RFC: Syntax for read- and write- access to properties (#15)

* RFC for property access modifier syntax.

Co-authored-by: Alexander McCord <11488393+alexmccord@users.noreply.github.com>

docs/syntax-property-access-modifiers.md

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+# Syntax for table property access modifiers
+
+## Summary
+
+We need syntax to match the semantics of read-only and write-only modifiers for table properties and indexers.
+
+## Motivation
+
+See the semantic RFCs for motivation:
+
+* [Read-only table properties](https://github.com/luau-lang/rfcs/blob/master/docs/property-readonly.md)
+* [Write-only table properties](https://github.com/luau-lang/rfcs/blob/master/docs/property-writeonly.md)
+
+## Design
+
+We will use the following syntax for describing a read or a write type of a property:
+
+```lua
+type ReadOnly = { read x: number }
+type WriteOnly = { write x: number }
+```
+
+A property will occasionally be both readable and writable, but using different
+types.  The author will have to duplicate the property name in this case:
+
+```lua
+type Foo = {
+    read p: Animal,
+    write p: Dog
+}
+```
+
+The tokens `read` and `write` are contextual.  They are still valid property names.
+
+```lua
+type Reader = { read: () -> number }
+type Writer = { write: (number) -> () }
+```
+
+Indexers can also be read-only or write-only.
+
+```lua
+type ReadOnlyMap<K, V> = { read [K]: V }
+type WriteOnlyMap<K, V> = { write [K]: V }
+
+type ReadAnimals = { read Animal }
+type WriteDogs = { write Dog }
+```
+
+Mixed indexers are allowed but heavily discouraged:
+
+```lua
+type MixedMap = { read [string]: Animal, write [string]: Dog }
+type MixedArray = { read Animal, write Dog }
+```
+
+Redundant record fields are still disallowed: Each field may have at most one
+read type and one write type:
+
+```lua
+type A = { read x: string, write x: "Hello" } -- OK
+type C = { read x: string, read x: "hello" }  -- ERROR
+type B = { x: string, read x: "hello" }       -- ERROR
+```
+
+We place no restriction on the relationship between the read and write type.
+The following is certainly a bad idea, but it is legal:
+
+```lua
+type T = { read n: number, write n: string }
+```
+
+This syntax is readable, matches the flavour of preexisting Luau syntax well,
+and is pretty easy to parse efficiently.
+
+## Drawbacks
+
+We expect it to be very commonplace to have table types containing read-only
+functions.  This syntax is a little bit verbose and maybe unintuitive for that
+use case.
+
+`read` and `write` are also very useful method names.  It's a little bit
+awkward to talk about a table that has a `read` or a `write` method:
+
+```lua
+type Reader = { read read: () -> number }
+type Writer = { read write: (number) -> () }
+```
+
+It is important to consider that this will be an issue for any keywords we might
+choose unless we were to take the step of picking something unlikely to be a
+useful property name. (ie something weird looking and ugly)
+
+Lastly, mixed indexers are very awkward both in syntax and semantics.
+
+## Alternatives
+
+The design space for syntax includes:
+
+* Names, symbols or attributes?
+* Modifier position?
+* How to parse or serialize properties with differing read- and write-types?
+
+### Names, symbols or attributes?
+
+We could use names for modifiers, such as
+
+* `get` and `set`
+* `const` and `mut`
+* `read` and `write`
+* `readonly` and `writeonly`
+
+One issue is that these are all valid identifiers, so if we want
+backward compatbility, they cannot be made keywords. This presents
+issues with code that uses the chosen names as type or property names,
+for instance:
+
+```lua
+  type set = { [any] : bool }
+  type ugh = { get set : set }
+```
+
+We could use symbols, for example
+
+* `+` and `-`
+* (Are there other obvious pairs of symbols?)
+
+We could use attributes, for example
+
+* `@read` and `@write`
+* ... as per "names for modifiers", only prefixed by `@` ...
+
+These both have the advantage of being unambiguous and easier to parse. Symbols
+are terser, whch is both good and bad.
+
+We decided not to use glyphs because they are more difficult to understand,
+don't contribute very much, and aren't very stylistically consistent with other
+Luau syntax.
+
+### Modifier position?
+
+For attributes, the position is given by the syntax of attributes, for example:
+
+```lua
+  type Vector2 = { @read x: number, @read y : Number }
+```
+
+For the other proposals, there are four possibilities, depending on whether the
+modifier is west-coast or east-coast, and whether it modifies the propertry name
+or the type:
+
+```lua
+  type Vector2 = { read x : number, read y : number }
+  type Vector2 = { x read : number, y read : number }
+  type Vector2 = { x : read number, y : read number }
+  type Vector2 = { x : number read, y : number read }
+```
+
+The east-coast options are not easy-to-read with names, but are
+easier with symbols, especially since `T?` is already postfix, for
+example
+
+```lua
+  type Foo = { p: number?+ }
+```
+
+### How do we talk about properties with differing read- and write-types?
+
+One corner case is that type inference may deduce different read- and
+write-types, which need to be presented to the user. For example the
+read-type of `x` is `Animal` but its write-type is `Dog` in the principal type of:
+
+```lua
+   function f(x)
+     let a: Animal = x.pet
+     x.pet = Dog.new()
+     return a
+   end
+```
+
+If we are adding the modifier to the property name, we can repeat the name, for example
+
+```lua
+  x : { read pet : Animal, write pet : Dog }
+```
+
+If we are adding the modifier to the property type, we can give both types, for example:
+```lua
+  x : { pet : read Animal + write Dog }
+```
+
+This syntax plays well with symbols for modifiers, for example
+```lua
+  x : { pet : +Animal -Dog }
+```