Add input validation

Input validation uses Malli coercion. Sadly enough, email is validated
with a regexp.

README.md

@@ -27,3 +27,5 @@ The following links proved more than useful when working on this assignment:
 
 * <https://practical.li/clojure-web-services/building-api/>
 * <https://github.com/metosin/reitit/blob/master/doc/ring/content_negotiation.md>
+* <https://ostash.dev/posts/2021-08-22-data-validation-in-clojure/>
+* <https://clojurecivitas.github.io/malli/elements_of_malli.html>

project.clj

@@ -7,6 +7,7 @@
                  [ring/ring-core "1.15.3"]
                  [ring/ring-jetty-adapter "1.15.3"]
                  [metosin/reitit "0.10.0"]
+                 [metosin/reitit-malli "0.10.0"]
                  [metosin/muuntaja "0.6.11"]]
   :main ^:skip-aot yohoho.core
   :target-path "target/%s"

src/yohoho/core.clj

@@ -2,10 +2,26 @@
   (:require [reitit.ring :as ring]
             [ring.adapter.jetty :as http-server]
             [reitit.ring.middleware.muuntaja :as muuntaja]
-            [muuntaja.core :as m])
+            [muuntaja.core :as m]
+            [reitit.coercion.malli :as malli]
+            [reitit.ring.coercion :as coercion])
   (:gen-class))
 
 
+;; Data schema ----------------------------------------------------
+;; Malli schemas for validation
+
+;; Validating an email address is hard. 
+;; Regexp shamelessly taken from https://emailregex.com/
+(def email-regexp #"(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|\"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*\")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])")
+
+(def Email [:re email-regexp])
+
+(def Item
+  [:map
+   [:name :string]
+   [:email Email]])
+
 
 ;; Handlers -------------------------------------------------------
 (defn ahoy-handler
@@ -52,11 +68,19 @@
    (ring/router
     [["/ahoy"  {:get ahoy-handler}]
      ["/items" {:get get-items-handler
-                :post post-items-handler}]]
+                :post {:handler post-items-handler
-    ;; Use muuntaja middleware to automatically decode JSON
+                       :parameters {:body Item}}}]]
-    {:data {:muuntaja m/instance
+    ;; Middlewares:
+    ;; - wrap-content-type-json: ensure POST routes are sent JSON payload
+    ;; - muuntaja middleware to automatically decode JSON
+    ;; - malli + coercion: handle data validation 
+    {:data {:coercion malli/coercion
+            :muuntaja m/instance
             :middleware [wrap-content-type-json
-                         muuntaja/format-middleware]}})
+                         muuntaja/format-middleware
+                         coercion/coerce-exceptions-middleware
+                         coercion/coerce-request-middleware
+                         coercion/coerce-response-middleware]}})
    ;; Default route: anything not explicitely handled should give a 404
    (ring/create-default-handler
     {:not-found (constantly {:status 404