Add Record, Tuple, Map, and Set

[nikomatsakis]

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Sebastien Markbage suggested the addition of four kinds of value types:

• Record
• Tuple
• Map
• Set

These are all things that conceptually could be built on what was written up till now but it would be a pain and likely far less efficient.

Records and tuples are both structural variations on the existing nominal types. So you could create a record or tuple without having to declare the type ahead of time. Creating a record like {f, g} would be roughly equivalent to dynamically creating a struct type like new ValueType({f: Any, g: Any}). Maps and sets are persistent maps/sets, they have a get API. Each of these would have a global wrapper Record and Tuple and hence would get a global prototype.

for in order for records should be consistent no matter the order of creation. map and set should be in the order of insertion.

[nikomatsakis]

cc @sebmarkbage

[sebmarkbage]

I created a separate repo for this proposal where I will expand on helpful prototypes of these built-ins:

https://github.com/sebmarkbage/ecmascript-immutable-data-structures