added STAY, stay-then-continue, and continue-then-stay selectors

src/clj/com/rpl/specter.cljx

@@ -146,6 +146,15 @@
     structure
     ))
 
+(defpath
+  ^{:doc "Stays navigated at the current point. Essentially a no-op selector."}
+  STAY
+  []
+  (select* [this structure next-fn]
+    (next-fn structure))
+  (transform* [this structure next-fn]
+    (next-fn structure)))
+
 (def ALL (comp-paths (i/->AllStructurePath)))
 
 (def VAL (i/->ValCollect))
@@ -235,6 +244,30 @@
                 ancestry))
       )))
 
+(defn stay-then-continue
+  "Navigates to the current element and then navigates via the provided path.
+   This can be used to implement pre-order traversal."
+  [& path]
+  (fixed-pathed-path [late path]
+    (select* [this structure next-fn]
+      (concat (next-fn structure)
+              (doall (mapcat next-fn (compiled-select late structure)))))
+    (transform* [this structure next-fn]
+      (compiled-transform late next-fn (next-fn structure))
+      )))
+
+(defn continue-then-stay
+  "Navigates to the provided path and then to the current element. This can be used
+   to implement post-order traversal."
+  [& path]
+  (fixed-pathed-path [late path]
+    (select* [this structure next-fn]
+      (concat (doall (mapcat next-fn (compiled-select late structure)))
+              (next-fn structure)))
+    (transform* [this structure next-fn]
+      (next-fn (compiled-transform late next-fn structure))
+      )))
+
 
 (defpath keypath [key]
   (select* [this structure next-fn]

test/com/rpl/specter/core_test.cljx

@@ -578,6 +578,17 @@
          (s/transform [s/ALL even?] inc s1))
       )))
 
+(deftest stay-continue-tests
+  (is (= [[1 2 [:a :b]] [3 [:a :b]] [:a :b [:a :b]]]
+         (s/setval [(s/stay-then-continue s/ALL) s/END] [[:a :b]] [[1 2] [3]])))
+  (is (= [[1 2 [:a :b]] [3 [:a :b]] [:a :b]]
+         (s/setval [(s/continue-then-stay s/ALL) s/END] [[:a :b]] [[1 2] [3]])))
+  (is (= [[1 2 3] 1 3]
+         (s/select (s/stay-then-continue s/ALL odd?) [1 2 3])))
+  (is (= [1 3 [1 2 3]]
+         (s/select (s/continue-then-stay s/ALL odd?) [1 2 3])))
+  )
+
 #+clj
 (deftest large-params-test
   (let [path (apply s/comp-paths (repeat 25 s/keypath))