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Practical Functional Programming Jeremy Fairbank @elpapapollo

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@testdouble helps improves how the world build software. testdouble.com

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bit.ly/programming-elm

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Functional programming:

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Pure Functional programming:

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Pure Total Functional programming:

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Pure Total Idempotent Functional programming:

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Pure Total Idempotent Monad Functional programming:

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Pure Total Idempotent Monad Functor Functional programming:

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Easier-to-reason-about- oriented programming

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Pure Total Idempotent Monad Functor

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5 Become a rockstar programmer with these tricks!

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Hard to test

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })

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const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })

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const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })

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const td = require('testdouble') describe('order details', () => { let originalLog = console.log let printOrderDetails let fetchJson beforeEach(() => { fetchJson = td.replace('./fetchJson') console.log = td.function('console.log') printOrderDetails = require('./printOrderDetails') }) afterEach(() => { td.reset() console.log = originalLog })

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it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })

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it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })

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it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })

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it('prints details for multiple orders', async () => { td .when(fetchJson('/orders?ids=1,2,3')) .thenResolve([ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ]) await printOrderDetails([1, 2, 3]) td.verify(console.log('Order 1 on 12/1/17 by Tucker')) td.verify(console.log('Order 2 on 11/25/17 by Sally')) td.verify(console.log('Order 3 on 3/30/18 by Joe')) }) })

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Side Effects

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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async function printOrderDetails(ids) { let url = `/orders?ids=${ids.join(',')}` let orders = await fetchJson(url) for (let i = 0; i < orders.length; i++) { let { id, date, customer } = orders[i] let details = `Order ${id} on ${date} by ${customer}` console.log(details) } }

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Less Predictable

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Harder to Test

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Functions

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const add = (x, y) => x + y add(2, 3) === 5 add(2, 3) === 5 add(2, 3) === 5

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const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)

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const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)

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const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)

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const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)

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const url = ids => `/orders?ids=${ids.join(',')}` const orderDetails = ({ id, date, customer }) => `Order ${id} on ${date} by ${customer}` const detailsForOrders = orders => orders.map(orderDetails)

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{ id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' }

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{ id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' } [ 'Order 1 on 12/1/17 by Tucker', ] map orderDetails

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[ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', ] orderDetails { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' } map

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orderDetails [ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', 'Order 3 on 3/30/18 by Joe', ] map { id: 1, date: '12/1/17', customer: 'Tucker' } { id: 2, date: '11/25/17', customer: 'Sally' } { id: 3, date: '3/30/18', customer: 'Joe' }

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it('uses the correct url', () => { expect(url([1, 2, 3])).toEqual('/orders?ids=1,2,3') })

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it('creates details for orders', () => { let orders = [ { id: 1, date: '12/1/17', customer: 'Tucker' }, { id: 2, date: '11/25/17', customer: 'Sally' }, { id: 3, date: '3/30/18', customer: 'Joe' }, ] expect(detailsForOrders(orders)).toEqual([ 'Order 1 on 12/1/17 by Tucker', 'Order 2 on 11/25/17 by Sally', 'Order 3 on 3/30/18 by Joe', ]) })

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async function printOrderDetails(ids) { let orders = await fetchJson(url(ids)) let details = detailsForOrders(orders) details.forEach(orderDetails => { console.log(orderDetails) }) }

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Hard to test Code breaks unexpectedly

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class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }

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class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }

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class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }

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class Person { constructor(name, hobbies) { this.name = name this.hobbies = hobbies } describe() { if (this.hobbies.length === 0) { return `${this.name} doesn't do much` } let hobbies = this.hobbies.splice(0, 2).join(' and ') return `${this.name} likes to do things such as ${hobbies}` } }

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let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music

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let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music

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let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music

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let person = new Person('Jeremy', [ 'programming', 'reading', 'playing music', ]) person.describe() // Jeremy likes to do things such as programming and reading person.describe() // Jeremy likes to do things such as playing music

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let hobbies = this.hobbies.splice(0, 2).join(' and ')

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Mutable Data

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Immutable Data Safety and consistency

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class Person { constructor(name, hobbies) { this.name = name this.hobbies = Object.freeze(hobbies) } // ... }

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class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } }

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class Person { // ... describe() { // ... let hobbies = this.hobbies.splice(0, 2).join(' and ') // ... } } TypeError: Cannot add/remove sealed array elements

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class Person { // ... describe() { // ... let hobbies = this.hobbies.slice(0, 2).join(' and ') // ... } }

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Create New State

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const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }

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const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }

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const create = (name, hobbies) => Object.freeze({ name, hobbies: Object.freeze(hobbies) }) function describe(person) { if (person.hobbies.length === 0) { return `${person.name} doesn't do much` } let hobbies = person.hobbies.slice(0, 2).join(' and ') return `${person.name} likes to do things such as ${hobbies}` }

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const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

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const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

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const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

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const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

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const addHobby = (hobby, person) => Object.freeze({ ...person, hobbies: Object.freeze([...person.hobbies, hobby]), })

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let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

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let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

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let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

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let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

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let person = create('Jeremy', []) person = addHobby('programming', person) person = addHobby('reading', person) describe(person) // Jeremy likes to do things such as programming and reading

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Hard to test Code breaks unexpectedly Too much code

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const greetInEnglish = name => `Hi, ${name}` const greetInSpanish = name => `Hola, ${name}`

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const greet = (greeting, name) => `${greeting}, ${name}` // .. const greetInEnglish = name => greet('Hi', name) const greetInSpanish = name => greet('Hola', name)

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') greetInEnglish('All Things Open') // Hi, All Things Open greetInSpanish('All Things Open') // Hola, All Things Open

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const greet = greeting => name => `${greeting}, ${name}` const greetInEnglish = greet('Hi') const greetInSpanish = greet('Hola') Curried Function Partial Application

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greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

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greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

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greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

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greet greeting name = greeting ++ ", " ++ name greetInEnglish = greet "Hi" greetInSpanish = greet "Hola"

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greetInEnglish "All Things Open" -- Hi, All Things Open greetInSpanish "All Things Open" -- Hola, All Things Open

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No content

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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const excitedGreeting = name => // ...

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const toUpper = string => string.toUpperCase() const greet = greeting => name => `${greeting}, ${name}` const exclaim = phrase => `${phrase}!` const excitedGreeting = name => exclaim(greet('Hi')(toUpper(name))) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, // ALL THINGS OPEN greet('Hi'), exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), // Hi, ALL THINGS OPEN exclaim, )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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const R = require('ramda') const excitedGreeting = name => R.pipe( toUpper, greet('Hi'), exclaim, // Hi, ALL THINGS OPEN! )(name) excitedGreeting('All Things Open') // Hi, ALL THINGS OPEN!

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Function Composition

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greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!

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greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!

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greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!

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greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!

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greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper -- ALL THINGS OPEN |> greet "Hi" |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!

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greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" -- Hi, ALL THINGS OPEN |> exclaim excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!

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greet greeting name = greeting ++ ", " ++ name exclaim phrase = phrase ++ "!" excitedGreeting name = name |> String.toUpper |> greet "Hi" |> exclaim -- Hi, ALL THINGS OPEN! excitedGreeting "All Things Open" -- Hi, ALL THINGS OPEN!

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function createOrder(shoppingCart, customer) { let total = 0 shoppingCart.items.forEach(item => { total += item.price }) shoppingCart.discounts.forEach(discount => { total *= 1 - discount }) total *= 1 + shoppingCart.tax total += shoppingCart.shippingCost total = Math.round(total * 100) / 100 return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }

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function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }

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Hard to test Code breaks unexpectedly Too much code Scary to refactor

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function createOrder(shoppingCart, customer) { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }

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const createOrder = customer => shoppingCart => { let total = R.pipe( addPrices(shoppingCart.items), addDiscounts(shoppingCart.discounts), addTax(shoppingCart.tax), add(shoppingCart.shippingCost), roundTotal, )(0) return { status: 'new', customer, total } }

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createOrder(myShoppingCart, 'Tucker')

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Strong Static Types

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createOrder shoppingCart customer = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }

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createOrder shoppingCart customer = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }

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createOrder shoppingCart customer = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }

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createOrder customer shoppingCart = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = "new" , customer = customer , total = total }

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-- TYPE MISMATCH ------------------------- Responsibilities.elm The 2nd argument to `createOrder` is not what I expect: 66| createOrder myShoppingCart "Tucker" ^^^^^^^^ This argument is a string of type: String But `createOrder` needs the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float }

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-- TYPE MISMATCH ------------------------- Responsibilities.elm The 2nd argument to `createOrder` is not what I expect: 66| createOrder myShoppingCart "Tucker" ^^^^^^^^ This argument is a string of type: String But `createOrder` needs the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float }

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-- TYPE MISMATCH ------------------------- Responsibilities.elm The 2nd argument to `createOrder` is not what I expect: 66| createOrder myShoppingCart "Tucker" ^^^^^^^^ This argument is a string of type: String But `createOrder` needs the 2nd argument to be: { b | discounts : List Float , items : List { a | price : Float } , shippingCost : Float , tax : Float }

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conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ String.fromFloat amount

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conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ String.fromFloat amount

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conference : String conference = "Lambda Squared" life : Int life = 42 greet : String -> String greet name = "Hello, " ++ name formatCurrency : Float -> String formatCurrency amount = "$" ++ String.fromFloat amount

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Custom Types

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type alias Item = { price : Float }

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type alias Item = { price : Float }

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type alias Item = { price : Float }

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type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }

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type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float }

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type alias Item = { price : Float } type alias ShoppingCart = { items : List Item , discounts : List Float , tax : Float , shippingCost : Float } type alias Order = { status : String , customer : String , total : Float }

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createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...

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createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = ...

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type alias Order = { status : String , customer : String , total : Float }

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type alias Order = { status : String , customer : String , total : Float }

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type Status = New | Processing | Shipped

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type Status = New | Processing | Shipped

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type Status = New | Processing | Shipped

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type Status = New | Processing | Shipped type alias Order = { status : Status , customer : String , total : Float }

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createOrder : ShoppingCart -> String -> Order createOrder shoppingCart customer = let total = addPrices shoppingCart.items 0 |> addDiscounts shoppingCart.discounts |> addTax shoppingCart.tax |> add shoppingCart.shippingCost |> roundTotal in { status = New , customer = customer , total = total }

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Hard to test Code breaks unexpectedly Too much code Scary to refactor Oh, null

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function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

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function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

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function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

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function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location }

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getBestFriendLocation(42, users) Uncaught TypeError: Cannot read property 'bestFriendId' of null

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function getBestFriendLocation(id, users) { let user = users.get(id) let friend = users.get(user.bestFriendId) return friend.location } null

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function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } }

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function getBestFriendLocation(id, users) { let user = users.get(id) if (user) { let friend = users.get(user.bestFriendId) return friend.location } } null

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function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } }

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function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) return friend.location } } null

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function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }

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No content

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if () {} if () {} if () {}

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type Maybe a = Nothing | Just a

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type Maybe a = Nothing | Just a

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type Maybe a = Nothing | Just a

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type Maybe a = Nothing | Just a

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42 Just 42 "Hi" Just "Hi"

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Just 42 [42] Nothing [ ]

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getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

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getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

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getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

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getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users

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getUserName : Id -> Dict Id User -> String getUserName id users = Dict.get id users Maybe User

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getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name

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getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name

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getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name user

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getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name

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-- MISSING PATTERNS ------------------------------ App.elm This `case` does not have branches for all possibilities: 31|> case Dict.get id users of 32|> Just user -> 33|> user.name Missing possibilities include: Nothing I would have to crash if I saw one of those. Add branches for them!

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getUserName : Id -> Dict Id User -> String getUserName id users = case Dict.get id users of Just user -> user.name Nothing -> ""

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function getBestFriendLocation(id, users) { let user = users.get(id) if (user && user.bestFriendId) { let friend = users.get(user.bestFriendId) if (friend) { return friend.location } } }

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getBestFriendLocation : Id -> Dict Id User -> Maybe Location getBestFriendLocation id users = case Dict.get id users of Just user -> case user.bestFriendId of Just bestFriendId -> case Dict.get bestFriendId users of Just friend -> friend.location Nothing -> Nothing Nothing -> Nothing Nothing -> Nothing

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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getBestFriendLocation id users = Dict.get id users |> Maybe.andThen (\user -> user.bestFriendId) |> Maybe.andThen (\fid -> Dict.get fid users) |> Maybe.andThen (\friend -> friend.location)

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P.S. You kinda, sorta just learned monads

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Hard to test Code breaks unexpectedly Too much code Scary to refactor Oh, null

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Hard to test Pure Functions Code breaks unexpectedly Too much code Scary to refactor Oh, null

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Code breaks unexpectedly Immutable Data Too much code Scary to refactor Oh, null Hard to test Pure Functions

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Too much code Curried, Composable Functions Scary to refactor Oh, null Code breaks unexpectedly Immutable Data Hard to test Pure Functions

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Scary to refactor Strong, Static Types Oh, null Too much code Curried, Composable Functions Code breaks unexpectedly Immutable Data Hard to test Pure Functions

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Oh, null Maybe and Monads Scary to refactor Strong, Static Types Too much code Curried, Composable Functions Code breaks unexpectedly Immutable Data Hard to test Pure Functions

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Thanks! bit.ly/ato-practical-fp Jeremy Fairbank @elpapapollo