Nested functors: Compiler can't determine type equality

[jrfondren]

@jbe

But still, isn’t a module instantiated twice with the same functor argument exactly the same module?

I don’t have a complete answer for you, but from Practical example of applicative vs generative functors? , that’s true if it’s an applicative functor vs. a generative one, and what you might be running into is discussed in Generative Functors , where there is a distinction to a functor over a module path vs. a functor over a module expression.