[Haskell-cafe] fast graph algorithms without object identities

It seems that algorithms on graphs can be implemented particularly efficient in low-level languages with pointers and in-place updates. E.g. topological sort needs only linear time, provided that dereferencing pointers requires constant time. I could simulate pointer dereferencings and pointer updates by Map yielding linear logarithmic time for topological sort. I wonder if it is possible to write a linear time topological sort using lazy evaluation, since the runtime system of Haskell implementations is a graph processor based on pointers.