TurtleSerializer baseUrl and prefix mappings effect

[koslambrou]

Say I create a RDF graph.

I'm trying to save the graph in a file and I want to set the baseUrl and prefix mappings.
However, the following won't set the baseUrl and will only put the default prefix mappings (not the ones I specified).

main :: IO ()
main = do
  let prefixMappings = ...
  let graph = ...
  let baseUri = Just $ "http://localhost:8080/resource"
  withFile "..." WriteMode (\h -> hWriteRdf (TurtleSerializer baseUri prefixMappings) h graph)

Seems like the only way to set the baseUrl and prefix mapping is through mkRdf or addPrefixMappings. If that's the case, what is the point of TurtleSerializer parameters ?

[koslambrou]

Actually, the 2nd parameter of TurtleSerializer correctly adds the specified prefixes in the saved file, but it also adds the defaults prefixes from Data.RDF.Namespace.standard_ns_mappings

[robstewart57]

Actually, the 2nd parameter of TurtleSerializer correctly adds the specified prefixes in the saved file, but it also adds the defaults prefixes from Data.RDF.Namespace.standard_ns_mappings

@koslambrou could you put together a small, compilable, example that demonstrates this? Thanks.

[koslambrou]

Here:

import Data.RDF (RDF)
import qualified Data.RDF as RDF
import qualified Data.RDF.Namespace as RDF
import qualified Data.Map as Map

test :: IO ()
test = do
  let emptyRdf = RDF.empty :: RDF RDF.TList
  let mappings = RDF.PrefixMappings $ Map.fromList [ ("mo", "http://purl.org/ontology/mo/") ]
  let baseUrl = "http://example.org/resource"
  RDF.writeRdf (TurtleSerializer Nothing mappings) emptyRdf

Expected output:

@base <http://example.com/resource> .
@prefix mo: <http://purl.org/ontology/mo/> .

Actual output:

@prefix dc: <http://purl.org/dc/elements/1.1/> .
@prefix dct: <http://purl.org/dc/terms/> .
@prefix ex: <http://www.example.org/> .
@prefix ex2: <http://www2.example.org/> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix mo: <http://purl.org/ontology/mo/> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

[koslambrou]

The actual output in my last comment is not on the latest commit.

Here's the actual output:

@prefix mo: <http://purl.org/ontology/mo/> .

There's just the @base missing.

[robstewart57]

Because in your code:

let baseUrl = "http://example.org/resource"
RDF.writeRdf (TurtleSerializer Nothing mappings) emptyRdf

You're not using baseUrl in the TurtleSerializer.

[koslambrou]

Oh right!

run :: RIO App ()
run = do
  let emptyRdf = RDF.empty :: RDF RDF.TList
  let mappings = RDF.PrefixMappings $ Map.fromList [ ("mo", "http://purl.org/ontology/mo/") ]
  let baseUrl = Just "http://example.org/resource"
  liftIO $ RDF.writeRdf (TurtleSerializer baseUrl mappings) emptyRdf

The actual output is still:

@prefix mo: <http://purl.org/ontology/mo/> .

[robstewart57]

Ah, thanks.

The optional argument representing the base URL in the Turtle parser is to use for resolving relative URLs in the document. This may be overridden in the document itself using the @base directive.

However in the Turtle serialisation, the optional base URL is used when serialising all URI nodes in the outputted RDF, i.e. normalising prefix:foo to be http://example.com/foo if the prefix namespace is http://example.com. That is, I'm not sure there's a need to include the @base in the serialised namespaces since no URIs in the outputted RDF are relative, they are all absolute.

[koslambrou]

The following Turtle output contains a relative URI, no?

@base <http://example.com/resource> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

</Movie> a rdfs:Class .

Isn't /Movie considered a relative URI since it should be resolved to http://example.com/resource/Movie with the base URI ?

Also, I don't understand the use case behind the optional base URL in the Turtle serialisation. Why would we want the convert a single prefix (e.g. prefix:foo) to absolute form (e.g. http://example.com/foo). Hope I make sense.