[Haskell-beginners] Question about data structures

[Brent Yorgey]

On Thu, Nov 25, 2010 at 09:12:05AM -0800, Russ Abbott wrote:
> Thanks, Patrick. I'm disappointed, though, that no one has
> actually responded to my question. It wasn't how to solve KenKen. It was how
> best to deal with quasi-mutable data structures.
> *
> -- Russ *

Someone did respond to your question, with a link to the data-accessor
package on Hackage (you could also take a look at fclabels or
lenses).  Do those help address your issue?

-Brent

> 
> 
> 
> On Thu, Nov 25, 2010 at 6:17 AM, Patrick LeBoutillier <
> patrick.leboutillier at gmail.com> wrote:
> 
> > Russ,
> >
> > If I understand correctly KenKen is something like Sudoku except that
> > the (more complicated) "cages"
> > constraints replace the usual "square" constraints.
> >
> > Have you seen these sudoku solvers:
> > http://www.haskell.org/haskellwiki/Sudoku ?
> >
> > Maybe you can get ideas there on how to attack the problem in a more
> > functional fashion.
> >
> >
> > Patrick
> >
> >
> > On Wed, Nov 24, 2010 at 9:58 PM, Russ Abbott <russ.abbott at gmail.com>
> > wrote:
> > > My previous two messages suggest a Haskell coding principle: distinguish
> > > between fixed and (quasi-)mutable data structures. (I know values don't
> > > change, but I hope you understand what I mean. The cage and the cell in
> > my
> > > previous example are quasi-mutable. They are conceptually mutable in the
> > > context of the problem.)  The fixed data structures can be organized any
> > way
> > > one wants. The quasi-/conceptually-mutable elements should be referred to
> > > symbolically and stored in a Map. The maps themselves should be stored at
> > a
> > > global level of the system's data structure so that it is easy to replace
> > > them when values change.
> > >
> > > -- Russ
> > >
> > >
> > > On Wed, Nov 24, 2010 at 5:54 PM, Russ Abbott <russ.abbott at gmail.com>
> > wrote:
> > >>
> > >> Actually using a Map does solve the problem. The Map has to be kept at
> > the
> > >> level of the Tree rather than have each leaf node point to it.  So
> > instead
> > >> of just a Tree one has, say (Map, Tree). Then when one wants to change
> > the
> > >> property of something associated with a leaf node, one can just change
> > the
> > >> map. The Tree is unchanged.
> > >>
> > >> -- Russ
> > >>
> > >>
> > >>
> > >> On Wed, Nov 24, 2010 at 2:02 PM, Russ Abbott <russ.abbott at gmail.com>
> > >> wrote:
> > >>>
> > >>> OK. So putting a Map at Leaf nodes doesn't solve the problem.
> > >>> (Apparently I haven't been able to communicate what I see as the
> > problem.)
> > >>> The problem that I'm trying to get to is the need to write excessive
> > code
> > >>> for something that would require a lot less code in an OO world.  It's
> > not a
> > >>> matter of execution time or space. It's a matter of the amount of code
> > one
> > >>> is required to write.
> > >>> -- Russ
> > >>>
> > >>>
> > >>> On Wed, Nov 24, 2010 at 1:52 PM, Daniel Fischer
> > >>> <daniel.is.fischer at web.de> wrote:
> > >>>>
> > >>>> On Wednesday 24 November 2010 22:12:37, Russ Abbott wrote:
> > >>>> > Cool. I wasn't aware of that notation.  It doesn't quite get to the
> > >>>> > issue though.
> > >>>> >
> > >>>> > The problem I'm concerned about is the need to define y in the first
> > >>>> > place. If one is chasing through a data structure and finds a need
> > to
> > >>>> > change something buried within it, it seems necessary to rebuild
> > >>>> > everything that includes the changed thing.
> > >>>>
> > >>>> In general, values are immutable, so you can't "change something
> > buried
> > >>>> within it". You have to build a new value containing some of the old
> > >>>> stuff
> > >>>> and a new part. Building the new value usually consists mostly of
> > >>>> copying a
> > >>>> couple of pointers (plus building the new part of course), so isn't
> > too
> > >>>> expensive normally.
> > >>>>
> > >>>> You can have mutable values in the IO or (ST s) monads, if you need
> > >>>> them.
> > >>>>
> > >>>> > That is, I can't change a
> > >>>> > component of somethingNew without creating y. The point is there's
> > >>>> > nothing about x that changed,
> > >>>>
> > >>>> The thing with the changed component is not x anymore.
> > >>>>
> > >>>> > and there may be nothing about (var1 x)
> > >>>> > that changed, and there may be nothing about var11 . var1 $ x that
> > >>>> > changed, etc. Yet one is apparently forced to keep track of and
> > >>>> > reconstruct all those elements.
> > >>>>
> > >>>> The compiler takes care of that.
> > >>>>
> > >>>> >
> > >>>> > Another example is to imagine a Tree in which the leaves contain
> > >>>> > "objects." If I want to change a property of one of those leaf
> > >>>> > objects,
> > >>>>
> > >>>> You can't in general, the thing with a different property is a
> > different
> > >>>> object.
> > >>>>
> > >>>> > I am forced to rebuild all the ancestor nodes of that leaf down to
> > >>>> > rebuilding the root.
> > >>>>
> > >>>> Yes (well, not you, the compiler does it), except if your tree
> > contains
> > >>>> mutable objects (IORefs/STRefs for example).
> > >>>>
> > >>>> >
> > >>>> > One way to avoid that is for the leaves to refer to their objects
> > >>>> > through a Map. Then changing a leaf object requires only that the
> > >>>> > value
> > >>>> > associated with key represented by the leaf be (re-)inserted into
> > the
> > >>>> > Map.  The Tree itself need not change at all.
> > >>>>
> > >>>> Oh, it will. If you change a Map, you get a new one, thus you get a
> > new
> > >>>> tree containing the new Map.
> > >>>>
> > >>>> >
> > >>>> > But that trick isn't always available.  In the example we are
> > talking
> > >>>> > about we can't make a Map where they keys are the instance variable
> > >>>> > names and the values are their values.  That would seem to do the
> > job,
> > >>>> > but since the values are of different types, we can't create such a
> > >>>> > Map.
> > >>>> >
> > >>>> > So now what?
> > >>>>
> > >>>> Well, what's the problem with the compiler copying some nodes?
> > >>>> Normally, that doesn't cost very much performance, if it does in your
> > >>>> case,
> > >>>> we'd need to know more to suggest the best way to go.
> > >>>>
> > >>>> > *
> > >>>> > -- Russ *
> > >>>> >
> > >>>
> > >>
> > >
> > >
> > > _______________________________________________
> > > Beginners mailing list
> > > Beginners at haskell.org
> > > http://www.haskell.org/mailman/listinfo/beginners
> > >
> > >
> >
> >
> >
> > --
> > =====================
> > Patrick LeBoutillier
> > Rosemère, Québec, Canada
> >

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