rdf4h RDF for Haskell

Fork me on GitHub


The rdf4h Haskell library is for querying structured data described with the the Resource Description Framework model, where data is a collection of <subject,predicate,object> triples:

Type level RDF graph representations

The RDF type is a data family, for which there are a number of instances. Those instances represent type level indexes that provide the programmer with the choice of underlying in-memory graph representation.

data family RDF a

-- function provided a (s,p,o) triples based RDF graph.
foo :: RDF TList -> Bool
foo rdfGraph = ...

-- function provided a hash based adjacency map RDF graph.
bar :: RDF AdjHashMap -> [Triple]
bar rdfGraph = ...

Those implementations differ in their in-memory representation of RDF graphs.

TList implementation

Given two triples:

1 (s: "http://example.com/rob_stewart",
   p: "http://xmlns.com/foaf/0.1/interest",
   o: "http://dbpedia.org/resource/Semantic_Web")
2 (s: "http://example.com/rob_stewart",
   p: "http://xmlns.com/foaf/0.1/interest",
   o: "http://dbpedia.org/resource/Haskell_(programming_language)"

The TList implementation just stores them as is, i.e.

[ Triple (UNode "http://example.com/rob_stewart")
         (UNode "http://xmlns.com/foaf/0.1/interest")
         (UNode "http://dbpedia.org/resource/Semantic_Web")
, Triple (UNode "http://example.com/rob_stewart")
         (UNode "http://xmlns.com/foaf/0.1/interest")
         (UNode "http://dbpedia.org/resource/Haskell_(programming_language)")


The adjacency hash map implementation has two hash map implementations:

  1. A hashed S key pointing to value that is another hash map, whose key is a hashed P pointing to a hash set of O values.

  2. A hashed O key pointing to value that is another hash map, whose key is a hashed P pointing to a hash set of S values.

So our two-triple graph is stored in SPO and OPS indexes:

hash "http://example.com/rob_stewart"                             == -1527025807618695980
hash "http://xmlns.com/foaf/0.1/interest"                         == -2021146143382594279
hash "http://dbpedia.org/resource/Semantic_Web"                   == 3091419593178925190
hash "http://dbpedia.org/resource/Haskell_(programming_language)" == -8705743210846359529

-- SPO map
(k: -1527025807618695980
 v: (k: -2021146143382594279
     v: [3091419593178925190,-8705743210846359529]))

-- OPS map
(k: -8705743210846359529
 v: (k: -2021146143382594279
 k: 3091419593178925190
 v: (k: -2021146143382594279

This makes querying AdjHashMap graphs with query very efficient, but modifying the graph with addTriple and removeTriple more expensive that the TList implementation, which just use (:) and filter respectively.

The TList and AdjHashMap data family instances represent application specific tradeoffs in terms of space and runtime performance. TList performs better for query, whilst AdjHashMap performs better for select and modifying triples in a graph with addTriple and removeTriple. See these criterion results for performance benchmarks, taken in November 2016.

Rdf type class

The following Rdf type class methods are optimised for each graph implementation.

class Rdf a where
  baseUrl           :: RDF a -> Maybe BaseUrl
  prefixMappings    :: RDF a -> PrefixMappings
  addPrefixMappings :: RDF a -> PrefixMappings -> Bool -> RDF a
  empty             :: RDF a
  mkRdf             :: Triples -> Maybe BaseUrl -> PrefixMappings -> RDF a
  addTriple         :: RDF a -> Triple -> RDF a
  removeTriple      :: RDF a -> Triple -> RDF a
  triplesOf         :: RDF a -> Triples
  uniqTriplesOf     :: RDF a -> Triples
  select            :: RDF a -> NodeSelector -> NodeSelector -> NodeSelector -> Triples
  query             :: RDF a -> Maybe Node -> Maybe Node -> Maybe Node -> Triples
  showGraph         :: RDF a -> String

The Data.RDF.Query module contains more utility query functions, here.

Building RDF graphs interactively

An RDF graph can be constructed with empty, and its triples contents modified with addTriple and removeTriple, e.g.:

{-# LANGUAGE OverloadedStrings #-}
module Main where
import Data.RDF

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

  -- add a triple to the empty graph
      triple1 = triple
        (unode "http://www.example.com/rob")
        (unode "http://xmlns.com/foaf/0.1/interest")
        (unode "http://dbpedia.org/resource/Scotch_whisky")
      graph1 = addTriple myEmptyGraph triple1

  -- add another triple to the graph
      triple2 = triple
        (unode "http://www.example.com/rob")
        (unode "http://xmlns.com/foaf/0.1/interest")
        (unode "http://dbpedia.org/resource/Haskell_(programming_language)")
      graph2 = addTriple graph1 triple2

  -- remove one of my interests
      graph3 = removeTriple graph2 triple1

  putStrLn (showGraph graph3)

Bulk RDF graphs with parsing and writing

RDF graphs can also be populated by parsing RDF content from strings, files or URLs:

class RdfParser p where
  parseString :: (Rdf a) => p -> T.Text -> Either ParseFailure (RDF a)
  parseFile   :: (Rdf a) => p -> String -> IO (Either ParseFailure (RDF a))
  parseURL    :: (Rdf a) => p -> String -> IO (Either ParseFailure (RDF a))

RDF graphs can also be serialised to handles with hWriteRdf:

class RdfSerializer s where
  hWriteRdf :: (Rdf a) => s -> Handle -> RDF a -> IO ()

E.g. to write an RDF graph to a file:

withFile "out.nt" WriteMode (\h -> hWriteRdf NTriplesSerializer h rdfGraph)

Supported RDF serialisation formats

The rdf4h library supports three RDF serialisations:

Serialisation Reading Writing

RDF query example

module Main where

import Data.RDF
import qualified Data.Text as T

eswcCommitteeURI, heldByProp :: T.Text
eswcCommitteeURI =
heldByProp       = "swc:heldBy"

-- full names of ESWC 2015 conference programme committee.
eswcCommitteeMembers :: RDF TList -> [T.Text]
eswcCommitteeMembers graph =
  let triples = query
                  (Just (unode eswcCommitteeURI))
                  (Just (unode heldByProp))
      memberURIs = map objectOf triples
  in map
     (\memberURI ->
              let (LNode (PlainL firstName)) =
                    objectOf $ head $
                        (Just memberURI)
                        (Just (unode "foaf:firstName"))
                  (LNode (PlainL lastName))  =
                    objectOf $ head $
                        (Just memberURI)
                        (Just (unode "foaf:lastName"))
              in (T.append firstName (T.append (T.pack  " ") lastName)))
main :: IO ()
main = do
  result <- parseURL
    (XmlParser Nothing Nothing)
  case result of
    Left err -> error "Unable to parse RDF content from that URL"
    Right rdfGraph -> do
      let eswcMemberNames = eswcCommitteeMembers rdfGraph
      mapM_ (putStrLn . T.unpack) eswcMemberNames

The list of the Extended Semantic Web Conference 2015 programme committee members is printed to standard out:

> main
Vadim Ermolayev
Karl Aberer
Giorgos Stoilos
Birgitta König-Ries
Giuseppe Rizzo


This library has two test suites:

  1. Property based tests of the API using QuickCheck. All tests pass.
  2. Unit tests provided by the W3C to test conformance of RDF parsers, of which there are currently 521. Some parsing tests fail currently.

To list the available tests that can be run in isolation using a pattern:

$ stack test --test-arguments "--list-tests"

To run specific test groups:

$ stack test --test-arguments="--pattern TList"

Please contribute!

Pull requests should be submitted to the rdf4h GitHub repository: https://github.com/robstewart57/rdf4h

It’d be great to have:

In more detail…

Fixes for failing W3C parser unit tests

Wanted: fix failing W3C parsing tests for the Turtle and RDF/XML serialisation formats.

The library does not pass all W3C RDF parsing specification tests. See the rdf4h library’s current pass rate on TravisCI. To see what currently fails:

$ git submodule update --init --recursive
$ git submodule foreach git pull origin gh-pages
$ stack test
      langtagged_string:                                                    OK
      lantag_with_subtag:                                                   OK
      minimal_whitespace:                                                   OK

70 out of 528 tests failed (0.50s)

High performance RDF graph implementations

Wanted: new Rdf type class instances providing new high performance RDF graph representations, beyond the three instances the library currently has.

The latest set of criterion benchmark results (November 2016) are here. To run the criterion benchmarks locally:

$ wget https://www.govtrack.us/data/rdf/bills.099.actions.rdf.gz
$ gzip -d bills.099.actions.rdf.gz
$ stack bench

Or to run just the benchmarks for a specific RDF implementation, use the --benchmark-arguments flag, e.g.:

$ stack bench --benchmark-arguments 'rdf4h/query/AdjHashMap'
Benchmark rdf4h-bench: RUNNING...
benchmarking rdf4h/query/AdjHashMap SPO
time                 506.4 ns   (505.0 ns .. 507.8 ns)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 505.8 ns   (504.4 ns .. 507.3 ns)
std dev              5.087 ns   (3.939 ns .. 6.874 ns)