ValueConverter for custom values

[hallvard]

I'm experimenting with a language with several kinds of literals (data types), besides the typical ones (string, int, boolean) e.g. UUIDs, time of day. When traversing the AST I need to check the rule and convert the parsed string to the corresponding value type. An alternative is to register a value converter, which allows the converted value to be part of the AST (if I understand this correctly). It's more robust to check the value type when traversing the AST than checking the rule name. However, ValueConverter seems to only support a small set of basic types, at least its convert method returns ValueType, which is a union of number, bigint, string, Date, boolean.

Is there a way to make it support custom ones?

[msujew]

Hey @hallvard,

we built the ValueConverter mainly to perform simple conversion processes during the parsing processes, which would be annoying to handle after parsing. Stuff like removing the quotes from strings and converting a parsed number into an actual number in the JavaScript runtime, etc.

What is your exact need here? Do you want to transform a parsed AST node into something different?

[hallvard]

I'm writing a generator, and the conversion can either be done in the generator (later) or in the value converter (earlier). One of my literals is time-of-day, e.g. 12:07 (in general INT ':' INT, but with obvious constraints). I was wondering if I could write a value converter and find the converted value in the AST when traversing it in my generator, instead of having the conversion code in the generator. This way, the generator is slightly less grammar-specific, it just needs to handle the various kinds of value, rather than know about the rules.

E.g. declare TimeOfDayLiteral returns TimeOfDay: INT ':' INT and then write a value converter for it. The result would be the value of the assignment where TimeOfDayLiteral is one possibility.

[msujew]

While you could return something like type TimeOfDay = `{string}:{string}`; in your value converter, the main issue is that the AST generator won't know about the type (meaning that the AST doesn't know about it). The AST generator can only generate the types known as ValueType as a field type.