Haskell - AD Reflection - How does it work?

I have seen the ad package and i understand how it does automatic differentiation by providing a different instance of the class Floating and then implementing the rules of derivatives.

But in the example

Prelude Debug.SimpleReflect Numeric.AD> diff atanh x
recip (1 - x * x) * 1

We see that it can represent functions as ASTs and show them as a string with variable names.

I wonder how they did that, because when i write:

f :: Floating a => a -> a
f x = x^2

No matter what instance I provide, i will get a function f :: Something -> Something and not a representation like f :: AST, or f :: String

The instance cannot "know" what the parameters are.

How they are able to do it ?