Add contravariant notebook

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>

contravariant.ipynb

@@ -0,0 +1,58 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Contravariant Functors"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 21,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "{-# LANGUAGE KindSignatures #-}\n",
+    "\n",
+    "-- (.)        :: (b -> c) -> (a -> b) -> a -> c\n",
+    "-- contraComp :: (a -> b) -> (b -> c) -> a -> c\n",
+    "\n",
+    "-- (.)        f g x = f (g x)\n",
+    "-- contraComp f g x = g (f x)\n",
+    "\n",
+    "-- instance Functor (a ->) where\n",
+    "--    fmap = (.)\n",
+    "\n",
+    "-- instance Contravariant (-> z) where\n",
+    "--    contramap = flip (.)\n",
+    "\n",
+    "newtype Predicate a = Predicate { getPredicate :: a -> Bool }\n",
+    "\n",
+    "newtype Op b a = Op { getOp :: a -> b }\n",
+    "  \n",
+    "class Contravariant (f :: * -> *) where\n",
+    "  contramap :: (a -> b) -> f b -> f a\n",
+    "  \n",
+    "instance Contravariant (Op a) where\n",
+    "    contramap f g = Op $ let bToa = getOp g in bToa . f"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Haskell",
+   "language": "haskell",
+   "name": "haskell"
+  },
+  "language_info": {
+   "codemirror_mode": "ihaskell",
+   "file_extension": ".hs",
+   "name": "haskell",
+   "pygments_lexer": "Haskell",
+   "version": "8.8.3"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 4
+}