In object-oriented design, dependency injection is a well-known design pattern, although it's a complicated solution to the problem of decoupling. Functional programming offers a simpler way. This talk examines dependency injection in object-oriented design, and explains how it's not required (nor desired) in functional programming. You don't need to know Haskell or F# to attend this session; relevant syntax will be explained just-in-time. Object-oriented examples will be in C#.

1. From dependency injection
to dependency rejection
Mark Seemann
http://blog.ploeh.dk
@ploeh

2. @ploeh

3. Object-oriented Functional
@ploeh

4. Object-oriented Functional
Dependency injection Partial application
Composition
@ploeh

5. Dependency injection is “really just
a pretentious way to say ‘taking an
argument’”
- Rúnar Bjarnason (@runarorama)
@ploeh

6. Example
Restaurant Reservation
@ploeh

7. Restaurant Reservation
Make a reservation
Date
Name
Email
Quantity
Submit
@ploeh

8. Restaurant Reservation
Make a reservation
Date 2
Name
Email
Quantity
Submit
@ploeh

9. Restaurant Reservation
Make a reservation
Date 20
Name
Email
Quantity
Submit
@ploeh

10. Restaurant Reservation
Make a reservation
Date 201
Name
Email
Quantity
Submit
@ploeh

11. Restaurant Reservation
Make a reservation
Date 2016
Name
Email
Quantity
Submit
@ploeh

12. Restaurant Reservation
Make a reservation
Date 2016-
Name
Email
Quantity
Submit
@ploeh

13. Restaurant Reservation
Make a reservation
Date 2016-1
Name
Email
Quantity
Submit
@ploeh

14. Restaurant Reservation
Make a reservation
Date 2016-12
Name
Email
Quantity
Submit
@ploeh

15. Restaurant Reservation
Make a reservation
Date 2016-12-
Name
Email
Quantity
Submit
@ploeh

16. Restaurant Reservation
Make a reservation
Date 2016-12-2
Name
Email
Quantity
Submit
@ploeh

17. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name
Email
Quantity
Submit
@ploeh

18. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name M
Email
Quantity
Submit
@ploeh

19. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Ma
Email
Quantity
Submit
@ploeh

20. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mar
Email
Quantity
Submit
@ploeh

21. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark
Email
Quantity
Submit
@ploeh

22. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark
Email
Quantity
Submit
@ploeh

23. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark S
Email
Quantity
Submit
@ploeh

24. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Se
Email
Quantity
Submit
@ploeh

25. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark See
Email
Quantity
Submit
@ploeh

26. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seem
Email
Quantity
Submit
@ploeh

27. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seema
Email
Quantity
Submit
@ploeh

28. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seeman
Email
Quantity
Submit
@ploeh

29. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email
Quantity
Submit
@ploeh

30. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email ma
Quantity
Submit
@ploeh

31. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mar
Quantity
Submit
@ploeh

32. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark
Quantity
Submit
@ploeh

33. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@
Quantity
Submit
@ploeh

34. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@p
Quantity
Submit
@ploeh

35. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@pl
Quantity
Submit
@ploeh

36. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@plo
Quantity
Submit
@ploeh

37. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@ploe
Quantity
Submit
@ploeh

38. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@ploeh
Quantity
Submit
@ploeh

39. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@ploeh.
Quantity
Submit
@ploeh

40. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@ploeh.d
Quantity
Submit
@ploeh

41. Restaurant Reservation
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@ploeh.dk
Quantity
Submit
@ploeh

42. Restaurant Reservation
POST JSON
@ploeh
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@ploeh.dk
Quantity 4
Submit

43. Restaurant Reservation
POST JSON
@ploeh
Make a reservation
Date 2016-12-29
Name Mark Seemann
Email mark@ploeh.dk
Quantity 4
Submit

44. Validate
Get reserved
seats from
DB
Decide
Save
reservation
Create
response
@ploeh

45. Object-oriented
Dependency injection
@ploeh

46. public class ReservationsController : ApiController
{
public ReservationsController(IValidator validator, IMapper mapper, IMaîtreD maîtreD)
{
this.validator = validator;
this.mapper = mapper;
this.maîtreD = maîtreD;
}
public IHttpActionResult Post(ReservationRequestDto dto)
{
var validationMsg = validator.Validate(dto);
if (validationMsg != "")
return this.BadRequest(validationMsg);
var r = mapper.Map(dto);
var id = maîtreD.TryAccept(r);
if (id == null)
return this.StatusCode(HttpStatusCode.Forbidden);
return this.Ok();
}
}
POST /reservations HTTP/1.1
{
"date": "2016-12-08",
"name": "Mark Seemann",
"email": "mark@ploeh.dk",
"quantity": 4
}
@ploeh

47. public class MaîtreD : IMaîtreD
{
public MaîtreD(int capacity, IReservationsRepository reservationsRepository)
{
this.capacity = capacity;
this.reservationsRepository = reservationsRepository;
}
public int? TryAccept(Reservation reservation)
{
var reservedSeats = reservationsRepository
.ReadReservations(reservation.Date)
.Sum(r => r.Quantity);
if (reservedSeats + reservation.Quantity <= capacity)
{
reservation.IsAccepted = true;
return reservationsRepository.Create(reservation);
}
return null;
}
}
@ploeh
public interface IMaîtreD
{
int? TryAccept(Reservation reservation);
}

48. How do I do
dependency
injection in
Scala?
You don’t,
because Scala
is a functional
language.
Comic courtesy of
@jrecursive and @hmemcpy @ploeh

49. Fine, it’s functional.
How do I inject
dependencies?
You use a Free Monad,
which allows you to
build a Monad from
any Functor.
Comic courtesy of
@jrecursive and @hmemcpy @ploeh

50. Did you just
tell me to go
fuck myself?
I believe I
did, Bob.
Comic courtesy of
@jrecursive and @hmemcpy @ploeh

51. Partial application
An attempt at dependency injection with a functional language
@ploeh

52. // int -> (DateTimeOffset -> Reservation list) -> (Reservation -> int) -> Reservation
// -> int option
let tryAccept capacity readReservations createReservation reservation =
let reservedSeats =
readReservations reservation.Date |> List.sumBy (fun x -> x.Quantity)
if reservedSeats + reservation.Quantity <= capacity
then createReservation { reservation with IsAccepted = true } |> Some
else None
@ploeh
Repository

53. // Reservation -> int option
let tryAcceptComposition reservation =
let read = DB.readReservations connectionString
@ploeh
string -> DateTimeOffset -> Reservation list

54. // Reservation -> int option
let tryAcceptComposition reservation =
let read = DB.readReservations connectionString
@ploeh
string -> DateTimeOffset -> Reservation list
connectionString

55. // Reservation -> int option
let tryAcceptComposition reservation =
let read = DB.readReservations connectionString
@ploeh
string -> DateTimeOffset -> Reservation listconnectionString

56. // Reservation -> int option
let tryAcceptComposition reservation =
let read = DB.readReservations connectionString
@ploeh
DateTimeOffset -> Reservation list

57. // Reservation -> int option
let tryAcceptComposition reservation =
let read = DB.readReservations connectionString
let create = DB.createReservation connectionString
tryAccept 10 read create reservation
@ploeh
Reservation -> int

58. // Reservation -> int option
let tryAcceptComposition reservation =
let read = DB.readReservations connectionString
let create = DB.createReservation connectionString
tryAccept 10 read create reservation
@ploeh

59. // Reservation -> int option
let tryAcceptComposition =
let read = DB.readReservations connectionString
let create = DB.createReservation connectionString
tryAccept 10 read create
@ploeh

60. // Reservation -> int option
let tryAcceptComposition =
let read = DB.readReservations connectionString
let create = DB.createReservation connectionString
tryAccept 10 read create
@ploeh

61. IL
@ploeh

62. internal class tryAcceptComposition@17 : FSharpFunc<Reservation, FSharpOption<int>>
{
public int capacity;
public FSharpFunc<Reservation, int> createReservation;
public FSharpFunc<DateTimeOffset, FSharpList<Reservation>> readReservations;
internal tryAcceptComposition@17(
int capacity,
FSharpFunc<DateTimeOffset, FSharpList<Reservation>> readReservations,
FSharpFunc<Reservation, int> createReservation)
{
this.capacity = capacity;
this.readReservations = readReservations;
this.createReservation = createReservation;
}
public override FSharpOption<int> Invoke(Reservation reservation)
{
return MaîtreD.tryAccept<int>(
this.capacity, this.readReservations, this.createReservation, reservation);
}
}
@ploeh

63. Is it Functional?
Partial
application
Dependency
injection
@ploeh

64. Same
return
value for
same input
No side-
effects
Pure
function
@ploeh

65. Pure
function
Impure
function
@ploeh

66. Sanity check
@ploeh

67. tryAccept :: Int -> (ZonedTime -> [Reservation]) -> (Reservation -> Int) -> Reservation
-> Maybe Int
tryAccept capacity readReservations createReservation reservation =
let reservedSeats = sum $ map quantity $ readReservations $ date reservation
in if reservedSeats + quantity reservation <= capacity
then Just $ createReservation $ reservation { isAccepted = True }
else Nothing
@ploeh
Pure

68. tryAcceptComposition :: Reservation -> IO (Maybe Int)
tryAcceptComposition reservation =
let read = DB.readReservations connectionString
create = DB.createReservation connectionString
in tryAccept 10 read create reservation
@ploeh
ZonedTime -> IO [Reservation]
Reservation -> IO Int
ZonedTime -> [Reservation] Reservation -> Int

69. Not Functional
Partial
application
Dependency
injection
@ploeh

70. @ploeh

71. Dependency injection
makes everything impure
@ploeh

72. Dependency Rejection
@ploeh

73. Unit
Dependencies
Indirect
input
Indirect
output
Direct
input
Direct
output
@ploeh

74. Unit
Dependencies
Indirect
input
Direct
input
Direct
output
@ploeh

75. // int -> (DateTimeOffset -> Reservation list) -> (Reservation -> int) -> Reservation
// -> int option
let tryAccept capacity readReservations createReservation reservation =
@ploeh

76. // int -> (DateTimeOffset -> Reservation list) -> Reservation
// -> Reservation option
let tryAccept capacity readReservations reservation =
@ploeh

77. // int -> (DateTimeOffset -> Reservation list) -> Reservation
// -> Reservation option
let tryAccept capacity readReservations reservation =
@ploeh

78. // int -> (DateTimeOffset -> Reservation list) -> Reservation -> Reservation option
let tryAccept capacity readReservations reservation =
@ploeh

79. // int -> (DateTimeOffset -> Reservation list) -> Reservation -> Reservation option
let tryAccept capacity readReservations reservation =
let reservedSeats =
readReservations reservation.Date |> List.sumBy (fun x -> x.Quantity)
if reservedSeats + reservation.Quantity <= capacity
then { reservation with IsAccepted = true } |> Some
else None
@ploeh

80. // Reservation -> int option
let tryAcceptComposition reservation =
match tryAccept 10 (DB.readReservations connectionString) reservation with
| None -> None
| Some r -> (DB.createReservation connectionString) r |> Some
@ploeh

81. // Reservation -> int option
let tryAcceptComposition reservation =
tryAccept 10 (DB.readReservations connectionString) reservation
|> Option.map (fun r -> DB.createReservation connectionString r)
@ploeh

82. // Reservation -> int option
let tryAcceptComposition reservation =
tryAccept 10 (DB.readReservations connectionString) reservation
|> Option.map ( DB.createReservation connectionString )
@ploeh

83. // Reservation -> int option
let tryAcceptComposition reservation =
tryAccept 10 (DB.readReservations connectionString) reservation
|> Option.map (DB.createReservation connectionString)
@ploeh

84. // Reservation -> int option
let tryAcceptComposition reservation =
reservation
|> tryAccept 10 (DB.readReservations connectionString)
|> Option.map (DB.createReservation connectionString)
@ploeh

85. Unit
Dependencies
Indirect
input
Direct
input
Direct
output
@ploeh

86. Unit
Dependencies
Direct
input
Direct
output
@ploeh

87. // int -> (DateTimeOffset -> Reservation list) -> Reservation -> Reservation option
let tryAccept capacity readReservations reservation =
@ploeh

88. // int -> ( Reservation list) -> Reservation -> Reservation option
let tryAccept capacity readReservations reservation =
@ploeh

89. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity readReservations reservation =
@ploeh

90. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity radReservations reservation =
@ploeh

91. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity rdReservations reservation =
@ploeh

92. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity rReservations reservation =
@ploeh

93. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

94. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

95. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

96. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

97. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

98. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

99. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

100. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

101. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

102. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

103. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

104. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

105. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

106. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

107. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

108. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
@ploeh

109. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
let reservedSeats =
reservations |> List.sumBy (fun x -> x.Quantity)
if reservedSeats + reservation.Quantity <= capacity
then { reservation with IsAccepted = true } |> Some
else None
@ploeh

110. // int -> Reservation list -> Reservation -> Reservation option
let tryAccept capacity reservations reservation =
let reservedSeats = reservations |> List.sumBy (fun x -> x.Quantity)
if reservedSeats + reservation.Quantity <= capacity
then { reservation with IsAccepted = true } |> Some
else None
@ploeh

111. // ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
let flip f x y = f y x
// Reservation -> int option
let tryAcceptComposition reservation =
reservation.Date
|> DB.readReservations connectionString
|> fun reservations -> tryAccept 10 reservations reservation
|> Option.map (DB.createReservation connectionString)
@ploeh

112. // ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
let flip f x y = f y x
// Reservation -> int option
let tryAcceptComposition reservation =
reservation.Date
|> DB.readReservations connectionString
|> fun reservations -> flip (tryAccept 10) reservation reservations
|> Option.map (DB.createReservation connectionString)
@ploeh

113. // ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
let flip f x y = f y x
// Reservation -> int option
let tryAcceptComposition reservation =
reservation.Date
|> DB.readReservations connectionString
|> -> flip (tryAccept 10) reservation
|> Option.map (DB.createReservation connectionString)
@ploeh

114. // ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
let flip f x y = f y x
// Reservation -> int option
let tryAcceptComposition reservation =
reservation.Date
|> DB.readReservations connectionString
|> flip (tryAccept 10) reservation
|> Option.map (DB.createReservation connectionString)
@ploeh

115. Sanity check
@ploeh

116. tryAccept :: Int -> [Reservation] -> Reservation -> Maybe Reservation
tryAccept capacity reservations reservation =
let reservedSeats = sum $ map quantity reservations
in if reservedSeats + quantity reservation <= capacity
then Just $ reservation { isAccepted = True }
else Nothing
@ploeh

117. tryAcceptComposition :: Reservation -> IO (Maybe Int)
tryAcceptComposition reservation = runMaybeT $
liftIO (DB.readReservations connectionString $ date reservation)
>>= MaybeT . return . flip (tryAccept 10) reservation
>>= liftIO . DB.createReservation connectionString
@ploeh

118. Object-oriented Functional
Dependency injection Partial application
Composition
@ploeh

119. Mark Seemann
http://blog.ploeh.dk
http://bit.ly/ploehralsight
@ploeh