TimPut/hsparql

hsparql includes a DSL to easily create queries, as well as methods to submit those queries to a SPARQL server, returning the results as simple Haskell data structures.

Introduction

hsparql includes a DSL to easily create queries, as well as methods to submit those queries to a SPARQL server, returning the results as simple Haskell data structures.

Select Queries

Take the following SPARQL query:

PREFIX dbpedia: <http://dbpedia.org/resource/>
PREFIX dbprop: <http://dbpedia.org/property/>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name ?page
WHERE {
  ?x dbprop:genre dbpedia:Web_browser
  ?x foaf:name ?name
  ?x foaf:page ?page
}

Can be generated using the following Haskell code:

simpleSelect :: Query SelectQuery
simpleSelect = do
    resource <- prefix "dbpedia" (iriRef "http://dbpedia.org/resource/")
    dbpprop  <- prefix "dbprop" (iriRef "http://dbpedia.org/property/")
    foaf     <- prefix "foaf" (iriRef "http://xmlns.com/foaf/0.1/")

    x    <- var
    name <- var
    page <- var

    triple_ x (dbpprop .:. "genre") (resource .:. "Web_browser")
    triple_ x (foaf .:. "name") name
    triple_ x (foaf .:. "page") page

    selectVars [name, page]

Construct Queries

Take the following SPARQL query:

PREFIX dbpedia: <http://dbpedia.org/resource/>
PREFIX dbprop: <http://dbpedia.org/property/>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX example: <http://www.example.com/>
CONSTRUCT {
  ?x example:hasName ?name
}
WHERE {
  ?x dbprop:genre dbpedia:Web_browser
  ?x foaf:name ?name
  ?x foaf:page ?page
}

Can be generated using the following Haskell code:

simpleConstruct :: Query ConstructQuery
simpleConstruct = do
    resource <- prefix "dbpedia" (iriRef "http://dbpedia.org/resource/")
    dbpprop  <- prefix "dbprop" (iriRef "http://dbpedia.org/property/")
    foaf     <- prefix "foaf" (iriRef "http://xmlns.com/foaf/0.1/")
    example  <- prefix "example" (iriRef "http://www.example.com/")

    x    <- var
    name <- var
    page <- var

    construct <- constructTriple x (example .:. "hasName") name

    triple_ x (dbpprop .:. "genre") (resource .:. "Web_browser")
    triple_ x (foaf .:. "name") name
    triple_ x (foaf .:. "page") page

    return ConstructQuery { queryConstructs = [construct] }

Describe Queries

Take the following SPARQL query:

DESCRIBE <http://dbpedia.org/resource/Edinburgh>

Can be generated using the following Haskell code:

simpleDescribe :: Query DescribeQuery
simpleDescribe = do
    resource <- prefix "dbpedia" (iriRef "http://dbpedia.org/resource/")
    uri <- describeIRI (resource .:. "Edinburgh")
    return DescribeQuery { queryDescribe = uri }

Ask Queries

Take the following SPARQL query:

PREFIX dbprop: <http://dbpedia.org/property/>
ASK { ?x dbprop:genre <http://dbpedia.org/resource/Web_browser> }

Can be generated using the following Haskell code:

simpleAsk :: Query AskQuery
simpleAsk = do
    resource <- prefix "dbpedia" (iriRef "http://dbpedia.org/resource/")
    dbprop  <- prefix "dbprop" (iriRef "http://dbpedia.org/property/")

    x <- var
    ask <- askTriple x (dbprop .:. "genre") (resource .:. "Web_browser")

    return AskQuery { queryAsk = [ask] }

Output Types

Select Queries

SELECT queries generate a set of sparql query solutions. See: http://www.w3.org/TR/rdf-sparql-XMLres/

selectExample :: IO ()
selectExample = do
  (Just s) <- selectQuery "http://dbpedia.org/sparql" simpleSelect
  putStrLn . take 500 . show $ s

Here's the respective type:

selectQuery :: EndPoint -> Query SelectQuery -> IO (Maybe [[BindingValue]])

Construct Queries

CONSTRUCT queries generate RDF, which is serialized in N3 in this package. See: http://www.w3.org/TR/rdf-primer/#rdfxml

constructExample :: IO ()
constructExample = do
  rdfGraph <- constructQuery "http://dbpedia.org/sparql" simpleConstruct
  mapM_ print (triplesOf rdfGraph)

Here's the respective type:

constructQuery :: EndPoint -> Query ConstructQuery -> IO MGraph

Describe Queries

DESCRIBE queries generate RDF, which is serialized in N3 in this package. See: http://www.w3.org/TR/rdf-sparql-query/#describe

describeExample :: IO ()
describeExample = do
  rdfGraph <- describeQuery "http://dbpedia.org/sparql" simpleDescribe
  mapM_ print (triplesOf rdfGraph

Here's the respective type:

describeQuery :: EndPoint -> Query DescribeQuery -> IO MGraph

Ask Queries

ASK queries inspects whether or not a triple exists. RDF is an open-world assumption. See: http://www.w3.org/TR/rdf-sparql-query/#ask

askExample :: IO ()
askExample = do
  res <- askQuery "http://dbpedia.org/sparql" simpleAsk
  putStrLn $ "result: " ++ (show (res::Bool))

Here's the respective type:

askQuery :: EndPoint -> Query AskQuery -> IO Bool

More examples

Some extra examples can be found in tests.

TODOs

• Opt for a unified Type representation This hsparql package and the RDF4H package use similar, but not identical, types for triples, namespaces, prefixes and so on. Ideally, one type representation for such concepts should be adopted for both packages.

• Develop a unified semantic web toolkit for Haskell Combining the RDF4H and hsparql packages seems like a sensible goal to achieve, to provide a semantic web toolkit similar to Jena for Java.