Inspired by haskell/mtl#5

This commit closes scalaz#1658

core/src/main/scala/scalaz/MonadReader.scala

@@ -15,6 +15,27 @@ trait MonadReader[F[_], S] extends Monad[F] { self =>
 
   ////
 
+  trait MonadReaderLaw extends MonadLaw {
+    def localPoint[A](f: S => S)(a: A)(implicit FA: Equal[F[A]]): Boolean =
+      FA.equal(local(f)(point(a)), point(a))
+
+    def localComposition[A](f: S => S, g: S => S)(fa: F[A])(implicit FA: Equal[F[A]]): Boolean =
+      FA.equal(local(f)(local(g)(fa)), local(f andThen g)(fa))
+
+    def localFAsk(f: S => S)(implicit FS: Equal[F[S]]): Boolean =
+      FS.equal(local(f)(ask), map(ask)(f))
+
+    def askIdempotence(implicit FS: Equal[F[S]]): Boolean =
+      FS.equal(ask, apply2(ask, ask)((_, b) => b))
+
+    def askFALeft[A](fa: F[A])(implicit FA: Equal[F[A]]): Boolean =
+      FA.equal(apply2(ask, fa)((_, b) => b), fa)
+
+    def askFARight[A](fa: F[A])(implicit FA: Equal[F[A]]): Boolean =
+      FA.equal(apply2(fa, ask)((a, _) => a), fa)
+  }
+
+  def monadReaderLaw = new MonadReaderLaw {}
 }
 
 object MonadReader {