Haskell — get TypeRep from concrete type instance

Question

I want to write a function with this type signature:

getTypeRep :: Typeable a => t a -> TypeRep

where the TypeRep will be the type representation for a, not for t a. That is, the compiler should automatically return the correct type representation at any call sites [to getTypeRep], which will have concrete types for a.

To add some context, I want to create a "Dynamic type" data type, with the twist that it will remember the top-level type, but not its parameter. For example, I want to turn MyClass a into Dynamic MyClass, and the above function will be used to create instances of Dynamic MyClass that store a representation of the type parameter a.