Reading GraphML in Haskell

Question

I'm trying to read a GraphML file containing a single directed Graph into a Haskell Data.Graph in order to run an analysis using the Math.Combinatorics.Graph module.

However, I can't find any module that allows me to read a GraphML file, producing a Data.Graph. One related module I found is ForSyDe.Backend.GraphML. However, this seems to be specific to the ForSyDe DSL and I currently can't think of a way to use it to read a plain Data.Graph.

Could you point me to a library allowing me to read GraphML, preferably with some example code on how to use it?

Answer 1

After more than a week of searching, I assume there is currently no GraphML parser library in existence. Therefore I wrote my own minimal parser.

Let's assume we have this GraphML:

<?xml version="1.0" encoding="UTF-8"?>
<graphml xmlns="http://graphml.graphdrawing.org/xmlns"  
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns
     http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd">
  <graph id="G" edgedefault="undirected">
    <node id="n0"/>
    <node id="n1"/>
    <node id="n2"/>
    <node id="n3"/>
    <edge id="e1" source="n0" target="n1"/>
    <edge id="e1" source="n1" target="n2"/>
    <edge id="e1" source="n1" target="n3"/>
    <edge id="e1" source="n3" target="n0"/>
  </graph>
</graphml>

I created this HXT-based parser that's able to parse a minimal subset of GraphML (just enough to create a Data.Graph of the above GraphML). The main function of the following file represents an example of how to use it: It prints the list of nodes in the graph (also see this related question ).

{-# LANGUAGE Arrows, NoMonomorphismRestriction #-}
import Text.XML.HXT.Core
import qualified Data.Graph as DataGraph

data Graph = Graph
  { graphId :: String,
    nodes :: [String],
    edges :: [(String, String)] -- (Source, target)
  }
  deriving (Show, Eq)

atTag tag = deep (isElem >>> hasName tag)

parseEdges = atTag "edge" >>>
  proc e -> do
      source <- getAttrValue "source" -< e
      target <- getAttrValue "target" -< e
      returnA -< (source, target)

parseNodes = atTag "node" >>>
  proc n -> do
      nodeId <- getAttrValue "id" -< n
      returnA -< nodeId

parseGraph = atTag "graph" >>>
  proc g -> do
      graphId <- getAttrValue "id" -< g
      nodes <- listA parseNodes -< g
      edges <- listA parseEdges -< g
      returnA -< Graph{graphId=graphId, nodes=nodes, edges=edges}

getEdges = atTag "edge" >>> getAttrValue "source"

-- Get targets for a single node in a Graph
getTargets :: String -> Graph -> [String]
getTargets source graph = map snd $ filter ((==source).fst) $ edges graph

-- Convert a graph node into a Data.Graph-usable
getDataGraphNode :: Graph -> String -> (String, String, [String])
getDataGraphNode graph node = (node, node, getTargets node graph)

-- Convert a Graph instance into a Data.Graph list of (node, nodeid, edge) tuples
getDataGraphNodeList :: Graph -> [(String, String, [String])]
getDataGraphNodeList graph = map (getDataGraphNode graph) (nodes graph)

main :: IO()
main = do
    graphs <- runX (readDocument [withValidate no] "foo.graphml" >>> parseGraph)
    --  Convert Graph structure to Data.Graph-importable tuple list
    let graphEdges = getDataGraphNodeList $ head graphs
    -- Convert to a Data.Graph
    let (graph, vertexMap) = DataGraph.graphFromEdges' graphEdges
    -- Example of what to do with the Graph: Print vertices
    print $ map ((\ (vid, _, _) -> vid) . vertexMap) (DataGraph.vertices graph)