How FP made Grammarly codebase error-Free & Predictable

How FP made Grammarly codebase error-Free & Predictable Anton Pets 1 October 2019

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

Grammarly - a writing assistant that helps make your communication clear and effective, wherever you type. 20M Daily Active Users 4

! Kyiv hiring! ” San Francisco, New York # Vancouver new! 5

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Grammarly Challenges 8

Text is difficult 9

Change is difficult ➡ 10

Our combo Object Oriented Design 11

Our combo Object Oriented Design 12

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TypeScript is… Args validation - free & out of the box 14

TypeScript is… 15

TypeScript is… 💚 16

TypeScript is… Confident refactoring 17

TypeScript is… Interfaces! 18

= ❤ Types + 19

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TypeScript FP libs Funfix fp-ts 23

TypeScript FP libs Funfix fp-ts 24

Functional Programming: Real Life Cases 25

💼 Case 1/4 26

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🧪 How to experiment ? 31

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Can we 🧪 better ? 36

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Pure Functions Deterministic: Same Input ➡ Same output No uncontrolled side effects 46

Pure Functions Deterministic: Same Input ➡ Same output No uncontrolled side effects 47

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⛳ What we achieved 49

⛳ Reliable almost nobody knows about 🧪 50

⛳ Extensible add ♾ number of 🧪 51

💼 Case 2/4 52

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🙏 Will not compile 58

🤔 But how to deal with null? 59

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🤔 Empty alert? Exception? … 61

Bad 62

Null 2) Undefined

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Option ≡ an array with 0…1 elements 69

map filter forEach contains … 70

map filter forEach contains … empty getOrElse fold … 71

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⛳ What we achieved 77

⛳ Reliable empty values are safe 78

⛳ Clean no ifs overload, empty values handled separately 79

💼 Case 3/4 80

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🤔 How to handle errors correctly? 94

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⛳ What we achieved 101

⛳ Clean & Confident no defensive programming 102

⛳ Single Responsibility Business logics is separated from error handling 103

💼 Case 4/4 104

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React lazy loading 👍 108

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https://dev.to

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🤔 How to build a good retry function?

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fp-ts 127

Promise in FP Task 128

Promise in FP Task 129

Promise in FP Task 130

Error + Result container Either<Error, Result> 131

Error + Result container Either<Error, Result> 132

Error + Result container Either<Error, React.Component> where type Error = TimeoutError | 404Error 133

Task returning Either Task<Either<Error, Result>> alias: TaskEither<Error, Result> 134

Task returning Either Task<Either<Error, Result>> () => Promise< Either<Error, Result> > 135

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https://github.com/gcanti/retry-ts by the author of fp-ts 139

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You can’t just stop speaking about FP

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Сode examples: done 168

⛳ What we achieved 169

⛳ Flexible retry Combine different retry policies 170

⛳ Predictable Know your errors in advance 171

⛳ Isolated side effects Combine complex tasks without execution 172

Problem - Solution Empty values ➡ 173 Option

Problem - Solution Empty values ➡ Option Exceptions ➡ Try (or Either) 174

Problem - Solution Empty values ➡ Option Exceptions ➡ Try (or Either) Async side effects ➡ Task / TaskEither 175

Remember?

Option Try, Either Task, TaskEither 177

One last thing FP is about composition: Function composition Monadic composition 178

Conclusions 179

One last last thing Composing Software: The Book E. Elliot Functional Programming in Scala P. Chiusano, R. Bjarnason grammarly.com/jobs 180