luau/rfcs/function-and-table-types.md

Add primitive function and table types

Summary

Add types for "real" functions and tables.

Motivation

Some APIs require "real" functions and tables, not just things that "look functiony" (e.g. tables with a __call__ metamethod) or "look tabley" (e.g. instances of classes). This RFC adds types for those.

Design

Add:

• a type table, inhabited by Luau tables (but not class instances), and
• a type function, inhabited by Luau functions (but not class methods or tables with metamethods).

Luau functions with known source and targe types are now an intersection type function & (T) -> U.

Luau tables with known properties are now an intersection type table & { p : T }.

We may want to provide syntax sugar function(T) -> U and table{ p : T } for these, since they will probably be quite common.

We should audit APIs to see which ones accept functiony or tably arguments, and which ones want real functions and tables.

Drawbacks

Another bit of complexity budget spent.

Alternatives

Stick with the current imprecision.