How FP Made Grammarly’s Codebase Error-Free & Predictable

How FP Made Grammarly’s Codebase Error-Free and Predictable Anton Pets @oopets 7 March 2020

Grammarly - a writing assistant that helps make your communication clear and effective, wherever you type. 3

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Grammarly Challenges 5

Text is difficult 6

Change is difficult ➡ 7

Our combo Object Oriented Design 8

Our combo Object Oriented Design 9

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TypeScript is… Args validation - free & out of the box 11

TypeScript is… Args validation - free & out of the box 12

TypeScript is… 13

TypeScript is… 💚 14

TypeScript is… Confident refactoring 15

TypeScript is… Interfaces! 16

TypeScript is… Interfaces! 17

= ❤ Types + 18

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TypeScript FP libs Funfix fp-ts 21

Functional Programming: Real-Life Cases 22

💼 Case 1/4 23

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😊 26

🧪 How to experiment? 27

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🤔 Can we 🧪 better ? 32

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Immutability 39

Pure Functions Deterministic: Same Input ➡ Same output No uncontrolled side effects 40

Pure Functions Deterministic: Same Input ➡ Same output No uncontrolled side effects 41

Pure Functions 42

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What we achieved ⛳ Reliable Almost nobody knows about 🧪 ⛳ Extensible Add ♾ number of 🧪 44

💼 Case 2/4 45

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🙏 Will not compile 50

🤔 But how to deal with null? 51

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🤔 Empty alert? Exception? … 53

Null 2) Undefined Bad 54

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Option ≡ an array with 0…1 elements 60

map filter forEach contains … empty getOrElse fold … 61

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What we achieved ⛳ Reliable Empty values are safe ⛳ Clean No ifs overload, empty values handled separately 65

💼 Case 3/4 66

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🤔 How to handle errors correctly? 79

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What we achieved ⛳ Clean & Confident No defensive programming ⛳ Singe Responsibility Business logics is separated from error handling 85

💼 Case 4/4 86

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🏋 89

React lazy loading 👍 90

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https://dev.to

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🤔 How to build a good retry function?

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Lets check github.com/gcanti/fp-ts 109

Promise in FP Task 110

Promise in FP Task 111

Promise in FP Task 112

Error + Result container Either<Error, Result> 113

Error + Result container Either<Error, Result> 114

Error + Result container Either<Error, React.Component> where type Error = TimeoutError | 404Error 115

Task returning Either Task<Either<Error, Result>> alias: TaskEither<Error, Result> 116

Task returning Either TaskEither<Error, Result> () => Promise< Either<Error, Result> > 117

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☑ ✅ 119

github.com/gcanti/retry-ts by the author of fp-ts 120

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You can’t just stop speaking about FP

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☑ ✅ ☑ 131

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☑ ☑ ☑ ☑ ✅ 141

Сode examples: done 146

What we achieved ⛳ Flexible Combine different retry policies ⛳ Predictable Know your errors in advance ⛳ Isolated side effects Combine complex tasks without execution 147

Problem - Solution Empty values ➡ 148 Option

Problem - Solution Empty values ➡ Option Exceptions ➡ Try (or Either) 149

Problem - Solution Empty values ➡ Option Exceptions ➡ Try (or Either) Async side effects ➡ Task / TaskEither 150

Remember?

Option Try, Either Task, TaskEither 152

One last thing FP is about composition: Function composition Monadic composition 153

Conclusions Twitter: @oopets 154

The last thing Composing Software: The Book E. Elliot Functional Programming in Scala P. Chiusano, R. Bjarnason Blog: https://dev.to/gcanti (fp-ts) 155