Fix multiple optionals

[RickMoynihan]

TPX: matcha: fix multiple optionals

Issue: #21

In short: handling of optional clauses in matcha queries was returning incorrect results (depending on order). Root cause of that was the use of core.logic/conda where core.logic/conde should be used. But that change necessitates extra processing of the the raw core logic results (for both select and build macros).

Summary of changes

build

We infer which projected vars in the constructed map are optional. Knowing that, and having a solution we either set the (top level!) keys to the value from the solution (a set, if multiple values were returned by core.logic) or remove those keys.

We also rewrite the query by extracting the optionals and appending them at the end of a query.

While this should make no difference to core.logic it could make a difference in a SPARQL query, so one needs to be aware that those two are not the same.

select

1. After conda -> conde change, we get different results.
2. We want the results to as close to what a SPARQL query would return.

For example, a SPARQL query returns:

(def DB1 [(->Triple :john :status :online)
            (->Triple :john :prop1 "A")
            (->Triple :john :prop1 "B")
            (->Triple :john :prop2 :x)
            (->Triple :john :prop2 :y)])

(select [?o ?p ?x]
    [[?o :status ?status]
     (optional [[?o :prop1 ?p]])
     (optional [[?o :prop2 ?x]])]
    DB1)
 
;; result we expect, and what we would get in SPARQL: 
([:john "B" :y] [:john "B" :x] [:john "A" :y] [:john "A" :x])

;; result we get, after `conde` change, but without processing the solutions:
([:john "B" _0] [:john _0 _1] [:john "A" _0] [:john _0 :y] [:john _0 :x])

;; And if we remove the :prop1 relations from DB1, we should get:
([:john _1 :y] [:john _1 :x])

;; but without processing the solutions from `core.logic` we get:
([:john _0 _1] [:john _0 :y] [:john _0 :x])

The changes to select macro are cleaning up the unbound variables.

[RickMoynihan]

Thanks a million for this @rosado it's great work! 🙇