What does "Recursive Type Being used with Different Parameters" mean?

[mheiber]

What does this error message mean? Why does it happen

type Foo<T> = {a: Foo<number>, b: T}
type Bar<T> = Foo<Foo<T>> -- TypeError: recursive type being used with different parameters

Is this restriction in order to avoid non-contractive types?

The reason that I'm asking is that I'm working on a type checker for Erlang, which also has expressive generic aliases and untagged unions. I'm really impressed with Luau's type aliases and wonder how you got so much expressivity without infinite loops in the checker like racket/typed-racket#1130 and josefs/Gradualizer#322. Most languages don't accept definitions like the following, which luau accepts:

type Tree<T> = 
    {}
    | Children<Tree<T>>

type Children<T> = {left: T, right: T}

Beautiful language, thanks for open-sourcing!

[zeux]

This is a side-effect of https://github.com/Roblox/luau/blob/master/rfcs/recursive-type-restriction.md. We introduced this to keep generic types from expanding indefinitely during generic argument substitution. @asajeffrey was looking into whether we can make the restriction a bit more lenient without introducing issues for type checker, see #86 (which is still a draft for now)

[mheiber]

Thank you for these resources.

Is this a bug?

type Swap1<A, B> = {rec: Swap1<B, A>} -- TypeError: Recursive type being used with different parameters

[asajeffrey]

That's an example of a type that's rejected by the current restriction, but it would be sound to admit. The problem is that is we admit types like that, it's very easy to construct types with exponential blowup.