[Haskell-cafe] Re: Updating doubly linked lists
apfelmus at quantentunnel.de
Fri Jan 2 05:50:26 EST 2009
S. Günther wrote:
>> Whether circular or not, sharing values by using a "back pointer" is
>> problematic for any update. Why not use a zipper instead?
> I looked into zippers before and the problem I had was that they never
> really matched the structure which I needed and which led me to think
> about this whole knot tying thing again.
What kind of structure do you need exactly?
> The things I read about them
> always assumed either a list like (i.e. linear) or a tree like (i.e. existence
> of a root) structure on the type to be plugged into the zipper.
Viewing the zipper as the derivative of a data type opens up more
That being said, every algebraic data types has a tree-like structure.
The extra invariants like
left . right = right . left
that the programmer imposes are what make them different from trees.
> So I just have to decide whether to use IORefs/Vars (clunky)
> or to implement zippers for the structure I need (probably too hard for me).
It's not too hard for you. You've got a whole haskell-cafe and #haskell
at your fingertips, after all. ;)
More information about the Haskell-Cafe