Add f.u.fn/=to, exposing c.l.Util/equivPred

Author: amalloy

src/flatland/useful/fn.clj

@@ -176,3 +176,19 @@
   (partial apply f))
 
 (def ap "A shorthand version of applied" applied)
+
+(defn =to
+  "Produces an equality predicate from a single object. ((=to x) y) is
+the same as (= x y), but if the returned function will be called many
+times it may be more efficient than repeated calls to =, because =to
+can short-circuit many irrelevant code paths based on knowing the type
+of x.
+
+Just a wrapper for clojure.lang.Util/equivPred."
+  [x]
+  (if (keyword? x) ;; a case myssteriously not covered in equivPred
+    (fn [y]
+      (identical? x y))
+    (let [p (clojure.lang.Util/equivPred x)]
+      (fn [y]
+        (.equiv p x y)))))

test/flatland/useful/fn_test.clj

@@ -90,3 +90,9 @@
         by-square (key-comparator :ascending square)]
     (is (= (sort-by square [-9 -5 1 -2])
            (sort by-square [-9 -5 1 -2])))))
+
+(deftest test-=to
+  (let [objs [1 :x "x" [5] nil (Object.) {:x 1} '((a b c) d)]]
+    (doseq [x objs
+            y objs]
+      (is (= (= x y) ((=to x) y))))))