I stumbled across Timothy Baldridge's excellent video
<http://www.youtube.com/watch?v=HHZ8iqswiCw>explaining how to incorporate
data sources into core.logic. It reinvigorated my interest in using
core.logic to query SQL RDBMS. I'm stumbling on a pretty simple thing, I
think. I've got a table that has three columns, a single primary key
column and two value columns. Using the pattern Tim outlines in his video,
I've got a relation function that takes three parameters, one for each
column and I'm trying to work through the case where the primary key is
ground and the value columns are lvars. This translates to a query of the
form SELECT v1, v2 FROM t WHERE pkey=?. Of course, this returns two values
that must be unified. That's where I'm stuck.
I know I want to return a substitution map, but I have two lvars to unify.
How do I merge the two substitution maps?
Sample code:
(defn pkey-v1-v2-o [pkey v1 v2]
(fn [a]
(let [pkey (walk a pkey)
v1 (walk a v1)
v2(walk a v2)]
(condp = [(not (lvar? pkey))
(not (lvar? v1))
(not (lvar? v2))]
[true false false ] (to-stream
(let [r (first (query ["SELECT v1, v2 FROM T
WHERE pkey=?" pkey]))]
(some-merge-fn (unify a v1 (:v1 r))
(unify a v2 (:v2 r)))))))))
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