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 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<p style=\"float: left;\"><a href=\"tuples.ipynb\" target=\"_blank\">Previous</a></p>\n",
    "<p style=\"float: right;\"><a href=\"higher-order-functions.ipynb\" target=\"_blank\">Next</a></p>\n",
    "<p style=\"text-align:center;\">Tour of Scala</p>\n",
    "<div style=\"clear: both;\"></div>\n",
    "\n",
    "# Class Composition with Mixins\n",
    "Mixins are traits which are used to compose a class."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "I'm an instance of class B\n",
      "I'M AN INSTANCE OF CLASS B\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mclass\u001b[39m \u001b[36mA\u001b[39m\n",
       "defined \u001b[32mclass\u001b[39m \u001b[36mB\u001b[39m\n",
       "defined \u001b[32mtrait\u001b[39m \u001b[36mC\u001b[39m\n",
       "defined \u001b[32mclass\u001b[39m \u001b[36mD\u001b[39m\n",
       "\u001b[36md\u001b[39m: \u001b[32mD\u001b[39m = ammonite.$sess.cmd0$Helper$D@36fa807"
      ]
     },
     "execution_count": 1,
     "metadata": {},
     "output_type": "execute_result"
    }
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   "source": [
    "abstract class A {\n",
    "  val message: String\n",
    "}\n",
    "class B extends A {\n",
    "  val message = \"I'm an instance of class B\"\n",
    "}\n",
    "trait C extends A {\n",
    "  def loudMessage = message.toUpperCase()\n",
    "}\n",
    "class D extends B with C\n",
    "\n",
    "val d = new D\n",
    "println(d.message)  // I'm an instance of class B\n",
    "println(d.loudMessage)  // I'M AN INSTANCE OF CLASS B"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Class `D` has a superclass `B` and a mixin `C`. Classes can only have one superclass but many mixins (using the keywords `extends` and `with` respectively). The mixins and the superclass may have the same supertype.\n",
    "\n",
    "Now let's look at a more interesting example starting with an abstract class:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mclass\u001b[39m \u001b[36mAbsIterator\u001b[39m"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "abstract class AbsIterator {\n",
    "  type T\n",
    "  def hasNext: Boolean\n",
    "  def next(): T\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The class has an abstract type `T` and the standard iterator methods.\n",
    "\n",
    "Next, we'll implement a concrete class (all abstract members `T`, `hasNext`, and `next` have implementations):"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mclass\u001b[39m \u001b[36mStringIterator\u001b[39m"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "class StringIterator(s: String) extends AbsIterator {\n",
    "  type T = Char\n",
    "  private var i = 0\n",
    "  def hasNext = i < s.length\n",
    "  def next() = {\n",
    "    val ch = s charAt i\n",
    "    i += 1\n",
    "    ch\n",
    "  }\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`StringIterator` takes a `String` and can be used to iterate over the String (e.g. to see if a String contains a certain character).\n",
    "\n",
    "Now let's create a trait which also extends `AbsIterator`."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mtrait\u001b[39m \u001b[36mRichIterator\u001b[39m"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "trait RichIterator extends AbsIterator {\n",
    "  def foreach(f: T => Unit): Unit = while (hasNext) f(next())\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This trait implements `foreach` by continually calling the provided function `f: T => Unit` on the next element (`next()`) as long as there are further elements (`while (hasNext)`). Because `RichIterator` is a trait, it doesn't need to implement the abstract members of AbsIterator.\n",
    "\n",
    "We would like to combine the functionality of `StringIterator` and `RichIterator` into a single class."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "defined \u001b[32mobject\u001b[39m \u001b[36mStringIteratorTest\u001b[39m"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "object StringIteratorTest extends App {\n",
    "  class RichStringIter extends StringIterator(\"Scala\") with RichIterator\n",
    "  val richStringIter = new RichStringIter\n",
    "  richStringIter foreach println\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The new class `RichStringIter` has `StringIterator` as a superclass and `RichIterator` as a mixin.\n",
    "\n",
    "With single inheritance we would not be able to achieve this level of flexibility.\n",
    "<p style=\"float: left;\"><a href=\"tuples.ipynb\" target=\"_blank\">Previous</a></p>\n",
    "<p style=\"float: right;\"><a href=\"higher-order-functions.ipynb\" target=\"_blank\">Next</a></p>\n",
    "<p style=\"text-align:center;\">Tour of Scala</p>\n",
    "<div style=\"clear: both;\"></div>"
   ]
  }
 ],
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