[Haskell] An Alternative Data.List.Zipper
dave at zednenem.com
Sun Jan 18 01:39:04 EST 2009
On Sat, Jan 17, 2009 at 9:13 PM, <roconnor at theorem.ca> wrote:
> On Sat, 17 Jan 2009, David Menendez wrote:
>> instance Applicative f => Applicative (Backwards f) where
>> pure = B . pure
>> f <*> x = B (unB f <**> unB x)
> probably should be f <*> x = B (unB x <**> unB f)
I always get that backwards.
> anyhow, this should be part of Control.Applicative.
I agree. But until it is, we'll have to make do.
>> This may be terminological confusion. I would have said that a
>> PointedList *is* a zipper.
> This is what I thought, and what how I initally advised Jeff. However after
> carefully reading both <http://en.wikibooks.org/wiki/Haskell/Zippers> and
> <http://www.cs.nott.ac.uk/~ctm/Dissect.pdf> (Clowns to the left of me,
> jokers to the right), it is clear that formally a zipper focuses on a
> sub-list / sub-tree rather than focusing on a particular node of a list or a
> tree. Hence zippers are not comonads.
Funny, I was just reading that paper earlier today.
You're right. I wasn't making enough of a distinction between zippers
and structures with one selected element. It's the latter which are
Dave Menendez <dave at zednenem.com>
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