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Re: [Haskell-cafe] writing graphs with do-notation

Subject: Re: [Haskell-cafe] writing graphs with do-notation
From: Neil Davies
Date: Sun, 13 Dec 2009 09:08:52 +0000

Surely there is somewhere in the haskell Twiki that something like this should live?

On 12 Dec 2009, at 21:00, Soenke Hahn wrote:


Some time ago, i needed to write down graphs in Haskell. I wanted to be able to write them down without to much noise, to make them easily maintainable. I came up with a way to define graphs using monads and the do notation. I thought this might be interesting to someone, so i wrote a small script to illustrate
the idea. Here's an example:

example :: Graph String
example = buildGraph $ do
   a <- mkNode "A" []
   b <- mkNode "B" [a]
   mkNode "C" [a, b]

In this graph there are three nodes identified by ["A", "B", "C"] and three edges ([("A", "B"), ("A", "C"), ("B", "C")]). Think of the variables a and b as outputs of the nodes "A" and "B". Note that each node identifier needs to be mentioned only once. Also the definition of edges (references to other nodes
via the outputs) can be checked at compile time.

The attachment is a little script that defines a Graph-type (nothing
elaborate), the "buildGraph" function and an example graph that is a little more complex than the above. The main function of the script prints the
example graph to stdout to be read by dot (or similar).

By the way, it is possible to define cyclic graphs using mdo (RecursiveDo).
I haven't come across something similar, so i thought, i'd share it.
What do
you think?

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