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(defn relate [& pairs]
(assert (even? (count pairs)) "relate requires an even number of arguments")
(->> pairs
(partition 2)
(map (fn [[k vs]] (map #(hash-map k %) vs)))
(apply map merge)))
(defn matches-specmap? [specmap m]
(fn [_ k spec]
(if (s/valid? spec (get m k))
(reduced false)))
(defn where
"Returns a set of the elements conforming to the given spec-map."
(filter (partial matches-specmap? specmap)))
([specmap xset]
(filter (partial matches-specmap? specmap) xset)))
(defn derive
"Maps a tuple operation over the relation, associng the result as a new attribute."
([dest-attr f]
(map (fn [tuple] (assoc tuple dest-attr (apply f [tuple])))))
([dest-attr f xrel & args]
(->> xrel
(map (fn [tuple] (assoc tuple dest-attr (apply f tuple args))))
(into #{}))))
(defn derive-k
"Corresponds to SELECT F(key, key2, ...)"
([dest-attr f ks]
(map (fn [tuple]
(let [param-fn (apply juxt ks)
;; @TODO combine juxt with flatten? then a single key-fn may return more than one paramater
params (param-fn tuple)]
(assoc tuple dest-attr (apply f params))))))
([dest-attr f ks xrel]
(into #{} (derive-k dest-attr f ks) xrel)))
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