>     > The Zipper technique is the generalization
>     > of this idea to arbitrary algebraic datatypes; it can be generalized
to
>     > higher-order data too (continuations).

I was too optimistic when I wrote this; someone actually e-mailed me asking
for a fuller explanation after I posted this, but I had to take this claim
back. (Unfortunately, I didn't correct myself in public.) At the time I had
been fooling around with the idea, but never figured out exactly how to
implement it.

> Is there some code around that illustrates the use of Zippers?  They
> sound very interesting.

I have never seen anyone use the technique in any distributed Haskell code.
Gerard Huet wrote the article that appeared in JFP, and I believe he
remarked that they had used the technique to represent ASTs in a structural
editor (perhaps in the Coq distribution?).

--FAC

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