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# Functional Programming is not just Lambda Functions
## David Sferruzza
### 2017-06-08
---
# About me
- [@d_sferruzza](https://twitter.com/d_sferruzza)
- [github.com/dsferruzza](https://github.com/dsferruzza)
- Head of Research and Development at [Startup Palace](https://www.startup-palace.com)
- PhD student in software engineering at *University of Nantes*
 <!-- .element: class="stretch" -->
---
# FP is for Functional Programming
> FP is a **declarative** programming paradigm, which means programming is done with **expressions**.
It is becoming more and more popular.
 <!-- .element: class="stretch" -->
---
# Declarative vs. imperative
> **Imperative programming**: use statements that change a program's state
>
> **Declarative programming**: express the logic of a computation without describing its control flow
- imperative: shopping instructions
- declarative: shopping list
---
# Lambda functions
*also called __anonymous function__*
```javascript
// ▼ regular function
function myFunction(foo) {
return foo + 1;
}
// ▼ lambda function
var myLambdaFunction = function(foo) {
return foo + 1;
}
// ▼ lambda function
var myLambdaFunction = foo => foo + 1;
```
---
# λ-calculus
- Alonzo Church (1930s)
- formal system for expressing computation
- equivalent to the Turing machine
 <!-- .element: class="stretch" -->
<small>See: [The Lambda Calculus and The JavaScript](http://fr.slideshare.net/normanrichards/the-lambda-calculus-and-the-javascript)</small>
---
# Learning FP is like...
- learning **new ways to think** about programming
- **writting better code** right now
- even with *non-functional* languages
 <!-- .element: class="stretch" -->
Languages that allow to use some **FP concepts**:
- FP languages (Haskell, Scala, Erlang, ...)
- a lot of *non-FP* languages \o/ (like JS)
---
# Concepts
Let's explore some FP concepts:
- referential transparency
- higher-order functions
- lazy evaluation
- immutability
and apply them with JS!
---
# Impure function
```javascript
function divideByTwo(number) {
var rightNow = new Date();
launchNuke(rightNow);
return number / 2;
}
```
This function has *observable* **side-effects** (obviously).
But this can't be guessed by looking to its signature.
So we need to be very careful when manipulating it!
 <!-- .element: class="stretch" -->
---
# Pure function
```javascript
function addFive(number) {
return number + 5;
}
```
This function has no side-effects.
It **always** returns the same result when given the same arguments.
```javascript
addFive(-4) === addFive(-4)
```
 <!-- .element: class="stretch" -->
---
# Referential transparency
> An expression is said to be referentially transparent if it can be replaced with its corresponding value without changing the program's behavior.
Using pure functions allows to **reason** on programs as if they were equations.
 <!-- .element: class="stretch" -->
---
# Advice
## Build your programs with pure functions wherever possible
Business logic should be reliable and easily testable.
## Isolate functions with side-effects
We want to know which functions can make side-effects, so we can be careful when calling them.
---
# Concepts
- ~~referential transparency~~
- higher-order functions
- lazy evaluation
- immutability
---
# Higher-order functions
An higher-order function is just a function that verifies at least one of the following properties:
- it takes one or more functions as arguments
- it returns a function as its result
 <!-- .element: class="stretch" -->
---
# Example
```javascript
function applyTwice(f, x) {
return f(f(x));
}
// ^ takes a function as argument
function timesThree(x) {
return x * 3;
}
applyTwice(timesThree, 7);
// timesThree(timesThree(7))
// timesThree(21)
// 63
```
---
# What's the point?
- having a good *separation of concerns*
- creating complex functions by composing simple functions
 <!-- .element: class="stretch" -->
---
# Play time!
Let's apply our knowledge of higher-order functions by manipulating JS arrays and magical ponies!
 <!-- .element: class="stretch" -->
---
# Context
```javascript
var ponies = [
{ name: "Rainbow Dash", color: "purple",
flying: true },
{ name: "Applejack", color: "orange",
flying: false },
{ name: "Fluttershy", color: "yellow",
flying: true },
];
```
We want to get a list of strings formatted like this:
`name (color)`
---
# Context
We will write a `displayPonies` function so that:
```javascript
displayPonies(ponies);
// [ "Rainbow Dash (purple)",
// "Applejack (orange)",
// "Fluttershy (yellow)" ]
```
---
# Imperative solution
```javascript
function displayPonies(ponies) {
for (var i = 0; i < ponies.length; i++) {
ponies[i] = ponies[i].name +
" (" + ponies[i].color + ")";
}
return ponies;
}
```
We are mixing 2 behaviors:
- iterating on an array
- transforming our data
---
# Array.map
```javascript
function displayPonies(ponies) {
function transformation(pony) {
return pony.name +
" (" + pony.color + ")";
}
return ponies.map(transformation);
}
```
- `Array.map` handles array iteration for us \o/
- we handle the transformation
---
# With ES6 syntax
Let's use *lambda functions* and *template strings*!
```javascript
ponies.map(p => `${p.name} (${p.color})`);
// [ "Rainbow Dash (purple)",
// "Applejack (orange)",
// "Fluttershy (yellow)" ]
```
 <!-- .element: class="stretch" -->
---
# Array.filter
```javascript
ponies.filter(function(pony) {
// If a pony cannot fly, return true
// If it can, return false
return !pony.flying;
});
// [ { name: "Applejack", color: "orange",
// flying: false } ]
```
---
# Let's chain!
```javascript
ponies.filter(function(pony) {
return pony.flying;
}).map(function(pony) {
return pony.name + " (" + pony.color + ")";
});
// [ "Rainbow Dash (purple)",
// "Fluttershy (yellow)" ]
```
*Reminder: `Array.map` and `Array.filter` are higher-order functions.*
---
# Array.reduce
```javascript
var people = [
{ name: "Bruce", age: 30 },
{ name: "Tony", age: 35 },
{ name: "Peter", age: 26 },
];
people.reduce(function(acc, cur) {
return acc + cur.age;
}, 0);
// --> 91
```
---
# Array.reduce
```javascript
function map(array, transformation) {
return array.reduce(function(acc, cur) {
acc.push(transformation(cur));
return acc;
}, []);
}
function filter(array, predicate) {
return array.reduce(function(acc, cur) {
if (predicate(cur)) acc.push(cur);
return acc;
}, []);
}
```
---
# Advice
Avoid using loops to manipulate arrays.
## Cheat sheet
If you have an array and you want to:
- apply a transformation to each of its elements (keeping their order/number): **map**
- remove some of its elements (keeping order and value of the others): **filter**
- scan it to build a new data structure: **fold/reduce**
---
# Concepts
- ~~referential transparency~~
- ~~higher-order functions~~
- lazy evaluation
- immutability
---
# Evaluation strategy
- **when** to evaluate the arguments of a function call?
- **what kind of value** to pass to the function?
```javascript
myFunction(lower("WHATEVER"))
// ^ When to evaluate this?
// What to give to the body of myFunction?
```
 <!-- .element: class="stretch" -->
---
# Strict evaluation
*eager evaluation, greedy evaluation*
- **when:** as soon as possible
- **what kind of value:**
- *call by value*
- *call by reference*
- *call by sharing*
- ...
---
# Non-strict evaluation
*lazy evaluation*
- **call by name:** arguments are substituted directly into the function body
- **call by need:** same, with *memoization* (≈ *if* an argument is evaluated, its value is cached)
- ...
---
# Lazy evaluation
Delays the evaluation of an expression until its value is needed.
 <!-- .element: class="stretch" -->
---
# What's the point?
- **optimization:** avoid useless computations
- **maintainability:**
- it is possible to express infinite data structures
- it is possible to define control structures as abstractions, instead of primitives
---
# Lo-Dash
> A JavaScript utility library delivering consistency, modularity, performance, & extras.
<https://lodash.com/>
- a JS library that allows to manipulate collections
- *lazy* since version 3
---
# Example
```javascript
var list = [0, 1, 2, 3, 4];
function addOne(nb) {
console.log(nb + ' + 1');
if (nb > 2) console.log('Slow');
return nb + 1;
}
function isSmall(nb) {
console.log(nb + ' smaller than 3?');
return nb < 3;
}
```
---
# Without Lo-Dash
```javascript
var result = list
.map(addOne)
.filter(isSmall)
.slice(0, 2);
```
```text
0 + 1
1 + 1
2 + 1
3 + 1
Slow
4 + 1
Slow
1 smaller than 3?
2 smaller than 3?
3 smaller than 3?
4 smaller than 3?
5 smaller than 3?
[ 1, 2 ]
```
<!-- .element: class="smallcode" -->
---
# With Lo-Dash
```javascript
var _ = require('lodash');
var result = _(list)
.map(addOne)
.filter(isSmall)
.take(2)
.value();
```
```text
0 + 1
1 smaller than 3?
1 + 1
2 smaller than 3?
[ 1, 2 ]
```
<!-- .element: class="smallcode" -->
---
# Without Lo-Dash
 <!-- .element: class="stretch" -->
---
# With Lo-Dash
 <!-- .element: class="stretch" -->
---
# Lazy evaluation
- separation of
- definition *→ how computations are defined?*
- execution *→ when values are computed?*
- *glue* that allows to assemble pieces of programs efficiently
**Pros:** can help us achieve better maintainability *and* performances
**Cons:** can introduce some *overhead* (depending on the technology)
---
# Concepts
- ~~referential transparency~~
- ~~higher-order functions~~
- ~~lazy evaluation~~
- immutability
---
# Immutability
> An immutable object is an object whose state cannot be modified after it is created.
 <!-- .element: class="stretch" -->
---
# Mutable in JS
- objects
- (which implies) arrays
```javascript
var object = {
a: 1,
b: 'BATMAN',
};
var alias = object;
object.a = 2;
object // { a: 2, b: 'BATMAN' }
alias // { a: 2, b: 'BATMAN' }
```
---
# const != immutable
```javascript
const object = {
a: 1,
b: 'BATMAN',
};
const alias = object;
object.a = 2;
object // { a: 2, b: 'BATMAN' }
alias // { a: 2, b: 'BATMAN' }
```
`const` prevent us from modifying **references**, not values!
*`const` is nice though ;)*
---
# Immutable.js
> Immutable persistent data collections for Javascript which increase efficiency and simplicity.
<https://facebook.github.io/immutable-js/>
- a library that gives us a API for immutable collections
- List, Stack, [Ordered]Map, [Ordered]Set, Record, ...
---
# Immutable.js
```javascript
const Immutable = require('immutable');
var map1 = Immutable.Map({a:1, b:2, c:3});
var map2 = map1.set('b', 50);
map1.get('b'); // 2
map2.get('b'); // 50
var map3 = map2.set('b', 2);
map1.equals(map3); // true
```
*Careful: do not confuse `Map` (data structure) and `map` (function)!*
---
# Pros
- readability/maintainability: 1 value per reference
- no side-effects
- *thread safe*
But what about performances?
 <!-- .element: class="stretch" -->
---
# Performances
Introduces some *overhead*, but the balance maintainability vs. performance is often good.
 <!-- .element: class="stretch" -->
---
# Advice
## Use `const`
And use `let` when you cannot use `const`.
Do not use `var`.
## Avoid to create/use mutable API
A function that modifies an object should return a new object, not modify it secretly.
```javascript
const list = [1, 2, 3];
list.push(4);
// ^ side-effect
```
---
# Concepts
- ~~referential transparency~~
- ~~higher-order functions~~
- ~~lazy evaluation~~
- ~~immutability~~
---
# Going further
- [Why Functional Programming Matters](http://www.cs.kent.ac.uk/people/staff/dat/miranda/whyfp90.pdf): why higher-order functions and lazy evaluation are great tools to write modular programs
- [JavaScript Alongé](https://leanpub.com/javascriptallongesix): a advanced book on JavaScript that explains some of the concepts we saw today
- [Is your programming language unreasonable?](http://fsharpforfunandprofit.com/posts/is-your-language-unreasonable/): why predictability of programming languages is important
---
# Questions?
 <!-- .element: class="stretch" -->
Twitter: [@d_sferruzza](https://twitter.com/d_sferruzza)
Slides are on GitHub:
[dsferruzza/conf-fp-is-not-just-lambdas](http://github.com/dsferruzza/conf-fp-is-not-just-lambdas)
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