Count distinct query

[koslambrou]

The following SPARQL query allows to fetch the number of distinct subjects in the database:

SELECT (COUNT(DISTINCT ?s) as ?count) WHERE {
  ?s ?p ?o .
}

However, there's doesn't seem to be a way to translate that query in HSparql.

An equivalent way would be to translate it as a sub query such as:

SELECT (COUNT(?s) as ?count) WHERE {
  SELECT DISTINCT ?s WHERE {
    ?s ?p ?o .
  }
}

Which can be translated to HSparql as:

countSubjsQuery :: Query SelectQuery
countSubjsQuery = do
    s <- var
    p <- var
    o <- var

    subQuery_ $ do
        triple_ s p o
        distinct_
        selectVars [s]

    c <- var
    select [count r `as` c]

Possible enhancement in order to use the distinct function with the count function to produce a SPARQL query similar to the first one ?