RFC: Type Namespaces

rfcs/type-namespaces.md

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+# Type Namespaces
+
+## Summary
+
+Add user syntax to define namespaces for type declarations.
+
+## Motivation
+
+When working with multiple modules in a project, it is common for users to use a separate module solely for containing type declarations to avoid having cyclic requires in the typechecker. One of these modules may look like this:
+
+```lua
+export type ShapeType = "Triangle" | "Square" | "Circle"
+
+export type ObjectParams = {
+	Name: string,
+	Shape: ShapeType,
+	Position: Vector3
+}
+
+return nil
+```
+
+An added benefit of this pattern is that, when the above module is required, a namespace is created for the exported types in the module's name:
+
+```lua
+local Types = require("Types")
+
+local MyShape: Types.ShapeType = "Circle"
+
+local Object = {
+	Name = "MyObject",
+	Shape = MyShape,
+	Position = Vector3.zero
+} :: Types.ObjectParams
+```
+This RFC aims to streamline defining type namespaces like the above without having to use modules as well as allowing for multi-dimensional namespaces to help organize large projects (i.e., `Types.Shape.ObjectParams`).
+
+## Design
+
+Namespaces could be added to any type declaration via a dot to discern between space name and type name:
+
+```lua
+type Space.Name = number
+
+local Foo: Space.Name = 1
+```
+
+This syntax could also allow for nested namespaces as well as generated namespaces for exported types:
+
+```lua
+export type Types.Space.Name<A, B> = A | B
+```
+
+## Drawbacks
+
+This syntax design may introduce repetition for multiple types as it is by declaration-basis. For example:
+
+```lua
+export type Space.Foo = number | string
+export type Space.Bar = "Hello" | "World"
+export type Space.Baz = boolean
+```
+
+Mistypes could confuse users as they create a new namespace for the type:
+
+```lua
+export type Space.A = number
+export type Spaze.B = string
+```
+
+Nested spaces could also confuse users since namespaces could be mingled with actual types:
+
+```lua
+local Foo: Space.Foo = 1
+local Bar: Space.Bar.Foo = true
+```
+
+## Alternatives
+
+Using the `in` keyword instead:
+
+```lua
+type Name in Space = number
+```
+
+Having overarching syntax for namespaces:
+
+```lua
+export Space
+	type foo = number,
+	type bar = string
+```