Presented at: Open Source South Carolina, 8/14/19 Presented by: Dean Hallman, CTO @ Foresight Data Systems Abstract: This talk will present a simplified understanding of Monads, as well as key constructs that build upon them, like Promises, Railway-Oriented Programming and Functional Reactive Programming.

1. MONADIC FUNCTIONAL
PROGRAMMING
Dean Hallman - August 2019
Twitter: @rdhallman,
Email: rdhallman@gmail.com
LinkedIn: www.linkedin.com/in/dean-hallman

2. F1() F2() F3() F4()
Function Composition
a b c d e
Higher-Order
Itemizer Functions
I2I1

3. F1 F2 F3 F4 F5 F6 F7 F8
V1 V2 V3 V4 V5 V6 V7
Vi
Vo
Functional Programming

4. F1 F2 F3 F4 F5 F6 F7 F8
V1 V2 V3 V4 V5 V6 V7
Vi
Vo
Functional Programming
Infix Notation:
let F = Compose(f1,f2,f3,f4)
F(a)
=> e
Prefix Notation:
(f4 (f3 (f2 (f1 a) ) ) )
=> e
Dot Notation??
f1(a).f2.f3.f4
=> e

5. F1 F2 F3 F4 F5 F6 F7 F8
V1 V2 V3 V4 V5 V6 V7
Vi
Vo
Monadic Functional Programming
M M M M M M M
M

6. Dot Chaining
Array
.of(1,2,3,4,5) # [1,2,3,4,5]
.filter((v) => v % 2 == 0) # [2, 4]
.map((v) => v * 3) # [6, 12]
.filter((v) => v % 4 == 0) # [12]
.length()
=> 1

7. TSA Scanner as Monad

8. F1 F2 F3 F4 F5 F6 F7 F8
V1 V2 V3 V4 V5 V6 V7
Vi
Vo
M M M M M M M
M
Cross-Functional State
NULL
Monadic Functional Programming:
The “Maybe Monad”
X X X

9. Maybe Monad
function getUserBanner(user)
{
return Maybe
.of(user)
.map(prop('accountDetails’))
.map(prop('address’))
.map(prop('province’))
.map(getProvinceBanner)
.default(“Property Undefined”);
}
Source: https://jrsinclair.com/articles/2016/marvellously-mysterious-javascript-maybe-monad/

10. The Rise of Inversion Of Control:
MTS, EJB & Transactional COM+

11. Inversion of Control &
Aspect-Oriented Programming
Main() Library
Main()
IoC Container
Before 2000 After 2000
Your Code Your CodeLibrary
PRE
POST
Event

12. Inversion of Control &
Aspect-Oriented Programming
Main() Library
Main()
IoC Container
Before 2000 After 2000
Your Code Your CodeLibrary
PRE
POST
Event

13. Inversion of Control &
Aspect-Oriented Programming
Main() Library
Main()
IoC Container
Before 2000 After 2000
Your Code Your CodeLibrary
PRE
POST
Event
F1F2F3F4F8
V1V2V3V4
ViVo

14. f f f f f f f f
V1 V2 V3 V4 V5 V6 V7
Vi
Vo
Super-Monadic Functional Programming
Run Time
Declaration
Time
.g1 .g2 .g3 .g4 .g5 .g6 .g7 .g8
Mf
Pipeline
Describe
Mf Mf Mf Mf Mf Mf Mf

15. F1 F2 F3 F4 F5 F6 F7 F8
V1 V2 V3 V4 V5 V6 V7
Vi
Vo
Monadic Functional Programming
M M M M M M M
M

16. The Monad Laws & Interface
• Left identity: return a >>= f ≡ f a
• Right identity: m >>= return ≡ m
if we take a value, put it in a default context with return and
then feed it to a function by using >>=, it’s the same as
just taking the value and applying the function to it.
if we have a monadic value and we use >>= to feed it
to return, the result is our original monadic value.
• Associativity: (m >>= f) >>= g ≡ m >>= (x -> f x >>= g)
when we have a chain of monadic function applications
with >>=, it shouldn’t matter how they’re nested.
• Required:
• unit
• bind
• Optional:
• of
• flatMap
• flatten, join

17. Fantasy Land Specification

18. Category Theory
a b c
f g

19. Category Theory
a b c
f g
a b c
f g
==

20. Category Theory
a b c
f g
M(a) M(b) M(c)
M(f) M(g)
==

21. Category Theory
a b c
f g
M(a) M(b) M(c)
M(f) M(g)

22. Category Theory
a b c
f g
M(a) M(b) M(c)
M(f) M(g)
unit, of
map, chain
flatten, join
map, chain

23. Category
Theory
a b c
f g
M(a) M(b) M(c)
M(f) M(g)
M(M(b)) M(M(c))
M(M(g))
map
flat, join
map
flat, join

24. Category
Theory
a b c
f g
M(a) M(b) M(c)
M(f) M(g)
M(M(b)) M(M(c))
M(M(g))
map
flat, join
map
M(M(d))
M(M(h))
map
d
M(d)
h
M(h)

25. Promise/A+ vs Monadic Promises
doSomethingAsync()
.then(doSomethingElseAsync())
.then(doAnotherThingAsync())
.then(doThirdThingAsync())
.catch(err => {
console.log(“Error: “ + err);
});

26. Monadic FP Complexity
◦ Learning Curve for the more advanced concepts
◦ Monadic lensing etc.

27. compose
F(x) F(x) F(x) F(x) F(x)
d
estack f
g
h
i
j
a
b
c
input
stack
stack
stack stack
stack
k
l
output
B, doc
Parameter Hiding & Monadic Lensing

28. F1 F2 F3 F4 F5 F6 F7 F8
V1 V2 V3 V4 V5 V6 V7
Vi
Vo
Monadic Functional Programming
M M M M M M M
M
Cross-Functional State
Surender
Thread
Surender
Thread

29. Railway-Oriented
Programming
◦ Extracting failure capture logic
◦ Either Monad
◦ Happy path
◦ Failure path

30. F1()
F2()
F3()
F4()
Recourse
Promise Waterfall with Recourse
a
b
c
d
e
Rejected

31. Promise Waterfall with Recourse
F1()
F2()
F3()
F4()
Recourse
a
b
c
d
e
Rejected
F1()
F2()
F3()
F4()
Recourse
f
g
h
i
Rejected
Recourse
Rejected

32. Recommendations & Questions
◦ “Oh Composable World”, by Brian Lansdorf