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.

1. Dependent Types
and other ideas for guaranteeing
correctness with types
Radek Pietruszewski
@radexp • radex.io

2. @radexp • radex.io
Static typing is nice

3. @radexp • radex.io
Expressing guarantees

4. @radexp • radex.io
Partial functions

5. @radexp • radex.io
let xs = [1, 2, 3]
xs[0] // => 1
xs[4] // => crash
xs[-1] // => crash

6. @radexp • radex.io
subscript (index: Int) -> Element
-a lot, ..., -1, 0, 1, 2, 3, 4, a lot

7. @radexp • radex.io
subscript (index: Int) -> Element
-a lot, ..., -1, 0, 1, 2, 3, 4, a lot

8. @radexp • radex.io
No compile time check
😕

9. @radexp • radex.io
Prefer
total functions
to partial functions

10. @radexp • radex.io
Optionals

11. @radexp • radex.io
Foo* vs Foo vs Foo?

12. @radexp • radex.io
Enums

13. @radexp • radex.io
func move(direction: String)
move("up")
move("down")
move("wat") // undefined

14. @radexp • radex.io
enum Direction {
case Up, Down, Left, Right
}
func move(direction: Direction)

15. @radexp • radex.io
enum SuggestionViewModel {
case Header(String)
case Suggestion(Suggestion)
}

16. @radexp • radex.io
Dependent Types

17. @radexp • radex.io
struct User {
var loggedIn: Bool
...
}
/// `user` must be logged in!
func doSomethingImportant(user: User)

18. @radexp • radex.io
/// `user` must be logged in!
func doSomethingImportant(user: User)
😕User(loggedIn: false)

19. @radexp • radex.io
User<loggedIn=true>

20. @radexp • radex.io
Validated
github.com/Ben-G/Validated

21. @radexp • radex.io
struct LoggedInValidator: Validator {
static func validate(value: User) -> Bool {
return value.loggedIn
}
}

22. @radexp • radex.io
struct LoggedInValidator: Validator {
static func validate(value: User) -> Bool {
return value.loggedIn
}
}

23. @radexp • radex.io
Validated<User, LoggedInValidator>
User<loggedIn=true>

24. @radexp • radex.io
typealias LoggedInUser =
Validated<User, LoggedInValidator>

25. @radexp • radex.io
let rawUser = User(loggedIn: false)
let loggedInUser = LoggedInUser(rawUser)

26. @radexp • radex.io
func doSomethingImportant(user: LoggedInUser)
🙂

27. @radexp • radex.io
Keep your functions total

28. @radexp • radex.io
github.com/Ben-G/Validated
bit.do/partial-functions