[Haskell-cafe] FGL problem: cannot acces data constructor NodeMap

[Immanuel Normann]

2010/7/24 Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>

> Once again cc-ing -cafe; Immanuel, can you please do "reply to all"
> rather than just "reply" in GMail? :p
>

Oh, yes I forgot - sorry!

Thanks for all the valuable infos :-)

Immanuel

>
> Immanuel Normann <immanuel.normann at googlemail.com> writes:
>
> > 2010/7/24 Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>
> >
> >> CC-ing haskell-cafe again:
> >>
> >> Immanuel Normann <immanuel.normann at googlemail.com> writes:
> >>
> >> > 2010/7/24 Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>
> >>
> >> > Some of my lnodes in the graph have by construction unique labels and
> I
> >> want
> >> > to lookup the lnode from a graph given only the label of that lnode.
> >>
> >> Well, from my understanding NodeMap doesn't do what you want, since it
> >> assumes that _all_ of the node labels are unique
> >
> >
> > I wouldn't bother when NodeMap maps also non-unique labels to single
> nodes,
> > because I only lookup nodes with unique labels, e.g. assume my label are
> of
> > the form
> > data Label = Unique String | Multi Label
>
> I'm of the suspicion that using NodeMaps in this fashion will have
> problem when considering nodes with the same label (even if you don't
> explicitly use it for those nodes), as NodeMap seems to assume that node
> labels are unique.
>
> For example, mkNode assumes node labels are unique.
>
> >> Furthemore, the way of using it seems to be analogous of how one would
> use a
> >> State monad
> >> rather than for actual interactive use throughout.
> >>
> >
> > Could you elaborate this further, please.
>
>
> http://hackage.haskell.org/packages/archive/fgl/5.4.2.3/doc/html/Data-Graph-Inductive-NodeMap.html#v%3Arun
> and
>
> http://hackage.haskell.org/packages/archive/fgl/5.4.2.3/doc/html/Data-Graph-Inductive-NodeMap.html#v%3Arun_
>
> If you just want a mapping from labels to nodes, you may be better off
> using your own custom Map (and maybe even defining a new graph type that
> has such a Map as part of it).
>
> The successor to FGL-5.x (whatever we'll end up calling it) will allow
> you to have custom node types rather than using Ints.  However, this
> will not work for you in this situation.
>
> The node type is just an _index_ type, analogous to the actual hash
> value in a hash{map,array} (as opposed to the value which has the
> hashing function applied upon it).  It's there to provide a unique
> identifier for a particular node.  Arguably, an abstract type would be
> even better as an Int as it will prevent you from doing silly things
> like adding node indices together, but that's beside the point.
>
> If you wish to find a node with a specific label, try using something
> like gsel:
> http://hackage.haskell.org/packages/archive/fgl/5.4.2.3/doc/html/Data-Graph-Inductive-Basic.html#v%3Agsel
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> IvanMiljenovic.wordpress.com
>
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