[Haskell-cafe] ANN: Monad.Reader special Poetry and Fiction Edition

[Brent Yorgey]

On Wed, Mar 16, 2011 at 03:03:20PM +0100, Yves Parès wrote:
> Concerning this exercise, would it be simply possible to take the most of
> lazy evaluation and build a graph?
> 
> A node would be:
> Node a = Node { data    :: a,
>                 parents :: [Node a],
>                 children :: [Node a] }
> 
> Then, whichever node you are on, you can still directly find its
> predecessors, i.e. your way back in the labyrinth.
> I find it more simple than a zipper (of course, now you cannot serialize it,
> due to the cross-references).

This kind of "knot-tying" approach is nice for static graphs.
However, update is quite complicated and definitely not constant time
(you essentially have to rebuild the entire graph).  A zipper allows
O(1) updates to the current location.

-Brent