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-#+TITLE: Functional Programming in JavaScript
-#+PROPERTY: :html-toplevel-hlevel 1
-#+PROPERTY: :with-toc 0
-* Why?
-
-Imperative programming is concerned with *how*
-
-Functional programming is concerned with *what*
-
-* Concepts
-
- - First-class Functions
- - Higher-order Functions
- - Recursion
- - Pure functions
- - Currying & Partial Application
-
-* Further concepts
-
- - Lazy Evaluation
- - Types & Data Structures
- - Category Theory
-
-* First-class functions
-
- - Have no restriction on their use
- - Are values
- - Enable the use of callback functions in JavaScript
-
-
-
-#+BEGIN_SRC js
-const fn = function () {
-  return 2
-}
-#+END_SRC
-
-
-* Higher-order functions
-
-Functions that operate on other functions are higher-order functions
-
-
-
-#+BEGIN_SRC js
-const succ = function (x) {
-  return x + 1
-}
-
-const arr = [1, 2, 3, 4]
-
-arr.map(succ)
-#+END_SRC
-
-Here, =Array.prototype.map= is the higher-order function
-
-* Higher-order functions (cont.)
-
-Functions that return functions are also higher-order functions
-
-
-
-#+BEGIN_SRC js
-function adder (n) {
-  return function (x) {
-    return n + x
-  }
-}
-
-const add1 = adder(1)
-#+END_SRC
-
-=adder= is a higher-order function
-
-* Pure functions
-
-Functions without side-effects
-
-#+BEGIN_SRC js
-const succ = (x) => x + 1
-
-console.log(succ(succ(1)))
-
-// could be optimised away by a compiler, e.g.:
-
-console.log(3)
-#+END_SRC
-
-* Recursion
-
-Functions that call themselves
-
-#+BEGIN_SRC js
-function fibonacci (n) {
-  switch (n) {
-    case 0:
-    case 1:
-      return 1
-    default: 
-      return fibonacci(n - 1) + fibonacci(n - 2)
-  }
-}
-#+END_SRC
-
-* Partial application
-
-The infamous =Function.prototype.bind= in JavaScript
-
-#+BEGIN_SRC js
-function add (x, y) {
-  return x + y
-}
-
-const add1 = add.bind(add, 1)
-
-add1(3) // = 4
-#+END_SRC
-
-* Partial application (cont.)
-
-After ES6 introduced arrow functions, partial application has become
-more popular
-
-#+BEGIN_SRC js
-const add = x => y => x + y
-#+END_SRC
-
-* Currying
-
-Related to partial application, but more implicit and general
-
-Translates */1/ function of arity /n/* to */n/ functions of arity /1/*
-
-#+BEGIN_SRC js
-function volume (w, d, h) {
-  return w * d * h
-}
-
-const vol = curry(volume)
-vol(10)(20)(30)
-// is strictly equivalent to
-volume(10, 20, 30)
-#+END_SRC
-
-* Easy Currying
-
-In order to make currying (and partial application) easier to use,
-move the *most important* argument to a function to the end:
-
-#+BEGIN_SRC js
-const badMap = (arr, fn) => arr.map(fn)
-const goodMap = (fn, arr) => arr.map(fn)
-const curriedBadMap = curry(badmap)
-const curriedGoodMap = curry(goodMap)
-
-const goodDoubleArray = goodMap(x => x * 2)
-const badDoubleArray = badMap(_, x => x * 2)
-#+END_SRC
-
-The bad version requires the curry function to support a magic
-placeholder argument and doesn't look as clean.
-
-* Practical Currying
-
-Currying is not automatic in JavaScript, as in other languages
-
-External tools don't (currently) to statically analyse curried
-functions
-
-Solution: Don't expose curried functions
-Instead, write functions as if currying were automatic
-
-* Functional composition
-
-Creating functions from other functions
-
-Usually provided by =compose= (right-to-left) and =pipe= (left-to-right)
-
-A very simple definition of =compose= for only two functions would look like this
-
-#+BEGIN_SRC js
-function compose (f, g) {
-  return function (...args) {
-    return f(g(...args))
-  }
-}
-#+END_SRC
-
-* Functional composition (cont.)
-
-#+BEGIN_SRC js
-const plusOne = x => x + 1
-const timesTwo = x => x * 2
-
-const plusOneTimesTwo = compose(timesTwo, plusOne)
-const timesTwoPlusOne = compose(plusOne, timesTwo)
-
-nextDoubled(3) // = (3 + 1) * 2 = 8
-timesTwoPlusOne(3) // = (3 * 2) + 1 = 7
-#+END_SRC
-
-* pipe
-
-What about =pipe=?
-
-=pipe= does the same thing, but runs the functions the other way around
-
-=pipe(f, g)= is the same as =compose(g, f)=
-
-* Point-free programming
-
-With currying and higher-order functions, we (often) don't need to declare function arguments
-
-#+BEGIN_SRC js
-const modulo = a => b => b % a
-const eq = a => b => a === b
-
-const isEven = x => eq(0)(modulo(2)(x))
-const isEvenPointFree = compose(eq(0), modulo(2))
-#+END_SRC
-
-* Further Resources
-
-- [[https://drboolean.gitbooks.io/mostly-adequate-guide/content/][Mostly adequate guide to FP (in javascript)]]
-- [[http://ramdajs.com/][Ramda]], a general-purpose FP library
-- [[https://sanctuary.js.org/][Sanctuary]], a JavaScript library for Haskellers