1. Scala pitfalls
VSUG meetup 22.05.2013
Manuel Bernhardt - manuel@delving.eu - @elmanu
Thursday, May 23, 13

2. Motivation
Thursday, May 23, 13

3. Motivation
Thursday, May 23, 13

4. Thursday, May 23, 13

5. Thursday, May 23, 13

6. Orders of ignorance
• 0th Order Ignorance: Lack of Ignorance
• 1st Order Ignorance: Lack of Knowledge
• 2nd Order Ignorance: Lack of Awareness
• 3rd Order Ignorance: Lack of a Suitably
Efficient Process
• 4th Order Ignorance: Meta Ignorance
Thursday, May 23, 13

7. Thursday, May 23, 13

8. One-liner of death
Thursday, May 23, 13

9. BIG SCREEN
I HAZ IT !
Thursday, May 23, 13

10. //
update
the
thumbnail
setThumbnaiL(id,
USERCOLLECTION,
collectionModel.thumbnail,
findThumbnailUrl(collectionModel.thumbnail,
collectionModel.objects),
updatedUserCollection.links.filter(_.linkType
==
Link.LinkType.THUMBNAIL).map(_.link).headOption)
Thursday, May 23, 13

11. //
update
the
thumbnail
setThumbnail(id,
USERCOLLECTION,
collectionModel.thumbnail,
findThumbnailUrl(collectionModel.thumbnail,
collectionModel.objects),
updatedUserCollection.links.filter(_.linkType
==
Link.LinkType.THUMBNAIL).map(_.link).headOption)
Thursday, May 23, 13

12. • hard to read
• hidden context switching
• debugging?
Thursday, May 23, 13

13. //
update
the
thumbnail
val
thumbnailUrl
=
findThumbnailUrl(
collectionModel.thumbnail,
collectionModel.objects
)
val
maybeThumbnailLink
=
updatedUserCollection.links
.filter(_.linkType
==
Link.LinkType.THUMBNAIL)
.map(_.link)
.headOption
setThumbnail(
id,
USERCOLLECTION,
collectionModel.thumbnail,
thumbnailUrl,
maybeThumbnailLink
)
Thursday, May 23, 13

14. • Scala Style guide
• Tools! Scalastyle, Scalariform
Thursday, May 23, 13

15. null
Thursday, May 23, 13

16. null
Thursday, May 23, 13

17. So many Options
Thursday, May 23, 13

18. val
user:
Option[User]
=
???
if
(user
==
None)
{
//
foo!
}
else
{
//
bar!
val
u
=
user.get
}
Thursday, May 23, 13

19. val
user:
Option[User]
=
???
if
(user.isDefined)
{
//
do
something
user-­‐full
}
else
{
//
oh
my,
what
now?
}
Thursday, May 23, 13

20. val
user:
Option[User]
=
???
user
match
{
case
Some(u)
=>
//
now
I'm
happy!
case
None
=>
//
well,
doh
}
Thursday, May 23, 13

21. • Option monad
• Option[T] ~= List of zero or one elements
Thursday, May 23, 13

22. val
maybeUser:
Option[User]
=
???
maybeUser
map
{
user
=>
user.email
}
getOrElse
{
"foo@bar.com"
}
Thursday, May 23, 13

23. val
maybeUser:
Option[User]
=
???
def
turnLeft(u:
User):
Option[User]
=
???
def
shake(u:
User):
Option[User]
=
???
def
shower(u:
User):
Option[User]
=
???
val
transformedUser:
Option[User]
=
maybeUser
.flatMap
(turnLeft)
.flatMap
(shake)
.flatMap
(shower)
Thursday, May 23, 13

24. Did you say implicit?
Thursday, May 23, 13

25. implicit
def
string2Int(s:
String):
Int
=
Integer.parseInt(s)
Thursday, May 23, 13

26. implicit
def
string2Int(s:
String):
Int
=
Integer.parseInt(s)
val
foo
=
"22"
val
bar
=
"some"
def
fooBar(i:
Int)
=
i
+
1
fooBar(foo)
fooBar(bar)
//
ouch
Thursday, May 23, 13

27. It’s all about design
Thursday, May 23, 13

28. • use implicit conversions wisely: specific
types
• use within the correct scope
• implicit lookup scope
Thursday, May 23, 13

29. A word on IDEs
Thursday, May 23, 13

30. Thursday, May 23, 13

31. • Scala is complex
• IDEs are getting there, but are not there
yet
• False negatives be especially misleading at
the beginning
Thursday, May 23, 13

32. Modularity
Thursday, May 23, 13

33. Thursday, May 23, 13

34. Cake pattern pitfalls
• Initialization order
• Initialization order
• Compilation time overhead (traits)
• Beautiful theory, less beautiful practice
Thursday, May 23, 13

35. Cake pattern pitfalls
• Initialization order
• Initialization order
• Compilation time overhead (traits)
• Beautiful theory, less beautiful practice
WAT?
Thursday, May 23, 13

36. Alternatives
• Subcut
• MacWire - macros!
• Guice, Spring Scala
Thursday, May 23, 13

37. Time is money
Thursday, May 23, 13

38. http://xkcd.com/303/
Thursday, May 23, 13

39. • Scala compilation is slow
• Incremental compilation about to get
better
• Get appropriate hardware
Thursday, May 23, 13

40. Useful links
• Scala style guide - http://docs.scala-lang.org/style/
• Scalastyle - http://www.scalastyle.org
• SBT-scalariform: https://github.com/sbt/sbt-scalariform
• Post on implicits - http://stackoverflow.com/questions/5598085/where-does-scala-look-for-
implicits
• Cake pattern - http://jonasboner.com/2008/10/06/real-world-scala-dependency-injection-di/
• Subcut - https://github.com/dickwall/subcut
• MacWire - https://github.com/adamw/macwire
• Play 2.1 MacWire demo - https://github.com/przemek-pokrywka/play-2.1-macwire-demo
• Orders of ignorance - http://www-plan.cs.colorado.edu/diwan/3308-s10/p17-armour.pdf
Thursday, May 23, 13

41. Questions?
• @elmanu
• manuel@delving.eu
• http://github.com/manuelbernhardt
Thursday, May 23, 13