Extend the syntax and semantics of functions to support explicit generic functions, which can bind type parameters as well as data parameters.
## Motivation
Currently Luau allows generic functions to be inferred but not given explicit type annotations. For example
```lua
function id(x) return x end
local x: string = id("hi")
local y: number = id(37)
```
is fine, but there is no way for a user to write the type of `id`.
## Design
Allow functions to take type parameters as well as function parameters, similar to Java/Typescript/...
```lua
function id<a>(x : a) : a return x end
```
Functions may also take generic type pack arguments for varargs, for instance:
```lua
function compose<a...>(... : a...) -> (a...) return ... end
```
This change is *not* only syntax, as explicit type parameters need to be part of the semantics of types. For example, we can define a generic identity function
```lua
local function id(x) return x end
local x: string = id("hi")
local y: number = id(37)
type Id = typeof(id)
```
and two functions
```lua
function f()
return id
end
function g()
local y
function oh(x)
if not(y) then y = x end
return y
end
return oh
end
```
The types of these functions are
```lua
f : () -> <a>(a) -> a
g : <a>() -> (a) -> a
```
so this is okay:
```lua
local i: Id = f()
local x: string = i("hi")
local y: number = i(37)
```
but this is not:
```lua
-- This assignment shouldn't typecheck!
local i: Id = g()
local x: string = i("hi")
-- This is unsound, since it assigns a string to a variable of type number
local y: number = i(37)
```
Currently, Luau does not have explicit type binders, so `f` and `g` have the same type. We propose making type binders part of the semantics of types as well as their syntax (so `f` and `g` have different types, and the unsound example does not typecheck).
We propose supporting type parameters which can be instantiated with any type (jargon: Rank-N Types) but not type functions (jargon: Higher Kinded Types) or types with constraints (jargon: F-bounded polymorphism).
## Drawbacks
This is a breaking change, in that examples like the unsound program above will no longer typecheck.
Types become more complex, so harder for programmers to reason about, and adding to their space usage. This is particularly noticeable anywhere the typechecker has exponential blowup, since small increases in type size can result in large increases in space or time usage.
Not having higher-kinded types stops some examples which are parameterized on container types, for example:
```lua
function g<c>(f : <a>(a) -> c<a>) : <b>(b) -> c<c<b>>
return function(x) return f(f(x)) end
end
```
Not having bounded types stops some examples like giving a type to the function that sums an non-empty array:
```lua
function sum(xs)
local result = x[0]
for i=1,#xs
result += x[i]
end
return result
end
```
## Alternatives
We did originally consider Rank-1 types, but the problem is that's not backward-compatible, as DataBrain pointed out in the [Dev Forum](https://devforum.roblox.com/t/luau-recap-march-2021/1141387/29), since `typeof` allows users to construct generic types even without syntax for them. Rank-1 types give a false positive type error in this case, which comes from deployed code.
We could introduce syntax for generic types without changing the semantics, but then there'd be a gap between the syntax (where the types `() -> <a>(a) -> a` and `<a>() -> (a) -> a` are different) and the semantics (where they are not). As noted above, this isn't sound.
Rather than using Rank-N types, we could use SML-style polymorphism, but this would need something like the [value restriction](http://users.cis.fiu.edu/~smithg/cop4555/valrestr.html) to be sound.
