"Empty set" ambiguity

[lann]

An "empty set" value {} has two possible interpretations: a flags value with zero flags set or a record value with all field values set to none (because "Record entries with the option-typed value none may be omitted").

The WAVE parser in this repo avoids this ambiguity because it is type-driven, but any alternative implementation that wanted to parse to an AST would need to address it.

I see two obvious options:

• Disallow {} for records. A record with all fields values set to none would need to specify at least one of those fields explicitly, e.g. {optional-field: none}
• Give {} its own formal rule (e.g. empty-set := '{' '}') and accept the AST ambiguity. AST ambiguity is already present for other Component Model value representations such as various number types and bools (vs variant case labels).

[sunfishcode]

Additional options:

• Change flags to eg. |read, write|. (I know that looks like a closure in Rust/Ruby/etc., but I guess in context it'll be fine.)
• Parse {_} as "record without explicit fields", and parse {} as flags.

[lann]

Another option: {:} as "record without explicit fields", which would lead to the nice property that records always contain a : between braces.

[lann]

Yet another option: require that empty flag sets be expressed as ∅ :smile:

[lann]

20 adopts {:} for "empty" records.