cordawyn / 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

    return SelectQuery { queryVars = [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 resource: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

TODOs

Opt for a unified Type representation

This hsparql package and the RDF4H [1] 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 [2] for Java.

[1] - https://github.com/amccausl/RDF4H [2] - http://incubator.apache.org/jena/