[Haskell-cafe] xml conduit

[Michael Sloan]

I'm no lens authority by any means, but indeed, it looks like something
like Cursor / Axis could be done with the lens zipper.

https://github.com/snoyberg/xml/blob/0367af336e86d723bd9c9fbb49db0f86d1f989e6/xml-enumerator/Text/XML/Cursor/Generic.hs#L38

This cursor datatype is very much like the (:>) zipper type (I'm linking to
old code, because that's when I understood it - the newer stuff is
semantically the same, but more efficient, more confusing, and less
directly relatable):

https://github.com/ekmett/lens/blob/f8dfe3fd444648f61b8594cd672c25e70c8a30ff/src/Control/Lens/Internal/Zipper.hs#L317

Which is built out of the following two datatypes:

1) parent (and the way to rebuild the tree on the way back up) is provided
by this datatype:

https://github.com/ekmett/lens/blob/f8dfe3fd444648f61b8594cd672c25e70c8a30ff/src/Control/Lens/Internal/Zipper.hs#L74

2) precedingSibling / followingSibling / node is provided by this datatype
(which is pretty much the familiar list zipper!):

https://github.com/ekmett/lens/blob/f8dfe3fd444648f61b8594cd672c25e70c8a30ff/src/Control/Lens/Internal/Zipper.hs#L317


One way that this would be powerful is that some of the Axis constructors
could return a zipper.  In particular, all of the axis yielding functions
except the following would be supported:

parent, precedingSibling, followingSibling, ancestor, descendent, orSelf,
check

This is because zippers can be used for modification, which doesn't work
out very well when you can navigate to something outside of your focii's
children.  If we have a new datatype, that represents a node's payload,
then we could conceivably represent all of the axis yielding operations
except for parent / ancestor.  However, those operations would be
navigations to payloads - further xml-hierarchy level navigation would be
impossible because you'd no longer have references to children.  (further
navigation into payloads on the other hand, would still be possible)

So, that's just my thoughts after looking at it a bit - I hope it's
comprehensible / helpful!  An XML zipper would be pretty awesome.

-Michael


On Sun, Feb 10, 2013 at 8:34 PM, Michael Snoyman <michael at snoyman.com>wrote:

>
>
>
> On Sun, Feb 10, 2013 at 8:51 PM, grant <thelff at hotmail.com> wrote:
>
>> Michael Snoyman <michael <at> snoyman.com> writes:
>>
>> >
>>
>> Hi Michael,
>>
>> Just one last thought. Does it make any sense that xml-conduit could be
>> rewritten as a lens instead of a cursor? Or leverage the lens package
>> somehow?
>>
>>
> That's a really interesting idea, I'd never thought about it before. It's
> definitely something worth playing around with. However, I think in this
> case the Cursor is providing a totally different piece of functionality
> than what lenses would do. The Cursor is really working as a Zipper,
> allowing you to walk the node tree and do queries about preceding and
> following siblings and ancestors.
>
> Now given that every time I'm on #haskell someone mentions zippers in the
> context of lens, maybe lens *would* solve this use case as well, but I'm
> still a lens novice (if that), so I can't really speak on the matter. Maybe
> someone with more lens experience could provide some insight.
>
> Either way, some kind of lens add-on sounds really useful.
>
> Michael
>
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