`index` type operators

Summary

This RFC proposes the addition of one type operator, index, which can be used to look up a specific property of another type (like TypeScript's Indexed Access Type).

Motivation

The primary motivation of this proposal is to allow Luau to define a type by accessing a property of another existing type. For instance, consider the following example code:

type Person = {
  age: number,
  name: string,
  alive: boolean
}

local bob: Person = {
  age = 22,
  name = "Bob",
  alive = true
}

local function doSmt(param: typeof(bob["age"])) -- param = number
  -- rest of code
end

type unionType = typeof(bob["age"]) | typeof(bob["name"]) | typeof(bob["alive"]) -- unionType = number | string | boolean

This is a valid Luau program; however, in order to define the type of Person["age"] we had to first declare a variable bob and utilize the typeof type operator. This is quite cumbersome when developers want to typecheck using the type of Person["age"] without having to declare a variable first. Additionally, in order to define the union type of all the properties of Person, current Luau requires an explicit list of each property using typeof.

The expected outcome of the index type operator is that it will enhance developer experience and allow Luau developers to more easily develop well-typed programs.

Design

The solution to this problem is a type operator, index, that can compute the type based on the static properties of Person. Formally, the index type operator will take in two arguments: the type to index and the type to index it with. This would allow us to instead write the following code:

type Person = {
  age: number,
  name: string,
  alive: boolean
}

local function doSmt(param: index<Person, "age">) -- param = number
  -- rest of code
end

type idxType = index<Person, keyof<Person>> -- idxType = number | string | boolean

type idxType2 = index<Person, "age" | "name"> -- idxType2 = number | string

Now, the type of doSmt()'s parameter can be defined without declaring a variable bob. Additionally, regardless of how the type Person grows, idxType will always be defined as the union of all the properties.

Error messages will be displayed for incorrect syntax. If the given type used to access a property is invalid,

type age = index<Person, "ager"> -- Error message: Property 'ager' does not exist on type 'Person'.

If the given value used to access a property is not a type,

local key = "age"
type age = index<Person, key> -- Error message: Type 'key' cannot be used as an index type.

Note: these errors will be part of the general type family reduction errors since index will be built into the type family system.

Implementation is straight forward: the type of the indexee will be determined (table, array, etc) -> search through the properties of the indexee and return the corresponding type of the indexer if it exists; otherwise, return an error or unknown type depending on the scope.

Drawbacks

A drawback to this feature is the possible increase in cost of maintenance. In the end, this RFC proposes adding another built-in type operators to the new type system. However, the addition of this feature may be worthwhile as the index type operator is a useful type feature that:

Alleviates the need to manually keep types in sync
Provides powerful way to access the properties of an object and perform various operations on them with other type operators
And ultimately, allows the community to write code with fewer errors and more safety

Alternatives

An alternative design can be depicted from the example below:

type Person2 = {
  age: number,
  name: string,
  alive: boolean,
  job: string
}

local function edgeCase(p: Person)
  type unknownType = index<typeof(p), "job">
end

In our current design, the program simply fails to reduce (and throws an error). However, it is worth noting that index<Person, "job"> can also reduce to type unknown if p is of type Person2 (this is allowed since tables support width subtyping; hence Person2 is a subtype of Person). In this design, we would need a way to determine if the indexee can be different at runtime. We could determine this through an implementation of more table types, specifically exact and inexact table types. Then, the program will have two cases when a indexee does not contain the indexer type:

If the indexee is an inexact table type, the expression is reduced to an unknown type.
If the indexee is an exact table type, the expression fails to reduce and throws and error.

Note: exact table types "indicates that the table has only the properties listed in its type" and inexact table type "indicates that the table has at least the properties listed in its type".

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index type operators