add compact navigator

CHANGES.md

@@ -2,6 +2,7 @@
 
 * Add `vtransform` variant of `transform` that takes in collected values as a vector in the first argument rather than spliced into argument list.
 * Add `vterminal` that takes in collected vals as vector in first argument rather than spliced into argument list.
+* Add `compact` navigator. After each step of navigation of its subpath, `compact` removes the collection if it's empty.
 * Change `terminal` to be a no-op on select codepath
 
 ## 1.0.5

src/clj/com/rpl/specter.cljc

@@ -1455,3 +1455,11 @@
     (cond-path (pred afn) STAY
                coll? [ALL-WITH-META p]
                )))
+
+(let [compact* (stay-then-continue (if-path empty? (terminal-val NONE)))]
+ (defdynamicnav compact
+   "During transforms, after each step of navigation in subpath check if the
+    value is empty. If so, remove that value by setting it to NONE."
+   [& path]
+   (interleave (repeat (count path) compact*) path)
+   ))

test/com/rpl/specter/core_test.cljc

@@ -1662,6 +1662,21 @@
   (is (= {:a 6} (vtransform [:a (s/putval 2) (s/putval 3)] (fn [vs v] (+ v (reduce + vs))) {:a 1})))
   )
 
+(deftest compact-test
+  (is (= {} (setval [:a (s/compact :b :c)] s/NONE {:a {:b {:c 1}}})))
+  (is (= {:a {:d 2}} (setval [:a (s/compact :b :c)] s/NONE {:a {:b {:c 1} :d 2}})))
+  (let [TREE-VALUES (recursive-path [] p (s/if-path vector? [(s/compact s/ALL) p] s/STAY))
+        tree [1 [2 3] [4 [[5] [[6]]]]]]
+    (is (= [2 4 6] (select [TREE-VALUES even?] tree)))
+    (is (= [1 [3] [[[5]]]] (setval [TREE-VALUES even?] s/NONE tree)))
+    )
+  (is (= [{:a [{:c 1}]}]
+         (setval [s/ALL (s/compact :a s/ALL :b)]
+           s/NONE
+           [{:a [{:b 3}]}
+            {:a [{:b 2 :c 1}]}])))
+  )
+
 #?(:clj
   (do
     (defprotocolpath FooPP)