Vijaya Prakash Kandel. iOS Software Engineer at Zalando, Germany

1. TITLE IN CAPITAL
LETTERS
VIJAYA P. KANDEL
@kandelvijaya
iOS Engineer
Why Monadic computation?

2. 2

3. 3
When does it end? 1 more feature

4. 4
Spot a problem!

5. 5
Problems
1. Composability
2. Error propagation
3. True Type signature

6. 6
World full of failing cases (PROBLEM)
Verify Monad propagates error and composes
2 Programming Paradigms
Deducing answer from Monad
Coding the Monad
TABLE OF CONTENTS

7. 7
Are you connected?
Is Wifi working?
A world of 2 possibilities.

8. 8
How do we handle failable
computation?

9. 9
The two halves
Imperative
Programming
Functional
Programming

10. 10
A simple game: MAD.A.DAM
1. M: Multiply input to a random Int
2. A: Add input to a random Int
3. D: Divide input by a random Int
4. Assume Swift doesn’t have Optional
Perform: MAD.A.DAM on a dice roll input.

11. 11
A simple game: MAD.A.DAM

12. 12
A simple game: MAD.A.DAM

13. 13
A simple game: MAD.A.DAM

14. 14
A simple game: MAD.A.DAM

15. 15
A simple game: MAD.A.DAM

16. 16
A simple game: MAD.A.DAM

17. 17
The true division

18. 18
Contextual Box
T Error
Result<T> Contextual Box

19. 19
Contextual Box
Result<Int>Div(a:)Int
Mul(a:)Int Int

20. 20
MAD.A.DAM

21. 21
MAD.A.DAM

22. 22
MAD.A.DAM

23. 23
MAD.A.DAM

24. 24
MAD.A.DAM

25. 25
T
Error
T
flatten()

26. 26
FLATTEN

27. 27
MAD.A.DAM

28. 28
RESULT MONAD

29. 29
RESULT MONAD
T
map
flatMap
T -> U U
T -> U U
Monad is a Contextual
Type that provides
linear binding and
composition along
with error
propagation.

30. Monad is a CONTEXTUAL TYPE that provides easy
BINDING/COMPOSITION without sacrificing Error Handling.

31. 31
RESULT MONAD
T
map
flatMap
T -> U U
T -> U U
Optional<T> is a
monad. That is why we
can chain stuff
together.
- map()
- flatMap()

32. 32
RESULT MONAD
T
map
flatMap
T -> U U
T -> U U
Collection<T> is a
monad.
- map()
- flatMap()

33. 33
RESULT MONAD
T
map
flatMap
T -> U U
T -> U U
Swift Try/Catch in
principle can be
implemented in terms
of a Result Monad.

34. 34
MAD.A.DAM

35. 35
VERIFICATION

36. 36
Verification: True Types

37. 37
Verification: Error Propagation

38. 38
Verification: Error Propagation

39. 39
Verification: Composability
Who cares error
handling Solution!
I Do! Monadic Solution!

40. 40
WAIT, What about the async
task that might fail?

41. 41
Async Monad

42. 42
Async Monad

43. 43
Promise / Future / Deferred / IO :: Monad
aTask: ( ( (T) -> Void)? -> Void )?
T
aTask
T

44. 44
Promise / Future / Deferred / IO :: Monad
aTask: ( ( (T) -> Void)? -> Void )?
aTask
T
then/mapT -> U aTaskaTask
T
U
T
->
U

45. 45
Promise / Future / Deferred / IO :: Monad
aTask: ( ( (T) -> Void)? -> Void )?
aTask
T
bind/ flatMapT -> Promise<U> aTaskaTask
T
U
T
->
U

46. 46
Promise / Future / Deferred / IO :: Monad
aTaskaTask
T
U
Evaluate and finally call into outer (U -> Void)
By intending to evaluate the inner
block and calling into outer block with
eventual value; we unpack a promise.
None of the Pomise are evaluated
unless someone calls `execute()` on
the outermost layer.
Thereby guarantee of order of
execution is applied.

47. 47
Async Monad: USAGE

48. 48
Async Monad: USAGE

49. 49
Async Monad: USAGE

50. 50
Async Monad: USAGE

51. 51
Async Monad: USAGE

52. 52
Async Monad: USAGE

53. 53
Async Monad: USAGE

54. 54
A good programmer is
someone who looks both
ways before crossing a
one-way street.” — Doug
Linder

55. 55
References
1. Monad Research Paper
http://homepages.inf.ed.ac.uk/wadler/papers/marktoberdorf/baast
ad.pdf
2. Railway Oriented Programming
https://fsharpforfunandprofit.com/rop/
3. Dr. Bartosz Category theory lecture
https://www.youtube.com/user/DrBartosz/videos
4. My blog for more detailed explanation
https://kandelvijaya.com/2017/08/13/promisein30lines/
5. Promise<T> from the talk
https://github.com/kandelvijaya/MPromise
6. Result<T> from the talk
https://github.com/kandelvijaya/ResultType

56. This presentation and its contents are strictly confidential. It may not, in
whole or in part, be reproduced, redistributed, published or passed on to
any other person by the recipient.
The information in this presentation has not been independently verified. No
representation or warranty, express or implied, is made as to the accuracy
or completeness of the presentation and the information contained herein
and no reliance should be placed on such information. No responsibility is
accepted for any liability for any loss howsoever arising, directly or
indirectly, from this presentation or its contents.
DISCLAIMER
56

57. Vijaya P. Kandel
vijaya.prakash.kandel@zalando.de
@kandelvijaya
20-11-2017
Fashion Store
iOS Engineer