luau/rfcs/constrained-generics.md

Constrained Generics

Summary

Allows you to give a function generic a constrained type.

Motivation

Right now, when you create a generic function, the generic paramter is assumed to be everything. This creates problems, especially with tables

local function SomethingWithAGenericTable<T>(tab: T)
  for _, v in t do --Cannot call non-function T (T is not a table)

  end
end

This constraining is mainly for concrete table types and functions, since types like number, string, and boolean are literal types.

Design

The way to implement this is to have new syntax under the generic's brackets: <T: typedef>. This would constrain the type from the generic def and give the typechecker better hints.

local function SomethingWithAGenericTable<T: {[any]: any}>(tab: T)
  for _, v in t do
    print(v)
  end
end

SomethingWithAGenericTable({meow = 5}) --OK
SomethingWithAGenericTable(5) --not OK

Drawbacks

This adds unusual syntax, as well as adding to the complexity of the language.

This also doesn't address how this should be approached around type packs.

Alternatives

Use something similar to Rust's where syntax:

local function SomethingWithAGenericTable<T>(tab: T) where T: {[string]: any}

Do nothing, and use the assert(type(T) == T) syntax, though this creates extra bytecode. It also casts the object as never, which removes the type errors, however no autocomplete information is generated.