parser: enable record types with optional fields

[HoneyryderChuck]

this enables richer definitions of record types, which can only be defined nowadays with Hash[Symbol, untyped].

To handle #504

[HoneyryderChuck]

@soutaro this is a draft proposal, as in, the optional fields are parsed and stored, but not used yet. If you agree with this, I can proceed and use it in inferences (may need some help with that eventually as well).

[soutaro]

@HoneyryderChuck Thanks! Let's go with the syntax.

Having different hash tables for required and optional fields would result breaking the order of the records, for Writer or rewriting features. I'd like to add #all_fields attribute and let #fields and #optional_fields fields filtered from #all_fields.

    class Record
      attr_reader fields: Hash[Symbol, [t, bool]]

      type loc = Location[bot, bot]

      def initialize: (fields: Hash[Symbol, t], location: loc?) -> void      # For compatibility with 3.4
                          | (all_fields: Hash[Symbol, [t, bool]], location: loc?) -> void

      include _TypeBase

      def fields: () -> Hash[Symbol, t]
      def optional_fields: () -> Hash[Symbol, t]?
    end

[HoneyryderChuck]

@soutaro can you check the changes? I believe this is what you were aiming for?

[soutaro]

Thanks @HoneyryderChuck! Looks great. Do you think it's ready to merge?

[soutaro]

Ah, I made a mistake to put ? at the end of the method type. It should return an empty hash.

Suggested change

	def optional_fields: () -> Hash[Symbol, t]?
	def optional_fields: () -> Hash[Symbol, t]

[HoneyryderChuck]

if you look at the implementation, nothing is returned if all_fields is the same as fields. I think that's fine, i.e. we can avoid creating that extra hash when not necessary. Or do you see any use for it?

[HoneyryderChuck]

Do you think it's ready to merge?

It depends: the optional field set is not yet being used in inferences. Do you think that's a separate change? If so, I'd say yes, it's ready to merge.

Separate change or not, will you do it, or shall I perhaps try it out? (pointers on what/where definitely welcome if so 🙏 )

[HoneyryderChuck]

@soutaro anything on the above?

[soutaro]

Thanks! 👍