allow sets to be used directly as selector (acts as filter)

src/clj/com/rpl/specter.clj

@@ -161,12 +161,16 @@
 (extend-type clojure.lang.AFn
   StructurePath
   (select* [afn structure next-fn]
-    (if (afn structure)
-      (next-fn structure)))
+    (filter-select afn structure next-fn))
   (transform* [afn structure next-fn]
-    (if (afn structure)
-      (next-fn structure)
-      structure)))
+    (filter-transform afn structure next-fn)))
+
+(extend-protocol StructurePath
+  clojure.lang.PersistentHashSet
+  (select* [aset structure next-fn]
+    (filter-select aset structure next-fn))
+  (transform* [aset structure next-fn]
+    (filter-transform aset structure next-fn)))
 
 (defn collect [& selector]
   (->SelectCollector select (comp-paths* selector)))

src/clj/com/rpl/specter/impl.clj

@@ -532,3 +532,12 @@
       (.paths this))
     ))
 
+(defn filter-select [afn structure next-fn]
+  (if (afn structure)
+    (next-fn structure)))
+
+(defn filter-transform [afn structure next-fn]
+  (if (afn structure)
+    (next-fn structure)
+    structure))
+

test/clj/com/rpl/specter/core_test.clj

@@ -369,3 +369,12 @@
   (is (= [] (transform FIRST inc [])))
   (is (= [] (transform LAST inc [])))
   )
+
+(defspec set-filter-test
+  (for-all+
+    [k1 gen/keyword
+     k2 (gen/such-that #(not= k1 %) gen/keyword)
+     k3 (gen/such-that (complement #{k1 k2}) gen/keyword)
+     v (gen/vector (gen/elements [k1 k2 k3]))]
+    (= (filter #{k1 k2} v) (select [ALL #{k1 k2}] v))
+    ))