This document summarizes a talk about using the Scalaz library in real-world Scala applications. It begins by acknowledging common criticisms of Scalaz as being too complex or not immediately useful. It then provides examples of how the speaker's company uses Scalaz for tasks like JSON mapping and handling state. The speaker advocates using Scalaz intentionally and delineating pure and impure code. They discuss strategies for learning Scalaz incrementally and as a team. The document argues that with pragmatic use, Scalaz can make code more modular, testable and help reason about concurrency without adding undue complexity.

1. REAL
W⊛RLD
SCALAZ
http://speakerdeck.com/u/jrwest/p/real-world-scalaz
changed talk title (“practical scalaz” by Chris Marshall http://skillsmatter.com/podcast/scala/practical-scalaz-2518)

2. “If you are thinking of using
Scalaz, stop now while you
still have your sanity"
* context: java devs learning scala
* source: http://zeroturnaround.com/blog/scala-sink-or-swim-part-1/#scalaz

3. “How does this help
you ship features or
improve your product?”
* context: commenting on the use of lenses
* source: https://twitter.com/#!/coda/status/167794305107361796)

4. “It will not immediately
help you write better
programs that are easy to
understand”
* hear/read this often
* source: http://zeroturnaround.com/blog/scala-sink-or-swim-part-1/#scalaz

5. “There are two types of
[libraries]: the ones people hate
and the ones nobody uses”
- unknown
* applies to scala too
* know correct attribution? contact me

6. HOW
&
WHY?
* those quotes pretty much boil down to these two questions:
* how: apply tools given by scalaz to real world problems? to handle human scale?
* why would we choose this solution over other ones?

7. @_jrwest
github.com/jrwest
blog.loopedstrange.com
* a little context before we talk about the “how & why?”

8. I
WORK @
* small team, < 15 engineers
* many have java bg, but not all, most w/ little to no scala experience
* most have oop bg, varying knowledge of FP topics

9. WE USE SCALA
{ a lot }
* 15+ public facing services supported by many other internal modules/libs
* most built on netty, also I/O bound
* originally java, now > 70% scala

10. libraryDependencies +=
“org.scalaz” %% “scalaz-core” % “6.0.3”
* almost all our services depend on scalaz
* usage levels and parts of scalaz used vary among services
* blocked on upgrading to 6.0.4 b/c of binary compatibility issues

11. SOME
EXAMPLES

12. JSON Mapping
def Fi
leMani
def re festJS
ad(jso ONR =
ma n: JVa new JS
nifest lue) = ONR[ Ap
NoId.a p I D= > F
field( pplyJS ileMan
jsonMa O N( ifest]
f i e l d( nifest {
)(json change K e y) ,
) LogEnt
} ryCach
edJson
} K e y)
* lift-json-scalaz (https://github.com/lift/framework/tree/master/core/json-scalaz)
* validationnel applicative, error accumulation = better user-facing errors & debugging
* composition

13. S @-@ A / State[S,A]
for
{
mbE
_ xis
<- ts
_ man <-
<- ife man
} y cha stF ife
iel nge ile stF
d m Log s - ile
bEx Ent = p s m
ist ryF ath emb
s ile er
s - pat
=? h
(pa
th,
mbE
xi sts
>|
fal
se
| t
rue
)
* easy manipulation of highly nested case classes (ex: file manifests)
* better implementation hiding
* rich libraries (e.g. MapLens, SetLens, & provided state actions)

14. sideEffect.pure[IO]
ble ,X]
[ Thr owa = {
io n NEL in g]) _))
al i dat st [ Str mai n] (
X ]=V : Li mDo
y pe V[ s (ke ys [Cu sto
t tch K ey
t .fe tch ain ]])
def fe buc ke tom Dom
ap ( Cus
ke y s.m pti on[
qu e nce ce[ V,O
.se seq uen
(_.
. map
}
* can’t guarantee purity, can delineate impurity
* examples: storing data in riak, uploading files to s3 and phonegap build svc
* composition of IO makes side-effecting code much more reusable

15. Kle
]
i sli
]
OptionW[A] Tree[A]
F[_
[M
[_]
le[
,A, Pure[P[_]
B] ]
ab
MAB[M[
_],A,B]
ld
Bind[Z[ [A]
_ ]] ro
Fo
Ze t[ A]
Tra ty Lis
ver Identity ] NonEmp
p [ A
e [T s u
Fun [_]] r o
ctor mig Appl
[F[_] Se y[A[_
] MA[ M[_ ],A] ]]
* we make use of a lot more as well
* data structures like Validation and NonEmptyList
* most type classes and “enrichments”

16. ITS NOT ALL OR NOTHING
* shades of grey, not black and white
* scalaz is really many libraries, use the parts you want
* we started using “encrichments” and some data structures, grew from there
--
original image: http://www.chicagonow.com/chicago-tough/files/2012/03/chipsallin.jpg

17. MAKE BORDERS
* don’t abuse scala’s flexibility & power
* toss pure/impure code into a salad bowl & mixing is a bad idea
* define borders where pure code meets impure code
--
original image: http://media.nowpublic.net/images//59/4/59455eabd625a15e2afb583dcfbb27ce.jpg

18. Public Interface
Implementation
Dependency Calls
* your code can be pure
* public interface is the border, it runs IO, throws exceptions, etc
* existing libraries probably aren’t pure -- many are java after all

19. Outside World
Business Logic
* communicate with clients via HTTP
Data Access
* the most important part -- your code -- make pure*
* perform side-effects like talk to database, read/write files, etc

20. Impure
Pure
Impure
* “the sandwich” - common pattern in our code
--
original image: http://fastfood.ocregister.com/files/2010/05/ice-cream-sandwich1.jpg

21. :)
* code has become much more DRY/modular and as a result more unit testable
* concurrent code becomes “simpler” to reason about when pure
* FP makes reasoning about bugs simpler, narrow down cause faster

22. :(
* scala hardsips: bin. compatability problems, tooling deficiencies, type inference limits
* import scalaz._; import Scalaz._ adds a lot of clutter in the current scope
* lack of docs/related content (in Scala!)

23. scalaz7
* not currently used in our production systems but one or two of my side projects
* much better for teaching (explicit type class instance usage -- w/o syntax ext.)
* addresses some of the issues we have like importing “the scalaz world”

24. “SCALE”?

25. “Human Scale”
* the complexity comments may be FUD, but that does not mean there is not stuff to learn here
* how do we help new employees learn scala & scalaz?

26. HIRE PEOPLE WHO
WANT TO LEARN
* hiring from all backgrounds instead of hiring a “scala or java developer” is much easier
* those people need to be open to exploring and learning new tech/methods

27. Shared Resources
* easy win, shares effort in finding good resources
* dropbox full of downloaded papers and books purchased by the company
* internal talks available on github

28. * code review/pull requests
LEARN TOGETHER
* face to face discussion on a whiteboard
* team talks on fridays, any one can attend, anyone can talk, neither mandatory

29. ON YOUR OWN
* RTFC -- in the end it all just code
* apply what you know, this is what really makes you learn it
* internal talks mostly make you aware of it, give you starting point, application is key

30. ITS NOT ALL OR NOTHING
* focus on understanding a part of the library
* break that part down, use it in the repl in a few cases (w/ a few types, examples, etc)
* as you learn more about each part and then more parts the bigger picture emerges
--
original image: http://www.chicagonow.com/chicago-tough/files/2012/03/chipsallin.jpg

31. Learn You a
??
* not necessary
* understanding syntax, basics can be useful while learning concepts until scala docs fill out
--
haskell logo: http://www.haskell.org/wikistatic/haskellwiki_logo.png

32. ASK #scalaz
FOR HELP

33. BREAK
RULES
* you may hear things you cannot do, “validation is not a monad”
* ignore and explore yourself -- these are the real learning opportunites
* understand how to implement validation monad and why no error accum. happens

34. PAYING
UPFRONT
* of course this have some upfront cost
* so does writing tests and similarly a good balance pays off

35. L3
L2
L1
A3
Level
A2
A1
Up
* scalaz covers most advanced features in scala language
* great guide for library and code design (to an extent). Scalaz guys have done serious amount of research in this regard

36. Thanks!
talk recap slide

37. SOME RESOURCES
• Atlassian’s Typeclassopedia - http://typeclassopedia.bitbucket.org
• Apocalisp Blog - http://apocalisp.wordpress.com/
• “Functional Programming in Scala” MEAP - http://manning.com/bjarnason/
• Eric Torreborre’s Blog - http://etorreborre.blogspot.com/
• StackMob Engineering Blog - http://www.stackmob.com/category/engineering/
• Learn You a Scalaz (my WIP, about to undergo major rework) -
https://github.com/jrwest/learn-you-a-scalaz
• Chris Marshall’s “Practical Scalaz” Talk - http://skillsmatter.com/podcast/scala/
practical-scalaz-2518