Easily compose operations such as addTriple, removeTriple, etc.

[koslambrou]

Question here.

In the rdf4h tutorial, adding triples to the graph is done the following way:

main :: IO ()
main = do
  -- empty list based RDF graph
  let myEmptyGraph = empty :: RDF TList

  triple1 = triple (unode "...") (unode "...") (unode "...")
  graph1 = addTriple myEmptyGraph triple1

  triple2 = triple (unode "...") (unode "...") (unode "...")
  graph2 = addTriple graph1 triple2

  graph3 = removeTriple graph2 triple1

  putStrLn (showGraph graph3)

Is it currently possible to simply combine those operations (addTriple, removeTriple) without creating temporary variables (graph1, graph2) ?

Something in monadic style would be nice. For example:

createGraph = do
  let triple1 = triple (unode "...") (unode "...") (unode "...")
  addTriple $ triple1
  addTriple $ triple (unode "...") (unode "...") (unode "...")
  removeTriple $ triple1

[koslambrou]

Here's the idea (the following code does not compile):

type RdfState rdfImpl m a = StateT (RDF rdfImpl) m a

addTriple :: (RDF.Rdf rdfImpl, Monad m)
              => RDF.Triple
               -> RdfState rdfImpl m ()
addTriple triple = do
  graph <- get
  put $ RDF.addTriple graph triple

main :: IO ()
main = do
  let emptyRdf = empty :: RDF TList
  graph <- execStateT createRdf emptyRdf
  withFile "myfile.ttl" WriteMode (\h -> hWriteRdf (TurtleSerializer Nothing mappings) h graph)

createRdf :: (MonadIO m) => RdfState TList m ()
createRdf = do
    addTriple $ RDF.triple (RDF.unode $ RDF.mkUri dbp "Movie1")
                                       (RDF.unode $ RDF.mkUri RDF.rdf "type")
                                       (RDF.unode $ RDF.mkUri dbo "Movie")
    addTriple $ RDF.triple (RDF.unode $ RDF.mkUri dbp "Movie2")
                                       (RDF.unode $ RDF.mkUri RDF.rdf "type")
                                       (RDF.unode $ RDF.mkUri dbo "Movie")

Is that something that could be integrated in the library ?

[koslambrou]

@robstewart57 Would you accept a PR with the RdfState data type in the module Data.RDF.State ? I've already implemented most of it in my projects.

[robstewart57]

@koslambrou Yes, I would accept a PR. Thanks!