This functor is essentially there because we use this same interface in different
contexts, with different types for 'a t
.
1) One use case for it is where 'a X.t = 'a Typerep.t
. These interfaces are then
part of the type witness built for a type containing a record or a variant in its
structure. traverse
will give a way of accessing the type representation for the
arguments of a variant or record type.
2) Another use case is for building "staged generic computations". In that case, the
type 'a X.t
is the type of the computation that is being built. traverse
returns the computation built for the argument. The interface no longer exports
the typerep of the arguments in hopes of enforcing that no typerep traversal
happens at runtime if the computation happen to be a function.