{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<p style=\"float: left;\"><a href=\"nested-functions.ipynb\" target=\"_blank\">Previous</a></p>\n",
    "<p style=\"float: right;\"><a href=\"case-classes.ipynb\" target=\"_blank\">Next</a></p>\n",
    "<p style=\"text-align:center;\">Tour of Scala</p>\n",
    "<div style=\"clear: both;\"></div>\n",
    "\n",
    "# Multiple Parameter Lists (Currying)\n",
    "\n",
    "Methods may define multiple parameter lists. When a method is called with a fewer number of parameter lists, then this will yield a function taking the missing parameter lists as its arguments. This is formally known as [currying](https://en.wikipedia.org/wiki/Currying).\n",
    "\n",
    "Here is an example, defined in [Traversable](https://docs.scala-lang.org/overviews/collections/trait-traversable.html) trait from Scala collections:"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "```scala\n",
    "def foldLeft[B](z: B)(op: (B, A) => B): B\n",
    "```\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "`foldLeft` applies a binary operator `op` to an initial value `z` and all elements of this traversable, going left to right. Shown below is an example of its usage. \n",
    "\n",
    "Starting with an initial value of 0, `foldLeft` here applies the function `(m, n) => m + n` to each element in the List and the previous accumulated value."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "55"
     ]
    },
    {
     "data": {
      "text/plain": [
       "\u001b[36mnumbers\u001b[39m: \u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m] = \u001b[33mList\u001b[39m(\u001b[32m1\u001b[39m, \u001b[32m2\u001b[39m, \u001b[32m3\u001b[39m, \u001b[32m4\u001b[39m, \u001b[32m5\u001b[39m, \u001b[32m6\u001b[39m, \u001b[32m7\u001b[39m, \u001b[32m8\u001b[39m, \u001b[32m9\u001b[39m, \u001b[32m10\u001b[39m)\n",
       "\u001b[36mres\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m"
      ]
     },
     "execution_count": 1,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "val numbers = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)\n",
    "val res = numbers.foldLeft(0)((m, n) => m + n)\n",
    "print(res) // 55"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Multiple parameter lists have a more verbose invocation syntax; and hence should be used sparingly. Suggested use cases include:\n",
    "\n",
    "#### Single functional parameter\n",
    "   In case of a single functional parameter, like `op` in the case of `foldLeft` above, multiple parameter lists allow a concise syntax to pass an anonymous function to the method. Without multiple parameter lists, the code would look like this:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "cmd1.sc:1: missing argument list for method foldLeft in trait LinearSeqOptimized\n",
      "Unapplied methods are only converted to functions when a function type is expected.\n",
      "You can make this conversion explicit by writing `foldLeft _` or `foldLeft(_)(_)` instead of `foldLeft`.\n",
      "val res1 = numbers.foldLeft(0, {(m: Int, n: Int) => m + n})\n",
      "                           ^Compilation Failed"
     ]
    },
    {
     "ename": "",
     "evalue": "",
     "output_type": "error",
     "traceback": [
      "Compilation Failed"
     ]
    }
   ],
   "source": [
    "numbers.foldLeft(0, {(m: Int, n: Int) => m + n})"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Note that the use of multiple parameter lists here also allows us to take advantage of Scala type inference to make the code more concise as shown below; which would not be possible in a non-curried definition."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mres1\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "numbers.foldLeft(0)(_ + _)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Above statement `numbers.foldLeft(0)(_ + _)` allows us to fix the parameter `z` and pass around a partial function and reuse it as shown below:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)List(1, 8, 27, 64, 125, 216, 343, 512, 729, 1000)"
     ]
    },
    {
     "data": {
      "text/plain": [
       "\u001b[36mnumbers\u001b[39m: \u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m] = \u001b[33mList\u001b[39m(\u001b[32m1\u001b[39m, \u001b[32m2\u001b[39m, \u001b[32m3\u001b[39m, \u001b[32m4\u001b[39m, \u001b[32m5\u001b[39m, \u001b[32m6\u001b[39m, \u001b[32m7\u001b[39m, \u001b[32m8\u001b[39m, \u001b[32m9\u001b[39m, \u001b[32m10\u001b[39m)\n",
       "\u001b[36mnumberFunc\u001b[39m: (\u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m], \u001b[32mInt\u001b[39m) => \u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m] => \u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m] = ammonite.$sess.cmd2$Helper$$Lambda$2487/1839209380@62413d6c\n",
       "\u001b[36msquares\u001b[39m: \u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m] = \u001b[33mList\u001b[39m(\u001b[32m1\u001b[39m, \u001b[32m4\u001b[39m, \u001b[32m9\u001b[39m, \u001b[32m16\u001b[39m, \u001b[32m25\u001b[39m, \u001b[32m36\u001b[39m, \u001b[32m49\u001b[39m, \u001b[32m64\u001b[39m, \u001b[32m81\u001b[39m, \u001b[32m100\u001b[39m)\n",
       "\u001b[36mcubes\u001b[39m: \u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m] = \u001b[33mList\u001b[39m(\u001b[32m1\u001b[39m, \u001b[32m8\u001b[39m, \u001b[32m27\u001b[39m, \u001b[32m64\u001b[39m, \u001b[32m125\u001b[39m, \u001b[32m216\u001b[39m, \u001b[32m343\u001b[39m, \u001b[32m512\u001b[39m, \u001b[32m729\u001b[39m, \u001b[32m1000\u001b[39m)"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "val numbers = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)\n",
    "val numberFunc = numbers.foldLeft(List[Int]())_\n",
    "\n",
    "val squares = numberFunc((xs, x) => xs:+ x*x)\n",
    "print(squares.toString()) // List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)\n",
    "\n",
    "val cubes = numberFunc((xs, x) => xs:+ x*x*x)\n",
    "print(cubes.toString())  // List(1, 8, 27, 64, 125, 216, 343, 512, 729, 1000)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Finally, `foldLeft` and `foldRight` can be used in any of the following terms,"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "attributes": {
     "classes": [
      "tut"
     ],
     "id": ""
    }
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "\u001b[36mnumbers\u001b[39m: \u001b[32mList\u001b[39m[\u001b[32mInt\u001b[39m] = \u001b[33mList\u001b[39m(\u001b[32m1\u001b[39m, \u001b[32m2\u001b[39m, \u001b[32m3\u001b[39m, \u001b[32m4\u001b[39m, \u001b[32m5\u001b[39m, \u001b[32m6\u001b[39m, \u001b[32m7\u001b[39m, \u001b[32m8\u001b[39m, \u001b[32m9\u001b[39m, \u001b[32m10\u001b[39m)\n",
       "\u001b[36mres3_1\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m\n",
       "\u001b[36mres3_2\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m\n",
       "\u001b[36mres3_3\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m\n",
       "\u001b[36mres3_4\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m\n",
       "\u001b[36mres3_5\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m\n",
       "\u001b[36mres3_6\u001b[39m: \u001b[32mInt\u001b[39m = \u001b[32m55\u001b[39m"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "val numbers = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)\n",
    "\n",
    "numbers.foldLeft(0)((sum, item) => sum + item) // Generic Form\n",
    "numbers.foldRight(0)((sum, item) => sum + item) // Generic Form\n",
    "\n",
    "numbers.foldLeft(0)(_+_) // Curried Form\n",
    "numbers.foldRight(0)(_+_) // Curried Form\n",
    "\n",
    "(0 /: numbers)(_+_) // Used in place of foldLeft\n",
    "(numbers :\\ 0)(_+_) // Used in place of foldRight"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "#### Implicit parameters\n",
    "To specify certain parameters in a parameter list as `implicit`, multiple parameter lists should be used. An example of this is:"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "```scala\n",
    "def execute(arg: Int)(implicit ec: ExecutionContext) = ???\n",
    "```\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<p style=\"float: left;\"><a href=\"nested-functions.ipynb\" target=\"_blank\">Previous</a></p>\n",
    "<p style=\"float: right;\"><a href=\"case-classes.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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