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Scala pitfalls

  1. 1. Scala pitfalls VSUG meetup 22.05.2013 Manuel Bernhardt - manuel@delving.eu - @elmanu Thursday, May 23, 13
  2. 2. Motivation Thursday, May 23, 13
  3. 3. Motivation Thursday, May 23, 13
  4. 4. Thursday, May 23, 13
  5. 5. Thursday, May 23, 13
  6. 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. 7. Thursday, May 23, 13
  8. 8. One-liner of death Thursday, May 23, 13
  9. 9. BIG SCREEN I HAZ IT ! Thursday, May 23, 13
  10. 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. 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. 12. • hard to read • hidden context switching • debugging? Thursday, May 23, 13
  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. 14. • Scala Style guide • Tools! Scalastyle, Scalariform Thursday, May 23, 13
  15. 15. null Thursday, May 23, 13
  16. 16. null Thursday, May 23, 13
  17. 17. So many Options Thursday, May 23, 13
  18. 18. val  user:  Option[User]  =  ??? if  (user  ==  None)  {    //  foo! }  else  {    //  bar!    val  u  =  user.get } Thursday, May 23, 13
  19. 19. val  user:  Option[User]  =  ??? if  (user.isDefined)  {    //  do  something  user-­‐full }  else  {    //  oh  my,  what  now? } Thursday, May 23, 13
  20. 20. val  user:  Option[User]  =  ??? user  match  {    case  Some(u)  =>  //  now  I'm  happy!    case  None  =>  //  well,  doh } Thursday, May 23, 13
  21. 21. • Option monad • Option[T] ~= List of zero or one elements Thursday, May 23, 13
  22. 22. val  maybeUser:  Option[User]  =  ??? maybeUser  map  {  user  =>    user.email }  getOrElse  {    "foo@bar.com" } Thursday, May 23, 13
  23. 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. 24. Did you say implicit? Thursday, May 23, 13
  25. 25. implicit  def  string2Int(s:  String):  Int  =    Integer.parseInt(s) Thursday, May 23, 13
  26. 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. 27. It’s all about design Thursday, May 23, 13
  28. 28. • use implicit conversions wisely: specific types • use within the correct scope • implicit lookup scope Thursday, May 23, 13
  29. 29. A word on IDEs Thursday, May 23, 13
  30. 30. Thursday, May 23, 13
  31. 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. 32. Modularity Thursday, May 23, 13
  33. 33. Thursday, May 23, 13
  34. 34. Cake pattern pitfalls • Initialization order • Initialization order • Compilation time overhead (traits) • Beautiful theory, less beautiful practice Thursday, May 23, 13
  35. 35. Cake pattern pitfalls • Initialization order • Initialization order • Compilation time overhead (traits) • Beautiful theory, less beautiful practice WAT? Thursday, May 23, 13
  36. 36. Alternatives • Subcut • MacWire - macros! • Guice, Spring Scala Thursday, May 23, 13
  37. 37. Time is money Thursday, May 23, 13
  38. 38. http://xkcd.com/303/ Thursday, May 23, 13
  39. 39. • Scala compilation is slow • Incremental compilation about to get better • Get appropriate hardware Thursday, May 23, 13
  40. 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. 41. Questions? • @elmanu • manuel@delving.eu • http://github.com/manuelbernhardt Thursday, May 23, 13

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