1.
Functional Programming,
Is It Worth It?
Andrew Rollins, Shift Conference, June 2016

2.
Me: Andrew Rollins
• http://www.andrewrollins.com
• Twitter: @andrew311
• Boston, USA
• Co-founder of Localytics
• Long time functional programmer
• Now at Sigma Prime Ventures (Boston VC)
Andrew Rollins, Split, June 2016

3.
Have you encountered…
monad
functor
applicative
and said “WTF?”
Andrew Rollins, Split, June 2016

4.
You are not alone.
Andrew Rollins, Split, June 2016

5.
Andrew Rollins, Split, June 2016
Learning FP is like Officer
Barbrady learning to read
(second season, fourth episode of South Park)

6.
Andrew Rollins, Split, June 2016
Officer Barbrady confronts his
illiteracy, learning to read so he
can solve a crime!

7.
Upon solving the crime, he’s
given a copy of this:
Andrew Rollins, Split, June 2016

8.
Leading to a memorable South
Park quote…
Andrew Rollins, Split, June 2016

9.
Andrew Rollins, Split, June 2016
Yes, at first I was happy to be
learning how to read.
It seemed exciting and
magical, but then I read this:
Atlas Shrugged by Ayn Rand.
I read every last word of this
garbage, and because of this
piece of s**t, I am never
reading again.

10.
Andrew Rollins, Split, June 2016
Officer Barbrady’s experience
sums up many people’s
experience with Functional
Programming.

11.
Andrew Rollins, Split, June 2016
Exciting at first…
But then you ecounter crazy
s**t, and you have an Officer
Barbrady moment.

12.
Andrew Rollins, Split, June 2016
Crazy stuff like…
•monads
•functors
•monoids
•applicatives
•category theory
•blah blah blah

13.
Andrew Rollins, Split, June 2016
I saw this happen across
engineering at Localytics

14.
Andrew Rollins, Split, June 2016
Does it have to be this way?
Is FP worth it?

15.
Andrew Rollins, Split, June 2016
I’m here to say…
Yes, FP is worth it!

16.
We can’t get hung up on this.

17.
Andrew Rollins, Split, June 2016
Look past the…
•monads
•functors
•monoids
•applicatives
•category theory
•blah blah blah

18.
And find the useful things…
Andrew Rollins, Split, June 2016
•pure functions
•higher order functions
•immutable data
•isolating side effects
•using types effectively
•and so on

19.
Andrew Rollins, Split, June 2016
My goal for the rest of this
presentation is to help you
overcome your Officer Barbrady
moment and find a path to
success in FP.

20.
Let’s get to it!
Andrew Rollins, Split, June 2016

21.
Tip
FP is about a core set of principles
Andrew Rollins, Split, June 2016

22.
Principles
•Pure functions
•Higher order functions
•Immutable data
•Isolation of side effects
Andrew Rollins, Split, June 2016

23.
Pure functions
•Purity is all about making functions self-contained and
repeatable
•foo(x) should NOT
• read or manipulate state outside the function
• change the caller’s version of x
•foo(x) should
• return the same results for the same argument, always
Andrew Rollins, Split, June 2016

24.
Why pure functions?
•Because purity makes a function easier to reason
about and test
•Purity guarantees that you aren’t dependent on state
changes outside of the function
•Purity means that your tests can be concerned with
simple input and output checks
Andrew Rollins, Split, June 2016

25.
Higher order functions
•HoF is about being able to pass around functions as
values
Andrew Rollins, Split, June 2016

26.
Why higher order functions?
•Because they let us write useful methods like map
•Most people use languages with HoF these days, so
this shouldn’t require much convincing
Andrew Rollins, Split, June 2016

27.
Immutable data
•Immutability is about locking down objects once they
are created
•An object should NOT update its internal state via
object methods
•State changes should be reflected in new objects
•A programmer should not change variables once
created
Andrew Rollins, Split, June 2016

28.
Why immutable data?
•Because immutability makes it easier to reason about
state in your program
•Variable x is always the same x
•Threads couldn’t possibly stomp on each others’ state
if the state is immutable
Andrew Rollins, Split, June 2016

29.
Isolation of side effects
•Isolation of side effects is about being disciplined with
regards to side effects and where they happen
•We recognize that code must have side effects to do
anything useful, this is about techniques which let us
minimize the surface area of side effects
Andrew Rollins, Split, June 2016

30.
Why isolation of side effects?
•Because functions without side effects are easier to
test and reason about
•If we isolate side effects to a smaller portion of code,
testing is easier
Andrew Rollins, Split, June 2016

31.
Read this for more about principles:
“Intro To Functional Programming”
by Marko Pavlovic
http://markonis.github.io/functional/programming/2016/04/
16/intro-to-functional-programming.html
Andrew Rollins, Split, June 2016

32.
Tip
When learning FP, it’s important to
separate the principles from the
tools
Andrew Rollins, Split, June 2016

33.
Principle
Measure twice, cut once
Tool
saw
Andrew Rollins, Split, June 2016

34.
Principle
Don’t use null
Tool
Option
Andrew Rollins, Split, June 2016

35.
Tools
• For comprehensions
• Pattern matching
• Functors
• Monads
• Monoids
• Applicatives
• Either
• Option
Andrew Rollins, Split, June 2016

36.
Law of the Instrument
"Give a small boy a hammer, and he
will find that everything he
encounters needs pounding.”
- Abraham Kaplan, in 1964
Andrew Rollins, Split, June 2016

37.
Law of the Instrument
“If all you have is a hammer,
everything looks like a nail.”
- Abraham Maslow, in 1966
Andrew Rollins, Split, June 2016

38.
Beware of this effect with…
• Functors
• Monads
• Monoids
• Applicatives
• Either
• Option
Andrew Rollins, Split, June 2016

39.
hammer = monad
Andrew Rollins, Split, June 2016

40.
Andrew’s Corollary to Law of the Instrument
“If all you have is a hammer, and someone
asks you to build a house, you’re gonna have
a bad time.”
- Andrew Rollins (me), in 2016
Andrew Rollins, Split, June 2016

41.
It is exceptionally hard to build a
house (application) when you only
have a hammer (monad)
Andrew Rollins, Split, June 2016

42.
Treat FP tools like any other tools in your
toolbox, and realize that it takes a set of tools
to build a house.
Andrew Rollins, Split, June 2016

43.
Tip
I like to think of functors, monads,
etc. as a special kind of container.
They are just codified patterns.
Andrew Rollins, Split, June 2016

44.
There are many kinds of containers:
linked list, hash, array, tree, queue
But they usually have common
interfaces:
Java Collection, Ruby Enumerable
Andrew Rollins, Split, June 2016

45.
Quoting Russ Bishop:
Functors are containers you can
call map on
Monads are containers you can
call flatMap on
“Functors, Applicatives, and Monads in Plain English”
http://www.russbishop.net/monoids-monads-and-functors
Andrew Rollins, Split, June 2016

46.
It’s a simplification, but it’ll get you
started.
Andrew Rollins, Split, June 2016

47.
Also check out:
“Functors, Applicatives, And Monads
in Pictures”
by Aditya Bhargava
http://adit.io/posts/2013-04-17-
functors,_applicatives,_and_monads_in_pictures.html
Andrew Rollins, Split, June 2016

48.
Tip
Use libraries, but beware.
Andrew Rollins, Split, June 2016

49.
It helps to read about “Typeclasses”,
especially for Haskell and Scala
“Learn You a Haskell, Types and Typeclasses”
http://learnyouahaskell.com/types-and-typeclasses
Andrew Rollins, Split, June 2016

50.
Many libraries use Typeclasses as an
alternative to typical object
inheritance and interfaces.
It takes time to become comfortable
with this pattern.
Andrew Rollins, Split, June 2016

51.
Wrapping up
You already FP
Andrew Rollins, Split, June 2016

52.
Any time you…
• Create a self-contained function without side effects
• Pass around functions as input / output to other functions
• Create immutable objects instead of mutating object state
• Use return values instead of exceptions
Andrew Rollins, Split, June 2016

53.
Andrew Rollins, Split, June 2016
Then why does the Barbrady
moment happen?
Because FP wanders into some
hard topics.

54.
“A monad is just a monoid in the category of
endofunctors, what's the problem?”
- James Iry
Andrew Rollins, Split, June 2016

55.
Andrew Rollins, Split, June 2016
Don’t let it get you down.
FP is just a style of
programming.
It might help you write better
code.
Don’t sweat the small stuff.

56.
Takeaways
•first focus on the principles
•read those links
•mess around with tools
•don’t get bogged down by tools
•try some popular libraries
Andrew Rollins, Split, June 2016

57.
Andrew Rollins, Split, June 2016
• Andrew Rollins
• Twitter: @andrew311
• http://andrewrollins.com
Happy programming!
Thank you!