rdf4h is a library for working with RDF in Haskell
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rdf4h - An RDF library for Haskell

Available on Hackage License BSD3 Build Status

rdf4h is a library for working with RDF in Haskell. It includes RDF serializers for the N-Triples and Turtle formats, and parsing support for RDF/XML, N-Triples and Turtle formats. The library provides an API for querying for triples containing a particular subject, predicate, or object, or selecting triples that satisfy an arbitrary predicate function.

RDF graph implementations

The RDF type class provides a way to implement RDF graphs with a variety of different concrete data structures. The idea is that the selection of RDF implementation instances provides a trade off to meet application requirements, e.g. querying mostly for subjects in an RDF grpah, or querying mostly predicates and objects in an RDF graph. The current RDF instances are:

  • HashMapSP, which is a map from (S,P) pairs to a list of O's.
  • HashMapS, which is a map from S to a map from P to a list of O's.
  • MapSP, same as HashMapSP, but uses a Data.HashMap rather than a Data.Map.
  • TriplesList, which is a standard list of Triple elements.
  • TriplesPatriciaTree, which stores triples in a patricia tree from the fgl library.

The RDF class provides the following methods, each of which are optimised for the underlying RDF graph representation:

class RDF rdf where
  baseUrl           :: rdf -> Maybe BaseUrl
  prefixMappings    :: rdf -> PrefixMappings
  addPrefixMappings :: rdf -> PrefixMappings -> Bool -> rdf
  empty             :: rdf
  mkRdf             :: Triples -> Maybe BaseUrl -> PrefixMappings -> rdf
  triplesOf         :: rdf -> Triples
  uniqTriplesOf     :: rdf -> Triples
  select            :: rdf -> NodeSelector -> NodeSelector -> NodeSelector -> Triples
  query             :: rdf -> Maybe Node -> Maybe Node -> Maybe Node -> Triples

How to query RDF graphs

The following RDF graph query functions are in the library:

fromEither            :: RDF rdf => Either ParseFailure rdf -> rdf
equalSubjects         :: Triple -> Triple -> Bool
equalPredicates       :: Triple -> Triple -> Bool
equalObjects          :: Triple -> Triple -> Bool
isEmpty               :: RDF rdf => rdf -> Bool
subjectsWithPredicate :: RDF rdf => rdf -> Predicate -> [Subject]
objectsOfPredicate    :: RDF rdf => rdf -> Predicate -> [Object]
isIsomorphic          :: (RDF rdf1, RDF rdf2) => rdf1 -> rdf2 -> Bool
isGraphIsomorphic     :: (RDF rdf1, RDF rdf2) => rdf1 -> rdf2 -> Bool
expandTriples         :: (RDF rdf) => rdf -> Triples
expandTriple          :: PrefixMappings -> Triple -> Triple
expandNode            :: PrefixMappings -> Node -> Node
expandURI             :: PrefixMappings -> T.Text -> T.Text

How to parse RDF data

The RdfParser is a type class for parsing RDF text in RDF/XML, N-Triples and Turtle formats using instances XmlParser NTriplesParser and TurtleParser. The API supports parsing from Haskell strings, files and URLs.

class RdfParser p where
  parseString :: RDF rdf => p -> Text   -> Either ParseFailure rdf
  parseFile   :: RDF rdf => p -> String -> IO (Either ParseFailure rdf)
  parseURL    :: RDF rdf => p -> String -> IO (Either ParseFailure rdf)

For example, to parse an NTriple file:

Right (rdf::TriplesList) <- parseFile NTriplesParser "literal.nt"

To parse Turtle at a URL:

Right (rdf::TriplesList) <- parseURL (TurtleParser Nothing Nothing)

How to serialise RDF data

The RdfSerializer is a type class for serialising RDF graphs in N-Triples and Turtle formats using instances NTriplesSerializer and TurtleSerializer. The API supports writing RDF serialisations to handlers for writing to files or sockets.

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

For example, to write to an NTriples file:

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

RDF instance benchmarks

The following benchmark results were obtained running bench/MainCriterion.hs at commit https://github.com/robstewart57/rdf4h/commit/3d86295b085ecfb9a944dc7b40bb668df38c2777 on 28 December 2015. The complete output from criterion are in benchmark-results/criterion-results-28.12.2015.txt.

To run the benchmark suite with cabal or stack:

$ cabal bench
$ stack bench

Results: mapping the [Triple] stream from the Turtle parser into each RDF instance:

HashMapS 19.7s
HashMapSP 19.5s
MapSP 19.6s
TriplesList 18.6s
TriplesPatriciaTree 22.0s

Benchmarking query results in microseconds:

HashMapS 29 2254 25640 0.5 0.3 4 0.5
HashMapSP 20080 10170 13120 0.4 10130 19810 20080
MapSP 9379 8518 1458 8 8639 9221 9379
TriplesList 11100 7600 9600 11100 7600 11100 1100
TriplesPatriciaTree 206600 178700 188300 206800 187400 174200 173600

Benchmarking select results in microseconds:

HashMapS 275100 326400 112900 263700 111100 96680 95630
HashMapSP 22080 24310 11830 24030 12210 15460 15690
MapSP 12820 14610 13230 14950 14270 13420 14160
TriplesList 12620 10730 9961 12760 11080 11790 13080
TriplesPatriciaTree 243300 238600 204100 251000 210300 209400 211400

Development wishlist

  • To pass 100% of the W3C parser unit tests for the XML, Turtle and NTriples parsers. The current pass rate is here: https://travis-ci.org/robstewart57/rdf4h

  • To improve the benchmarked performance of the existing RDF instances, and to add new optimised instances of RDF. Pull requests of this kind especially welcome!

  • Add new benchmarks to the criterion benchmark suite in the bench/ directory. It currently benchmarks all RDF instances for query and select functions in the library. The benchmarks could cover more the library's API.

  • The current RDF type class does not currently support the addition and removal of triples. The introduction of an API for allowing this would need a careful design, and a lot of new tests in the testsutie.

When adding a new RDF implementation, put your RDF type class instance in an appropriately named Haskell module in Data/RDF/Graph/. Now add your instance to the library testsuite and the criterion benchmarks.

  1. Testsuite add your instance to the testsuite by adding an appropriate file in testsuite/tests/Data/RDF/Graph/ and hook in your instance to Tests.hs here.

  2. Benchmarks add your instance to the criterion benchmarks in MainCriterion.hs here.


{-# LANGUAGE OverloadedStrings #-}

import Data.RDF

rdfGraph1 :: IO TriplesList
rdfGraph1 = fmap fromEither (parseFile NTriplesParser "test1.nt")

rdfGraph2 :: IO TriplesList
rdfGraph2 = fmap fromEither (parseFile NTriplesParser "test2.nt")

example :: IO ()
example = do
  g1 <- rdfGraph1
  g2 <- rdfGraph2
  let node1 = lnode $ PlainL "foo"
  putStrLn $ "Subjects of g1: " ++ show (map subjectOf (triplesOf g1))
  putStrLn $ "RDF contains literal 'foo': " ++ show (rdfContainsNode g1 node1)
  putStrLn $ "Isomorphism test: " ++ show (isIsomorphic g1 g2)
  putStrLn $ "Unsorted triples: " ++ show (triplesOf g2)
  putStrLn $ "Sorted triples: "   ++ show ((sort . triplesOf) g2)
  putStrLn $ "Query: " ++ show (query g1 Nothing Nothing (Just node1))


If you use the cabal-install Haskell library installer, then:

$ cabal update
$ cabal install rdf4h

Or just supply rdf4h in the build-depends field in the .cabal file of your executable or library. The same applies for users of the stack tool.

Running tests

Writing tests is highly encouraged. The library contains two kinds of tests:

  1. W3C parser unit tests for the XML, Turtle and NTriples formats using HUint. These tests remain in sync with https://github.com/w3c/rdf-tests .

  2. Property based API tests for RDF graph querying using QuickCheck.

These tests are unifed with the tasty test framework. It provides a way of running all rdf4h library tests or just specific test groups.

If you've never initialised the rdf-tests repository in your rdf4h clone:

$ git submodule update --init --recursive

To run the parser tests against the latest W3C test files:

$ git submodule foreach git pull origin gh-pages

To run all library tests with stack or cabal:

$ stack test
$ cabal configure --enable-tests && cabal build && cabal test

To run specific test groups:

$ stack test --test-arguments="--pattern HashMapSP"
$ cabal test --test-options="--pattern HashMapSP"

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

$ stack test --test-arguments "--list-tests"
$ cabal test --test-option "--list-tests" --show-details=streaming

For this outputted entry rdf4h tests/N-Triples tests/nt-syntax-file-02

$ stack test --test-arguments="--pattern \"N-Triples tests/nt-syntax-file-02\""
rdf4h-2.0.0: test (suite: test-rdf4h, args: --pattern "N-Triples tests/nt-syntax-file-02")
rdf4h tests
  N-Triples tests
    nt-syntax-file-02: OK (0.02s)
All 1 tests passed (1.06s)


Please use the GitHub issue tracker to report any bugs you might find. Also feel free to use the GitHub issue tracker to raise new discussions about the API design or proposals for additions to the library.

Related Haskell Packages

  • hsparql is a DSL for programmatic creation and execution of SPARQL queries. It makes use of the RDF type class in rdf4h, allowing the two packages to be combined easily.
  • swish is a toolkit for RDF inference and for implementing RDF file processors. It explores Haskell as "a scripting language for the semantic web".