Totally Typed Domain

Where Dependent types meets domain driven design

Using the power of types to encode domain constraints into code.

As presented at ITAKE

Transcript

1. Totally Typed Domain When dependent types meets DDD
2. Why? Type driven development and Dependent types look cool! Model the domain closely ! Make impossible states impossible ! Some guarantees
3. Type ... Define ... Refine
4. Dependent types
5. ?Holes
6. Nice compiler Interactive Strict but helpful
7. Proof search
8. %default total
9. Unit tests It compiles so it's obviously correct right? NO
10. Unit tests It compiles so it's obviously correct right? YES Unit tests implemented in the type system
11. Small types Big types ~= inheritance Small types ~= composition Compose well
12. Ok fancy types, but all inputs are string Validate user input => Strong typing Strong typing => enforces validation A type can be a permission to do something
13. Tic Tac Toe placeMark : (freePos: List Pos) -> Pos-> List Pos vs
 
 placeMark : (freePos: Vect (S n) Pos) // available positions, (S n) means n+1 
 -> (p: Pos) // position to be marked
 -> (Elem p freePos) // proof that position p is in freePos
 -> Vect n Pos // remaining positions, 1 less than input
14. TypeDD ~ TestDD
15. Fast feedback? Dynamic No compilation Can hack Static All branches Type mistakes Autocomplete
16. Recursive types
17. Sometimes the types are clean But not the code
18. Conclusions ! Types are proofs ! Compilers ~ tests ! Small types ! Qualified types <=> encoded knowledge ! Feedback ! Compilers will evolve