[Haskell-cafe] writing graphs with do-notation

[Neil Davies]

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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