The document discusses the differences between hexagonal architecture and functional architecture for software design. Hexagonal architecture separates infrastructure from business logic but allows business code with side effects, while functional architecture requires business code be free of side effects. Both approaches separate infrastructure from business concerns and support testability, but functional architecture more strictly avoids technical leaks into the domain and is better suited for functional programming languages. The document raises questions about how to handle sequential I/O operations in a functional style.

1. Hexagonal or Functional Core?
Thomas PIERRAIN
@tpierrain
Bruno BOUCARD
@brunoboucard

2. First: our business

3. Suggesting seats in Theaters
“I want to find 3 seats
for show Z”

4. Suggesting seats in Theaters
“I want to find 3 seats
for show Z”
SEATS SUGGESTIONS API

5. Suggesting seats in Theaters
AUDITORIUM SEATING
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API

6. Suggesting seats in Theaters
AUDITORIUM SEATING
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API

7. Suggesting seats in Theaters
AUDITORIUM SEATING
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API

8. Suggesting seats in Theaters
AUDITORIUM SEATING
API
SEATS AVAILABILITY
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API
“I also need to know
which seats are
already reserved for
show Z”

9. Suggesting seats in Theaters
AUDITORIUM SEATING
API
SEATS AVAILABILITY
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API
“I also need to know
which seats are
already reserved for
show Z”

10. Suggesting seats in Theaters
AUDITORIUM SEATING
API
SEATS AVAILABILITY
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API
“I also need to know
which seats are
already reserved for
show Z”
OUR CORE DOMAIN

11. Suggesting seats in Theaters
AUDITORIUM SEATING
API
SEATS AVAILABILITY
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API
“I also need to know
which seats are
already reserved for
show Z”

12. Suggesting seats in Theaters
AUDITORIUM SEATING
API
SEATS AVAILABILITY
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API
“I also need to know
which seats are
already reserved for
show Z”
“Ok. You can have seats
A1, A2, A3 in pricing tier 1,
C4, C5, C6 in pricing tier 2
…”

13. Suggesting seats in Theaters
AUDITORIUM SEATING
API
SEATS AVAILABILITY
API
“I want to find 3 seats
for show Z” “I need the
auditorium seating
of show Z”
SEATS SUGGESTIONS API
“I also need to know
which seats are
already reserved for
show Z”
“Ok. You can have seats
A1, A2, A3 in pricing tier 1,
C4, C5, C6 in pricing tier 2
…”

14. Suggesting seats in Theaters

15. API architecture
(2 options)

17. Source: http://coder.efelti.com/2018/02/introduction-of-functional-programming.html

19. Hexagonal in motion

27. Functional core in motion

34. From Hexagonal to Functional core
(live coding)

35. Suggesting seats in Theaters

36. More Functional?

37. https://blog.ploeh.dk/2018/03/26/the-maybe-functor/

38. Makes decisions
Imperative Shell acts upon those decisions
Functional Core - Theater

39. How to cope with multiple
I/Os in sequence?

41. Make decisions
Acts upon those decisions
Functional Core – Train reservation

42. Feature Hexagonal
Architecture
Functional
Architecture
Separate Infra from Business Yes Yes
Business Code without side effect
Business Decision are in Business part
Offers good ability to test
Technical concerns never leak from domain
Hexagonal or Functional Core?

43. Feature Hexagonal
Architecture
Functional
Architecture
Separate Infra from Business Yes Yes
Business Code without side effect No* Yes
Business Decision are in Business part
Offers good ability to test
Technical concerns never leak from domain
* By its requirements Functional Architecture is more extreme (code without mutation) and therefore
more addressable with functional language.
The hexagonal architecture is more favorable to the OOP
Hexagonal or Functional Core?

44. Feature Hexagonal
Architecture
Functional
Architecture
Separate Infra from Business Yes Yes
Business Code without side effect No* Yes
Business Decision are in Business part Yes Yes
Offers good ability to test
Technical concerns never leak from domain
Hexagonal or Functional Core?
* By its requirements Functional Architecture is more extreme (code without mutation) and therefore
more addressable with functional language.
The hexagonal architecture is more favorable to the OOP

45. Feature Hexagonal
Architecture
Functional
Architecture
Separate Infra from Business Yes Yes
Business Code without side effect No* Yes
Business Decision are in Business part Yes Yes
Offers good ability to test Yes Yes
Technical concerns never leak from domain
Hexagonal or Functional Core?
* By its requirements Functional Architecture is more extreme (code without mutation) and therefore
more addressable with functional language.
The hexagonal architecture is more favorable to the OOP

46. Feature Hexagonal
Architecture
Functional
Architecture
Separate Infra from Business Yes Yes
Business Code without side effect No* Yes
Business Decision are in Business part Yes Yes
Offers good ability to test Yes Yes
Technical concerns never leak from domain No Yes
Hexagonal or Functional Core?
* By its requirements Functional Architecture is more extreme (code without mutation) and therefore
more addressable with functional language.
The hexagonal architecture is more favorable to the OOP

47. Some references
The Maybe functor - Mark Seemann
https://blog.ploeh.dk/2018/03/26/the-maybe-functor/
Functional architecture: a definition - Mark Seemann
https://blog.ploeh.dk/2018/11/19/functional-architecture-a-definition/
https://thinkbeforecoding.com/post/2018/01/25/functional-core
Functional Core - Jérémie Chassaing
https://www.destroyallsoftware.com/screencasts/catalog/functional-core-imperative-shell
Functional Core, Imperative Shell – Gary Bernhardt

48. Thank you!