Added merge-with core fn (#874)

Hi,

can you please consider patch to add support for the `merge-with` core
fn. It resolves #860.

I have added a couple of test cases for the same.

Thanks

---------

Co-authored-by: ikappaki <[email protected]>

Author: ikappaki

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  * Added support for binding destructuring in `for` bindings (#774)
  * Added `==` as an alias to `=` (#859)
  * Added custom exception formatting for `basilisp.lang.compiler.exception.CompilerException` and `basilisp.lang.reader.SyntaxError` to show more useful details to users on errors (#870)
+ * Added `merge-with` core function (#860)
 
 ### Changed
  * Cause exceptions arising from compilation issues during macroexpansion will no longer be nested for each level of macroexpansion (#852)

src/basilisp/core.lpy

@@ -3258,6 +3258,21 @@
              (transient {})
              maps))))
 
+(defn merge-with
+  "Merge maps together from left to right as by :lpy:fn:`conj`. If a
+  duplicate key appears in a map, the resulting value of that key in
+  the merged result will be the result of calling ``(f current-val
+  next-val)``."
+  [f & maps]
+  (when (some identity maps)
+    (reduce (fn [acc kv]
+              (let [k (first  kv)
+                    v (second kv)]
+                (if (contains? acc k)
+                  (update acc k f v)
+                  (assoc acc k v))))
+            (first maps) (apply concat (rest maps)))))
+
 (defn trampoline
   "Trampoline ``f`` with starting arguments. If ``f`` returns a function (as determined
   by :lpy:fn:`fn?`), call its return value with no arguments, repeating that process

tests/basilisp/prompt_test.py

@@ -40,6 +40,7 @@ def completions(self) -> Iterable[str]:
             "max",
             "max-key",
             "merge",
+            "merge-with",
             "meta",
             "methods",
             "min",
@@ -77,6 +78,7 @@ def patch_completions(self, completions: Iterable[str]):
                     "memfn",
                     "memoize",
                     "merge",
+                    "merge-with",
                     "meta",
                     "methods",
                     "min",

tests/basilisp/test_core_fns.lpy

@@ -1096,6 +1096,12 @@
     {:a 1 :b 2} [:a :b] [1 2]
     {:a 0 :b 1} [:a :b] (range)))
 
+(deftest merge-with-test
+  (is (= nil (merge-with conj)))
+  (is (= nil (merge-with str nil)))
+  (is (= {:a 0 :b "235" :c "46" :d 7 :z 8} (merge-with str {:a 0 :b 2} {:b 3 :c 4 :z 8} {:b 5 :c 6 :d 7})))
+  (is (= {:a 0 :b [2 5] :c 7} (merge-with conj {:a 0 :b [2]} nil {:b 5 :c 7}))))
+
 (deftest trampoline-test
   (let [a (atom [])]
     (trampoline (fn [v]