Dependent Types and other ideas for guaranteeing correctness with types

Radek Pietruszewski @radexp • radex.io

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Static typing is nice

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Expressing guarantees

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Partial functions

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let xs = [1, 2, 3]

xs[0] // => 1 xs[4] // => crash xs[-1] // => crash

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subscript (index: Int) -> Element

-a lot, ..., -1, 0, 1, 2, 3, 4, a lot

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subscript (index: Int) -> Element

-a lot, ..., -1, 0, 1, 2, 3, 4, a lot

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No compile time check

😕

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Prefer total functions to partial functions

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Optionals

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Foo* vs Foo vs Foo?

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Enums

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func move(direction: String)

move("up") move("down") move("wat") // undefined

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enum Direction { case Up, Down, Left, Right }

func move(direction: Direction)

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enum SuggestionViewModel { case Header(String) case Suggestion(Suggestion) }

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Dependent Types

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struct User { var loggedIn: Bool ... }

/// `user` must be logged in! func doSomethingImportant(user: User)

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/// `user` must be logged in! func doSomethingImportant(user: User)

😕User(loggedIn: false)

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User<loggedIn=true>

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Validated

github.com/Ben-G/Validated

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struct LoggedInValidator: Validator { static func validate(value: User) -> Bool { return value.loggedIn } }

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struct LoggedInValidator: Validator { static func validate(value: User) -> Bool { return value.loggedIn } }

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Validated<User, LoggedInValidator>

User<loggedIn=true>

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typealias LoggedInUser = Validated<User, LoggedInValidator>

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let rawUser = User(loggedIn: false)

let loggedInUser = LoggedInUser(rawUser)

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func doSomethingImportant(user: LoggedInUser)

🙂

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Keep your functions total

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github.com/Ben-G/Validated

bit.do/partial-functions