Allow validation key to be a function to handle password/confirm password case

project.clj

@@ -1,5 +1,5 @@
 (defproject valip "0.2.0"
   :description "Functional validation library"
-  :dependencies [[org.clojure/clojure "1.2.0"]
+  :dependencies [[org.clojure/clojure "1.3.0"]
                  [commons-validator "1.3.1"]
-                 [clj-time "0.3.0"]])
+                 [clj-time "0.4.0"]])

src/valip/core.clj

@@ -5,13 +5,16 @@
   "Performs a validation on a key in a map using the supplied predicate
   function. A {key [error]} map is returned if the predicate returns false;
   nil is returned if the predicate returns true, or if the supplied value does
-  not match the predicates preconditions (i.e. throws an AssertionError)."
+  not match the predicates preconditions (i.e. throws an AssertionError).
+
+  Key may be a function taking a map and returning a value.
+  It's error would be associated with :* in the error map."
   [key pred? error]
   (fn [value-map]
-    (let [value (value-map key)]
+    (let [value (key value-map)]
       (try
         (if-not (pred? value)
-          {key [error]})
+          {(if (keyword? key) key :*) [error]})
         (catch AssertionError _)))))
 
 (defn merge-errors
@@ -23,7 +26,10 @@
   "Validate a map of values using the supplied validations. Each validation
   is represented as a vector containing [key predicate? error] values. A map
   is returned for all the keys that failed their predicates, in the form:
-  {key [errors]}. If no predicates return false, nil is returned."
+  {key [errors]}. If no predicates return false, nil is returned.
+
+  A validation key may be a function taking a map and returning a value.
+  It's error would be associated with :* key in the error map."
   [value-map & validations]
   (->> validations
        (map (partial apply validation-on))

test/valip/test/core.clj

@@ -15,3 +15,10 @@
            [:x #(> % 18) "must be greater than 18"])
          {:x ["must be greater than 18"]
           :y ["must be present"]})))
+
+(deftest validate-key-as-fn
+  (is (= (validate {:password "secret" :confirm-password ""}
+                   [identity #(apply = (map % [:password :confirm-password])) "passwords must match"])
+         {:* ["passwords must match"]}))
+  (is (nil? (validate {:password "secret" :confirm-password "secret"}
+                      [#(map % [:password :confirm-password]) #(apply = %) "passwords must match"]))))