as expression operands.

Today, literals (quoted and unquoted) can appear in 3 positions:

• as variant keys,
• as option values,
• as expression operands.

The first two positions are common and well justified. OTOH, I posit that the third position is very rare, except numeric values. In fact, I'm not aware of strong use-cases for unquoted non-numeric literals as expression operands. Furthermore, I'm concerned about the risk of confusing such unquoted literals for keywords.

Forbidding unquoted non-numeric literals as expression operands will provide two benefits:

• Since our syntax uses keywords positioned at the front of a declaration, unquoted literals as expression operands may be confused for novel keywords. Consider:

  {now :datetime} vs. {|now| :datetime}
  {error :msgref} vs. {|error| :msgref}
  

  |now| and |error| are clearly recognizable as literal values.

• It would open new opportunities for introducing new placeholder or expression syntax: {foo}. See Open/close design: A familiar HTML-like syntax as an alternative #516 for an example use-case.

As a reminder, {5 :number} would still be valid under this proposal, as well as :func opt=unquoted and when unquoted.

ABNF-wise, I think we'd need the following two changes:

• Split unquoted into unquoted-string and unquoted-numeric.
• Change literal-expression to "{" [s] (quoted / unquoted-numeric) [s annotation] [s] "}"