Dependent Types

Dependent Types (and other ideas for guaranteeing correctness with types)

A few strategies for protecting yourself from your future self's mistakes. Write more robust code by expressing constraints in the type signature and avoiding partial functions. Give your compiler the tools it needs to assist you. Let it help you.

Transcript

1. Dependent Types and other ideas for guaranteeing correctness with types Radek Pietruszewski @radexp • radex.io • nozbe.com
2. @radexp • radex.io • nozbe.com Static typing is nice
3. @radexp • radex.io • nozbe.com Expressing guarantees
4. @radexp • radex.io • nozbe.com Partial functions
5. @radexp • radex.io • nozbe.com let xs = [1, 2, 3] xs[0] // => 1 xs[4] // => crash xs[-1] // => crash
6. @radexp • radex.io • nozbe.com let xs = [1, 2, 3] xs[n] = {some value; n ∈ ⟨0,2⟩ ¯\_(ϑ)_/¯; n<0 ⋁ n>2
7. @radexp • radex.io • nozbe.com subscript (index: Int) -> Element -a lot, ..., -1, 0, 1, 2, 3, 4, a lot
8. @radexp • radex.io • nozbe.com subscript (index: Int) -> Element -a lot, ..., -1, 0, 1, 2, 3, 4, a lot
9. @radexp • radex.io • nozbe.com Partial function! subscript (index: Int) -> Element
10. @radexp • radex.io • nozbe.com No compile time check
11. @radexp • radex.io • nozbe.com Prefer total functions to partial functions
12. @radexp • radex.io • nozbe.com Optionals
13. @radexp • radex.io • nozbe.com Foo* vs Foo vs Foo?
14. @radexp • radex.io • nozbe.com Enums
15. @radexp • radex.io • nozbe.com func move(direction: String) move("up") move("down") move("wat") // undefined
16. @radexp • radex.io • nozbe.com enum Direction { case Up, Down, Left, Right } func move(direction: Direction)
17. @radexp • radex.io • nozbe.com enum SuggestionViewModel { case Header(String) case Suggestion(Suggestion) }
18. @radexp • radex.io • nozbe.com Dependent Types
19. @radexp • radex.io • nozbe.com struct User { var loggedIn: Bool ... } /// `user` must be logged in! func doSomethingImportant(user: User)
20. @radexp • radex.io • nozbe.com /// `user` must be logged in! func doSomethingImportant(user: User) User(loggedIn: false)
21. @radexp • radex.io • nozbe.com User<loggedIn=true>
22. @radexp • radex.io • nozbe.com Validated github.com/Ben-G/Validated
23. @radexp • radex.io • nozbe.com struct LoggedInValidator: Validator { static func validate(value: User) -> Bool { return value.loggedIn } }
24. @radexp • radex.io • nozbe.com struct LoggedInValidator: Validator { static func validate(value: User) -> Bool { return value.loggedIn } }
25. @radexp • radex.io • nozbe.com Validated<User, LoggedInValidator> User<loggedIn=true>
26. @radexp • radex.io • nozbe.com typealias LoggedInUser = Validated<User, LoggedInValidator>
27. @radexp • radex.io • nozbe.com let rawUser = User(loggedIn: false) let loggedInUser = LoggedInUser(rawUser)
28. @radexp • radex.io • nozbe.com func doSomethingImportant(user: LoggedInUser)
29. @radexp • radex.io • nozbe.com Keep your functions total
30. @radexp • radex.io github.com/Ben-G/Validated bit.do/partial-functions