Neat
Surely there is somewhere in the haskell Twiki that something like
this should live?
Neil
On 12 Dec 2009, at 21:00, Soenke Hahn wrote:
Hi!
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?
Sönke
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