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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
|