Pangraph.GraphML.Writer does not write VertexID etc.

[debug-ito]

I don't think Pangraph.GraphML.Writer writes VertexID of a Vertex or edgeID, source or target of an Edge. This makes the result GraphML data invalid.

[snowleopard]

@debug-ito Thanks for the report -- could you give a simple example with an expected and produced output?

[debug-ito]

Sure.

{-# LANGUAGE OverloadedStrings #-}
module Main (main) where

import qualified Data.ByteString as B
import Pangraph (makePangraph, makeVertex, makeEdge)
import qualified Pangraph.GraphML.Writer as GraphML

main :: IO ()
main = B.putStrLn $ GraphML.write graph
  where
    (Just graph) = makePangraph vertices edges
    vertices = [ makeVertex "foo" [],
                 makeVertex "bar" [],
                 makeVertex "buzz" []
               ]
    edges = [ makeEdge ("foo", "bar") [],
              makeEdge ("bar", "buzz") [],
              makeEdge ("buzz", "foo") []
            ]

Got:

<?xml version="1.0" encoding="UTF-8"?>
<graphml xmlns="http://graphml.graphdrawing.org/xmlns" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <graph id="G" edgedefault="undirected">
    <node/>
    <node/>
    <node/>
    <edge/>
    <edge/>
    <edge/>
  </graph>
</graphml>

Expected:

<?xml version="1.0" encoding="UTF-8"?>
<graphml xmlns="http://graphml.graphdrawing.org/xmlns" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <graph id="G" edgedefault="undirected">
    <node id="foo"/>
    <node id="bar"/>
    <node id="buzz"/>
    <edge source="foo" target="bar"/>
    <edge source="bar" target="buzz"/>
    <edge source="buzz" target="foo"/>
  </graph>
</graphml>

[snowleopard]

@debug-ito Great, thank you!

I'm not sure I'll find time for this soon, so PRs are welcome.

(Or, perhaps, @thisiswhereitype could have a look.)

[debug-ito]

OK, I'll make a pull-request.

[debug-ito]

Wait, I just checked test/GraphML.hs. That test deals with Pangraph.Examples.SampleGraph.sampleGraph, which is defined as:

smallGraph :: Pangraph
smallGraph = fromJust graph
  where
    graph =
      makePangraph
        [makeVertex "n0" [("id","n0")]
        ,makeVertex "n1" [("id","n1")]
        ,makeVertex "n2" [("id","n2")]]
        [makeEdge ("n0", "n2") 
          [("source","n0"),("target","n2")]]

So, according to this test, users of Pangraph are supposed to make Vertex with the explicit id attribute and Edge with the explicit source and target attributes, aren't they? Is that the expected usage of Pangraph?

[snowleopard]

@debug-ito The main purpose of Pangraph is to serve as a bridge for converting between various graph formats. It's internal graph representation is not really intended to be used for graph description -- for that you can use other graph libraries, which may be more suited for the task. With that in mind, I think the answer to your question

Is that the expected usage of Pangraph?

is negative: No, we do not expect the internal graph representation of Pangraph to be used, and it may change in future.

However, it is still important to have good documentation for the library to avoid confusion. So, if you think that the library's README or Haddock documentation can be improved, please let us know. Thank you!