docs: explain type aliasing (#799)

docs/aliasing.md

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+# Type aliasing rules in Teal
+
+## The general rule
+
+In Teal we can declare new types with user-defined names. These are called
+_nominal types_. These nominal types may be unique, or aliases.
+
+The `local type` syntax produces a new _nominal type_. Whenever you assign to
+it another user-defined nominal type, it becomes a _type alias_. Whenever you
+assign to it a type constructor, it becomes a new unique type. Type
+constructors are syntax constructs such as: block constructors for records,
+interfaces and enums (e.g. `record` ... `end`); function signature
+declarations with `function()`; applications of generics with `<>`-notation;
+declarations of array, tuple or map types with `{}`-notation; or a primitive
+type name such as `number`.
+
+Syntax such as `local record R` is a shorthand to `local type R = record`, so
+the same rules apply: it declares a new unique type.
+
+Nominal types are compared against each other _by name_, but type aliases are
+considered to be equivalent.
+
+```lua
+local record Point3D
+   x: number
+   y: number
+   z: number
+end
+
+local record Vector3D
+   x: number
+   y: number
+   z: number
+end
+
+local p: Point3D = { x = 1.0, y = 0.3, z = 2.5 }
+
+local v: Vector3D = p -- Teal compile error: Point3D is not a Vector3D
+
+local type P3D = Point3D
+
+local p2: P3D
+
+p2 = p  -- ok! P3D is a type alias type Point3D
+p = p2  -- ok! aliasing works both ways: they are effectively the same type
+```
+
+Nominal types are compared against non-nominal types _by structure_, so that
+you can manipulate concrete values, which have inferred types. For example,
+you can assign a plain function to a nominal function type, as long as the
+signatures are compatible, and you can assign a number literal to a nominal
+number type.
+
+```lua
+local type MyFunction = function(number): string
+
+-- f has a nominal type
+local f: MyFunction
+
+-- g is inferred a structural type: function(number): string
+local g = function(n: number): string
+   return tostring(n)
+end
+
+f = g  -- ok! structural matched against nominal
+g = f  -- ok! nominal matched against structural
+```
+
+You can declare structural types for functions explicitly:
+
+```lua
+local type MyFunction = function(number): string
+
+-- f has a nominal type
+local f: MyFunction
+
+-- h was explicitly given a structural function type
+local h: function(n: number): string
+
+f = h  -- ok!
+h = f  -- ok!
+```
+
+By design, there is no syntax in Teal for declaring structural record types.
+
+## Some examples
+
+Type aliasing only happens when declaring a new user-defined nominal type
+using an existing user-defined nominal type.
+
+```lua
+local type Record1 = record
+   x: integer
+   y: integer
+end
+
+local type Record2 = Record1
+
+local r1: Record1
+assert(r1 is Record2) -- ok!
+```
+
+This does not apply to primitive types. Declaring a type name with the same
+primitive type as a previous declaration is not an aliasing operation. This
+allows you to create types based on primitive types which are distinct from
+each other.
+
+```lua
+local type Temperature = number
+
+local type TemperatureAlias = Temperature
+
+local type Width = number
+
+local temp: Temperature
+
+assert(temp is TemperatureAlias)  -- ok!
+assert(temp is Width)             -- Teal compile error: temp (of type Temperature) can never be a Width
+```
+
+Like records, each declaration of a function type in the program source code
+represents a distinct type. The `function(...):...` syntax for type
+declaration is a type constructor.
+
+```lua
+local type Function1 = function(number): string
+
+local type Function2 = function(number): string
+
+local f1: Function1
+
+assert(f1 is Function2) -- Teal compile error: f1 (of type Function2) can never be a Function1
+```
+
+However, user-defined nominal names referencing those function types can be
+aliased.
+
+```lua
+local type Function1 = function(number): string
+
+local type Function3 = Function1
+
+local f1: Function1
+assert(f1 is Function3) -- ok!
+```