# Representing cyclic data structures efficiently in Haskell

Hal Daume t-hald@microsoft.com
Mon, 7 Jul 2003 08:15:56 -0700

>From reading your message, it seems you want a graph library of sorts.
I've used FGL extensively and found it to be very nice.  It even
provides both a purely functional interface as well as an
imperative-esque interface.  It also comes with a bunch of standard
algorithms built in, though I don't know enough about circuitry stuff to
know whether those would be useful or not.

If FGL seems like overkill to you, I have my own little mini graph
library which basically looks like:

- An ADT, Node:  newtype Node =3D Node Int deriving ...
- The 'Graph n e' data structure, containing:
- a FiniteMap from n -> Node
- an (growable?) array of type IOArray (Node,Node) e

the array represents the edges in the top half (i do some index fiddling
to get this efficient) and the finitemap represents the node labellings.
It's a pretty stupid interface and works a lot better when the graphs
are fixed size (that way you don't need to grow the array).  But it
works.

--
Hal Daume III                                   | hdaume@isi.edu
"Arrest this man, he talks in maths."           | www.isi.edu/~hdaume

> -----Original Message-----
> Sent: Monday, July 07, 2003 8:04 AM
> Subject: Representing cyclic data structures efficiently in Haskell
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>=20
> What is the best way to represent cyclic data structures in Haskell?
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> Lists are trivial. Trees are trivial. But what if the thing you're=20
> trying to represent is shaped like an electronic circuit, with a=20
> (possibly large) number of nodes connected by multiple arrows=20
> to other=20
> nodes? In an imperative language like C++, I'd probably just=20
> model the=20
> circuit directly, with objects representing nodes and pointers=20
> representing links between nodes. A good way to model this in=20
> less obvious, however.
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> A simplistic approach would be to represent such a structure=20
> as a list=20
> of nodes, where each node contained a list of other connected nodes.=20
> Containing the actual nodes within the sub-lists probably=20
> isn't feasible=20
> -- some kind of reference would be necessary in order to get around=20
> chicken-and-egg problems with instantiating the data=20
> structure. But, in=20
> this case, it's not so easy to see how one could 'walk' the structure=20
> efficiently, because moving from node to node would involve quite=20
> horrible (possibly slow) lookups. In C++, I'd simply follow a=20
> pointer,=20
> which would be an extremely fast operation.
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> I would also need to implement efficient indexes into the=20
> data structure=20
> -- flat searching will be too slow for non-trivial amounts of=20
> data. In=20
> C++, I'd implement one or more external (probably STL-based) indexes=20
> that point into the main data structure.
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> How do people typically solve this problem at the moment? I=20
> know a few=20
> people out there have written hardware compilers in Haskell=20
> and similar=20
> languages, so there should be some experience of this in the=20
> community.=20
> I can probably work something out soon enough, but=20
> reinventing the wheel=20
> is never a great idea.
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> --=20
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