I'm a Haskell beginner. I have RDF XML from Project Gutenberg that looks like this:
<?xml version="1.0" encoding="utf-8"?>
<rdf:RDF xml:base="http://www.gutenberg.org/"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:dcterms="http://purl.org/dc/terms/"
xmlns:cc="http://web.resource.org/cc/"
xmlns:dcam="http://purl.org/dc/dcam/"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xmlns:pgterms="http://www.gutenberg.org/2009/pgterms/"
>
<cc:Work rdf:about="">
<rdfs:comment>Archives containing the RDF files for *all* our books can be downloaded at
http://www.gutenberg.org/wiki/Gutenberg:Feeds#The_Complete_Project_Gutenberg_Catalog</rdfs:comment>
<cc:license rdf:resource="https://creativecommons.org/publicdomain/zero/1.0/"/>
</cc:Work>
<pgterms:ebook rdf:about="ebooks/20">
<pgterms:bookshelf>
<rdf:Description rdf:nodeID="N3f8445072d8e4499b2646626f94866e0">
<rdf:value>Poetry</rdf:value>
<dcam:memberOf rdf:resource="2009/pgterms/Bookshelf"/>
</rdf:Description>
</pgterms:bookshelf>
<dcterms:hasFormat>
<pgterms:file rdf:about="http://www.gutenberg.org/ebooks/20.rdf">
<dcterms:modified rdf:datatype="http://www.w3.org/2001/XMLSchema#dateTime">2017-03-16T05:01:13.615047</dcterms:modified>
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<dcterms:isFormatOf rdf:resource="ebooks/20"/>
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<rdf:Description rdf:nodeID="N735ba077c8424051b6470a92682aaa5e">
<dcam:memberOf rdf:resource="http://purl.org/dc/terms/IMT"/>
<rdf:value rdf:datatype="http://purl.org/dc/terms/IMT">application/rdf+xml</rdf:value>
</rdf:Description>
</dcterms:format>
</pgterms:file>
</dcterms:hasFormat>
<dcterms:issued rdf:datatype="http://www.w3.org/2001/XMLSchema#date">1991-10-01</dcterms:issued>
<dcterms:title>Paradise Lost</dcterms:title>
<dcterms:subject>
<rdf:Description rdf:nodeID="Ne259525c666c4886a996acbdddca0682">
<rdf:value>PR</rdf:value>
<dcam:memberOf rdf:resource="http://purl.org/dc/terms/LCC"/>
</rdf:Description>
</dcterms:subject>
<dcterms:hasFormat>
<pgterms:file rdf:about="http://www.gutenberg.org/files/20/20.txt">
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<rdf:value rdf:datatype="http://purl.org/dc/terms/IMT">application/x-mobipocket-ebook</rdf:value>
</rdf:Description>
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<dcterms:language>
<rdf:Description rdf:nodeID="N91273d0bffc74be393cda307d2b05137">
<rdf:value rdf:datatype="http://purl.org/dc/terms/RFC4646">en</rdf:value>
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<rdf:Description rdf:nodeID="N5e35fb378b37483ca6ef7a08f27cf936">
<dcam:memberOf rdf:resource="http://purl.org/dc/terms/LCSH"/>
<rdf:value>Eve (Biblical figure) -- Poetry</rdf:value>
</rdf:Description>
</dcterms:subject>
<dcterms:license rdf:resource="license"/>
<dcterms:hasFormat>
<pgterms:file rdf:about="http://www.gutenberg.org/ebooks/20.html.images">
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<rdf:Description rdf:nodeID="N1857bba1f5484e3d84846e1a554ec593">
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</rdf:Description>
</dcterms:format>
</pgterms:file>
</dcterms:hasFormat>
<dcterms:publisher>Project Gutenberg</dcterms:publisher>
<dcterms:rights>Public domain in the USA.</dcterms:rights>
<dcterms:creator>
<pgterms:agent rdf:about="2009/agents/17">
<pgterms:deathdate rdf:datatype="http://www.w3.org/2001/XMLSchema#integer">1674</pgterms:deathdate>
<pgterms:webpage rdf:resource="http://en.wikipedia.org/wiki/John_Milton"/>
<pgterms:name>Milton, John</pgterms:name>
<pgterms:birthdate rdf:datatype="http://www.w3.org/2001/XMLSchema#integer">1608</pgterms:birthdate>
</pgterms:agent>
</dcterms:creator>
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<rdf:value>Text</rdf:value>
</rdf:Description>
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<rdf:Description rdf:nodeID="N202624c4b5994d39a3ab8bf0a2a31d95">
<dcam:memberOf rdf:resource="http://purl.org/dc/terms/LCSH"/>
<rdf:value>Adam (Biblical figure) -- Poetry</rdf:value>
</rdf:Description>
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<dcterms:hasFormat>
<pgterms:file rdf:about="http://www.gutenberg.org/ebooks/20.html.noimages">
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</pgterms:file>
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<pgterms:bookshelf>
<rdf:Description rdf:nodeID="Nec598f664c934ed49ba3c0168ef09615">
<rdf:value>Banned Books from Anne Haight's list</rdf:value>
<dcam:memberOf rdf:resource="2009/pgterms/Bookshelf"/>
</rdf:Description>
</pgterms:bookshelf>
<dcterms:hasFormat>
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<dcterms:isFormatOf rdf:resource="ebooks/20"/>
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<dcterms:subject>
<rdf:Description rdf:nodeID="Nb489692851fa496d96b1a7fdf7a71b21">
<dcam:memberOf rdf:resource="http://purl.org/dc/terms/LCSH"/>
<rdf:value>Fall of man -- Poetry</rdf:value>
</rdf:Description>
</dcterms:subject>
<pgterms:downloads rdf:datatype="http://www.w3.org/2001/XMLSchema#integer">2088</pgterms:downloads>
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<rdf:Description rdf:nodeID="Naa6849a7660b4039baadec8af58f0c58">
<dcam:memberOf rdf:resource="http://purl.org/dc/terms/LCSH"/>
<rdf:value>Bible. Genesis -- History of Biblical events -- Poetry</rdf:value>
</rdf:Description>
</dcterms:subject>
<dcterms:hasFormat>
<pgterms:file rdf:about="http://www.gutenberg.org/files/20/20.zip">
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</rdf:Description>
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<dcterms:isFormatOf rdf:resource="ebooks/20"/>
<dcterms:modified rdf:datatype="http://www.w3.org/2001/XMLSchema#dateTime">2011-03-02T06:34:42</dcterms:modified>
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</pgterms:ebook>
<rdf:Description rdf:about="http://en.wikipedia.org/wiki/John_Milton">
<dcterms:description>Wikipedia</dcterms:description>
</rdf:Description>
</rdf:RDF>
And I want to turn this information into a regular Haskell data structure that I can query and manipulate. For example, I might want to query the title of this work, or get all of its Wikipedia URLs.
I notice that there's an RDF library in Haskell, rdh4h and that it has an XML parser. But I can't make heads or tails of the documentation, and there doesn't seem to be a tutorial around anywhere.
Another thing I've thought about doing is just importing all these RDF/XML files into a database of some sort, and then querying that database somehow using Haskell. But I'm not sure what database is appropriate, or whether that's even possible.
Of course, I could just treat this as XML data, ignoring the RDF aspect, but that seems like a ton of work, and I'd have to write some really long data structure for every thing in this XML file I wanted to get out.
Does anyone have any ideas about how to query data like this using Haskell?
I notice that there's an RDF library in Haskell, rdh4h and that it has an XML parser. But I can't make heads or tails of the documentation, and there doesn't seem to be a tutorial around anywhere.
Here is my attempt to make heads or tails of the documentation. (Take it with a grain of salt, as I know nothing about RDF and didn't actually try to use the library.)
If we open the docs for Data.RDF, the top level module, we find a trio of seemingly relevant functions: parseString, parseFile and parseURL. The documentation for parseString, for instance, is:
parseString :: Rdf a => p -> Text -> Either ParseFailure (RDF a)
Parse RDF from the given text, yielding a failure with error message or the resultant RDF.
To call it, then, we need to supply a p and a Text (the string to be parsed). But what is p? If we scroll upwards a bit, we'll note parseString is a method of the RdfParser class. The instances list -- which helps a lot to make sense of type classes -- shows that XmlParser is an instance of RdfParser. That looks useful!
If we now follow the link to the XmlParser documentation entry, we learn it has an exposed (or "public", if you will) constructor:
XmlParser (Maybe BaseUrl) (Maybe Text)
We can further follow the links to learn that BaseUrl is just a newtype around Text. There seems to be no useful documentation about what the arguments to the constructor are supposed to be, though. Little else remains but to appeal to the source code of the module, also reachable through links. Surprisingly enough, that reveals useful documentation, associated to the functions there. This is the relevant instance of RdfParser:
-- |'XmlParser' is an instance of 'RdfParser'.
instance RdfParser XmlParser where
parseString (XmlParser bUrl dUrl) = parseXmlRDF bUrl dUrl
parseFile (XmlParser bUrl dUrl) = parseFile' bUrl dUrl
parseURL (XmlParser bUrl dUrl) = parseURL' bUrl dUrl
The Haddock comment here is redundant; however, there is useful information in the Haddock comments to parseURL'...
-- |Parse the document at the given location URL as an XML document, using an optional @BaseUrl@
-- as the base URI, and using the given document URL as the URI of the XML document itself.
--
-- The @BaseUrl@ is used as the base URI within the document for resolving any relative URI references.
-- It may be changed within the document using the @\@base@ directive. At any given point, the current
-- base URI is the most recent @\@base@ directive, or if none, the @BaseUrl@ given to @parseURL@, or
-- if none given, the document URL given to @parseURL@. For example, if the @BaseUrl@ were
-- @http:\/\/example.org\/@ and a relative URI of @\<b>@ were encountered (with no preceding @\@base@
-- directive), then the relative URI would expand to @http:\/\/example.org\/b@.
--
-- The document URL is for the purpose of resolving references to 'this document' within the document,
-- and may be different than the actual location URL from which the document is retrieved. Any reference
-- to @\<>@ within the document is expanded to the value given here. Additionally, if no @BaseUrl@ is
-- given and no @\@base@ directive has appeared before a relative URI occurs, this value is used as the
-- base URI against which the relative URI is resolved.
--p
-- Returns either a @ParseFailure@ or a new RDF containing the parsed triples.
parseURL' :: (Rdf a) =>
Maybe BaseUrl -- ^ The optional base URI of the document.
-> Maybe T.Text -- ^ The document URI (i.e., the URI of the document itself); if Nothing, use location URI.
-> String -- ^ The location URI from which to retrieve the XML document.
-> IO (Either ParseFailure (RDF a))
-- ^ The parse result, which is either a @ParseFailure@ or the RDF
-- corresponding to the XML document.
parseURL' bUrl docUrl = _parseURL (parseXmlRDF bUrl docUrl)
... and parseXmlRDF:
-- |Parse a xml T.Text to an RDF representation
parseXmlRDF :: (Rdf a)
=> Maybe BaseUrl -- ^ The base URL for the RDF if required
-> Maybe T.Text -- ^ DocUrl: The request URL for the RDF if available
-> T.Text -- ^ The contents to parse
-> Either ParseFailure (RDF a) -- ^ The RDF representation of the triples or ParseFailure
parseXmlRDF bUrl dUrl xmlStr = case runParseArrow of
(_,r:_) -> Right r
_ -> Left (ParseFailure "XML parsing failed")
where runParseArrow = runSLA (xreadDoc >>> isElem >>> addMetaData bUrl dUrl >>> getRDF) initState (T.unpack xmlStr)
initState = GParseState { stateGenId = 0 }
These Haddock comments don't show up in the actual documentation because the functions they belong to aren't exported.
All in all, I'd say the documentation of this library could be improved. In such cases, though, knowing your way around Hackage docs can soften the blow.
So I tried
parsed <- parseURL (XmlParser Nothing Nothing) testText, but it saysAmbiguous type variable ‘a0’ arising from a use of ‘parseURL’ prevents the constraint ‘(Rdf a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be.
The error is telling you that you have to specify what a is in...
parseURL :: Rdf a => p -> String -> IO (Either ParseFailure (RDF a))
... either by using it somewhere that demands a concrete type or by adding a type annotation.
Further following links in the documentation shows that Rdf is a class with two instances (TList and AdjHashMap) and that RDF is a data family. That being so, you want something like:
parsed <- parseURL (XmlParser Nothing Nothing) testText :: IO (Either ParseFailure (RDF TList))
(Note how the type annotation matches the result type specified by the signature of parseURL.)
Alternatively, enabling ScopedTypeVariables makes it possible to write:
parsed :: Either ParseFailure (RDF TList) <- parseURL (XmlParser Nothing Nothing) testText