Eq & Ord instance for Cursor.

[kindaro]

I observe there are functions in the record for Cursor (precedingSibling' & followingSibling') that prevent me from deriving an Eq or Ord instance and thus, for example, from using Cursor with an ordinary Set, or applying many ordinary list processing functions such as nub. I notice, further, that the only way to construct a Cursor, a function toCursor' found in Text.XML.Cursor.Generic, can accept various functions to put in precedingSibling' & followingSibling', but its only invocation from toCursor specializes both of them to id.

In the light of these observations, could it be possible to simplify the definition of Cursor so as to make a derived Eq instance possible? Or, if not, could you suggest some other way of defining a custom Eq instance that makes sure any further operations on the equal cursors will have equal results? It would be nice if an Ord instance gets to be possible as well.

So far, I had to to toMarkup my cursors and only then compare. This is somewhat uncomfortable. Upon receiving directions, I could attempt to my ability at composing a pull request.

[kindaro]

I looked better into the code and yes, we cannot just remove these functions from Cursor because they appear to constitute a major part of the "zipper" algorithm used in the definition of toCursor'. What we can do, and what I actually tried just now, is evaluate a cursor as it comes out of toCursor, and then tediously fix the axes that used to evaluate it themselves. After about a dozen small and trivial modifications, the module compiles and the package passes the tests, with Cursor now featuring derived Eq & Ord instances, as desired.

It can benefit from some cleanup but I am generally ready to submit a pull request. However, if there are programs in the wild that make use of the inner workings of Cursor, they will definitely break.

[snoyberg]

Due to the knot tying, Cursor is a cyclic data structure. Have you tried using the Eq and Ord instances you've defined? Do they work, or do they loop infinitely?