Improving Code with Union Types in Elm

Talk given at Boston Elm meetup May 2017.

A more polished version of this was turned into two blog posts:

- https://thoughtbot.com/blog/booleans-and-enums
- https://thoughtbot.com/blog/modeling-with-union-types

Transcript

1. Improving Your Code With Union Types
2. What is "good code"?
4. Commonly accepted attributes · Expressive · Easier to change · Easier to read · More modular · More correct
6. Terminology
7. Sum types (AKA Enums) type Suit = Hearts | Spades | Diamonds | Clubs
8. 4 different suits
9. Product types (AKA Tagged values) type Card = Card Value Suit
10. 13 values x 4 Suits = 52
12. Combining them together type Coloring = Colored | Grayscale type Card = Card Value Suit | Joker Coloring
13. (13 x 4) + 2 = 54
14. Type variables type Maybe a = Nothing | Just a
15. What are they good for?
16. Let's talk about Booleans
17. Three-State Boolean problem type alias Order = { id : Int , delivery : Maybe Bool }
18. Three states 1. Just True 2. Just False 3. Nothing
19. Sum type type Delivery = Delivery | Pickup | NoneSelected type alias Order = { id : Int , delivery : Delivery }
20. Improvements · ✅ Easier to change · ✅ Easier to read · ❌ More modular · ❌ More correct
22. Double Dependant Boolean type alias Order = { id : Int , delivery : Bool , thirdParty : Bool }
23. Four states · True True · True False · False True · False False
24. What does this even mean { id = 1 , delivery = False , thirdParty = True }
25. Sum type type Delivery = Delivery | Pickup | ThirdPartyDelivery | NotSelected type alias Order = { id : Int , delivery : Delivery }
26. Improvements · ✅ Easier to change · ✅ Easier to read · ❌ More modular · ❌ More correct
27. Boolean Flags
28. Less readable npc : Character npc = Character True True False
29. More readable npc : Character npc = Character Immortal Female Stationary
30. Looks like type Mortality = Mortal | Immortal type Gender = Male | Female type Mobility = Stationary | Mobile type alias Character = { mortality : Mortality , gender : Gender , mobility : Mobility }
31. More extensible type Gender = Female | Male | NonBinary
32. Improvements · ✅ Easier to change · ✅ Easier to read · ❌ More modular · ❌ More correct
33. Why not Strings?
34. How Many Mobilities? type Mobility = Mobile | Stationary
35. How Many Mobilities? type alias Mobility = String
36. Case statements move : Int -> Character -> Character move distance character = case character.mobility of "Mobile" -> { character | position = character.position + distance } "Stationary" -> character _ -> character
37. Sum type gives error -- MISSING PATTERNS ------------------------------------------------------------ This `case` does not have branches for all possibilities. 11|> case character.mobility of 12|> Mobile -> 13|> { character | position = character.position + distance } 14|> 15|> Stationary -> 16|> character You need to account for the following values: Flying
38. Typos npc : Character npc = Character "Mobil" 55
39. Sum type gives error -- NAMING ERROR ---------------------------------------------------------------- Cannot find variable `Mobil` 20| Character Mobil 55 ^^^^^ Maybe you want one of the following? Mobile
40. Improvements · ✅ Easier to change · ✅ Easier to read · ❌ More modular · ✅ More correct
41. Primitive Obsession
43. Dangerous primitives type alias User = { name : String , age : Int , bankBalance : Int , salary : Int }
44. Working with Cash addBalance : User -> Int -> User addBalance user amount = { user | bankBalance = bankBalance + amount }
45. Invalid operation payday : User -> User payday user = addBalance user user.age
46. Custom type type Dollar = Dollar Int type alias User = { name : String , age : Int , bankBalance : Dollar , salary : Dollar }
47. Using Dollar addBalance : User -> Dollar -> User addBalance user dollarAmount = let (Dollar balance) = user.bankBalance (Dollar amount) = dollarAmount in { user | bankBalance = Dollar (balance + amount) }
48. Invalid operation payday : User -> User payday user = addBalance user user.age
49. Compiler error -- TYPE MISMATCH --------------------------------------------------------------- The 2nd argument to function `addBalance` is causing a mismatch. 21| addBalance user user.age ^^^^^^^^ Function `addBalance` is expecting the 2nd argument to be: Dollar But it is: Int
51. Pain points 1. Unwrap value 2. Add the integers 3. Re-wrap value
52. Sound familiar?
53. Mapping 1. Unwrap value 2. Apply function 3. Re-wrap value
54. Mapping Maybe doubleMaybe : Maybe Int -> Maybe Int doubleMaybe maybe = Maybe.map (\n -> n * 2 ) maybe
55. Summing two Maybes sumMaybe : Maybe Int -> Maybe Int -> Maybe Int sumMaybe m1 m2 = Maybe.map2 (+) m1 m2
56. We need that If only there was a Dollar.map2
57. Mapping to the rescue module Dollar exposing (Dollar, map2) map2 : (Int -> Int) -> Dollar -> Dollar -> Dollar map2 function (Dollar d1) (Dollar d2) = Dollar (function d1 d2)
58. Using Dollar addBalance : User -> Dollar -> Dollar addBalance user amount = { user | bankBalance = Dollar.map2 (+) user.bankBalance amount }
59. Getting clever module Dollar exposing (Dollar, map2) sum : Dollar -> Dollar -> Dollar sum = map2 (+)
60. General utility · map · map2 · sum · subtract · multiply · divide
61. Improvements · ✅ Easier to change · ✅ Easier to read · ❌ More modular · ✅ More correct
62. Opaque types
63. Start with this module Point exposing (Point) type alias Point = { x : Int, y : Int }
64. Use in in Code path : List Point path = [ Point 1 1, , Point 100 100 ]
65. Now you want to add 3D module Point exposing (Point) type alias Point = { x : Int, y : Int, z : Int}
66. Broken code
67. Wrapping in a type module Point exposing (Point, fromXY) type Point = Point { x : Int, y : Int } fromXY : (Int, Int) -> Point fromXY (x, y) = Point { x = x, y = y }
68. Adding 3D module Point exposing (Point, fromXY) type Point = Point { x : Int, y : Int, z : Int } fromXY : (Int, Int) -> Point fromXY (x, y) = Point { x = x, y = y, z = 0 }
69. Improvements · ✅ Easier to change · ✅ Easier to read · ✅ More modular · ❌ More correct
70. The shape of data
71. Primitives type alias Card = { suit : String , value : Int }
72. Looks like Card "Hearts" 2
73. How Many of this type of card?
74. This is valid Card "Starfish" 1999
75. How do you represent the Joker?
76. You have to force it Card "Joker" 0 or Card "Joker" Nothing
77. Union types allow multiple shapes type Card = Card Value Suit | Joker Coloring
78. Improvements · ✅ Easier to change · ✅ Easier to read · ❌ More modular · ✅ More correct · ✅ More expressive
79. HTTP
80. Maybe type alias Model = { lastOrder : Maybe Order }
81. Explore the shape of data type RemoteData e a = NotAsked | Loading | Failure e | Success a
82. Improved Model type alias Model = { lastOrder : RemoteData Http.Error Order }
83. Improvements · ✅ Easier to change · ✅ Easier to read · ❌ More modular · ❌ More correct · ✅ More expressive
84. Improve your code with Union Types
85. Questions?
86. Slides are on GitHub https://github.com/JoelQ/adts-elm-meetup
87. About Me · Developer at thoughtbot · @joelquen on Twitter · @JoelQ on GitHub