scala/Covariance-and-contravariance.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Covariance"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mtrait \u001b[36mMyOption\u001b[0m\n",
       "defined \u001b[32mclass \u001b[36mMySome\u001b[0m\n",
       "defined \u001b[32mclass \u001b[36mMyNone\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "sealed trait MyOption[A]\n",
    "case class MySome[A](get: A) extends MyOption[A]\n",
    "case class MyNone[A]() extends MyOption[A]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mf\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def f(a: MyOption[Any]) = a"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres6\u001b[0m: \u001b[32mMyOption\u001b[0m[\u001b[32mAny\u001b[0m] = MySome(1)"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "f(MySome[Any](1))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "ename": "",
     "evalue": "",
     "output_type": "error",
     "traceback": [
      "\u001b[31mMain.scala:27: type mismatch;",
      " found   : cmd6.this.$ref$cmd4.MySome[Int]",
      " required: cmd5.INSTANCE.$ref$cmd4.MyOption[Any]",
      "Note: Int <: Any, but trait MyOption is invariant in type A.",
      "You may wish to define A as +A instead. (SLS 4.5)",
      "f(MySome[Int](1))",
      "             ^\u001b[0m"
     ]
    }
   ],
   "source": [
    "f(MySome[Int](1))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mtrait \u001b[36mMyOption2\u001b[0m\n",
       "defined \u001b[32mclass \u001b[36mMySome2\u001b[0m\n",
       "defined \u001b[32mclass \u001b[36mMyNone2\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "sealed trait MyOption2[+A]\n",
    "case class MySome2[A](get: A) extends MyOption2[A]\n",
    "case class MyNone2[A]() extends MyOption2[A]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mf\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def f(a: MyOption2[Any]) = a"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres9\u001b[0m: \u001b[32mMyOption2\u001b[0m[\u001b[32mAny\u001b[0m] = MySome2(1)"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "f(MySome2[Int](1))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Contravariance"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mtoS\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def toS(a: Any) = a.toString"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres11\u001b[0m: \u001b[32mString\u001b[0m = \u001b[32m\"List(1, 2, 3)\"\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "toS(List(1,2,3))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mfunction \u001b[36mf\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "def f(a: String => String, s: String) = a(s)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres13\u001b[0m: \u001b[32mString\u001b[0m = \u001b[32m\"abc\"\u001b[0m"
      ]
     },
     "metadata": {},
     "output_type": "display_data"
    }
   ],
   "source": [
    "// This works even though f is String => String\n",
    "f(toS, \"abc\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "To understand why take a look at [Function1](http://scala-lang.org/files/archive/api/2.11.8/#scala.Function1):\n",
    "```\n",
    "trait Function1[-T1, +R] extends AnyRef\n",
    "```\n",
    "\n",
    "which is contravariant in input."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "#### Acknowledgement\n",
    "Thanks to [@deaktator](https://github.com/deaktator) for explaining these to me"
   ]
  }
 ],
 "metadata": {
  "anaconda-cloud": {},
  "kernelspec": {
   "display_name": "Scala 2.11",
   "language": "scala211",
   "name": "scala211"
  },
  "language_info": {
   "codemirror_mode": "text/x-scala",
   "file_extension": ".scala",
   "mimetype": "text/x-scala",
   "name": "scala211",
   "pygments_lexer": "scala",
   "version": "2.11.8"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}