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Scala dreaded _

         François Armand
Too many use for _ ?




© Normation 2010 -                          2
Too many use for _ ?




© Normation 2010 -                          3
It's always a kind of joker


  1/ “Everything”




  2/ “Give me a variable name but
                     – I don't care of what it is
                     – and/or
                     – don't want to pollute my namespace with it”




© Normation 2010 -                                                   4
1. Import all

   Java “import *”




© Normation 2010 -                   5
2. var initialization to default value

   Highly discourage in idiomatic Scala:
                     – Prefer val against var
                     – Try to always initialize a variable




© Normation 2010 -                                           6
3. Unused variables


   In higher order functions




© Normation 2010 -                         7
3. Unused variables

   In pattern matching




© Normation 2010 -                         8
4. Anonymous parameters

   In higher order functions




© Normation 2010 -                             9
4. Anonymous parameters

   In partial functions




© Normation 2010 -                             10
5. Don't import name in namespace




© Normation 2010 -                                   11
6. Syntactic sugar for existential type
   Existential type ?
   http://stackoverflow.com/questions/1031042/scalas-existential-types

   Java:
                     – Type erasure and raw types
                     – “?” generic wildcard




   Scala




© Normation 2010 -                                                       12
6. Syntactic sugar for existential type


   And in pattern matching




© Normation 2010 -                                   13
And some more _ with an other char


   1) Accessors in tuples
                       • ("foo", "bar")._2  // getter for “bar”
   2) _*
          In repeated parameter (Java 1.5 varargs)
                       • String.format("one: %s two: %s", "1" , "2")
                       • String.format("one: %s two: %s", Seq("1","2"):_*)
          In XML pattern matching
                       • See Scala XML book http://www.scala­lang.org/node/131 
          In pattern matching, as sequence wildcard
                       • Search for UnapplySeq documentation
   3) In identifier name ending with an operator
                       • ex: “query?” not valide name, use “query_?”
   4) In setter name
                       • _=
     http://stackoverflow.com/questions/1392862/scala-overriding-a-non-abstract-def-with-a-var/1393389#1393389




© Normation 2010 -                                                                                               14
Q&A
                   Thanks !




© Normation 2010
                              15

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Scala dreaded underscore

  • 1. Scala dreaded _ François Armand
  • 2. Too many use for _ ? © Normation 2010 - 2
  • 3. Too many use for _ ? © Normation 2010 - 3
  • 4. It's always a kind of joker 1/ “Everything” 2/ “Give me a variable name but – I don't care of what it is – and/or – don't want to pollute my namespace with it” © Normation 2010 - 4
  • 5. 1. Import all Java “import *” © Normation 2010 - 5
  • 6. 2. var initialization to default value Highly discourage in idiomatic Scala: – Prefer val against var – Try to always initialize a variable © Normation 2010 - 6
  • 7. 3. Unused variables In higher order functions © Normation 2010 - 7
  • 8. 3. Unused variables In pattern matching © Normation 2010 - 8
  • 9. 4. Anonymous parameters In higher order functions © Normation 2010 - 9
  • 10. 4. Anonymous parameters In partial functions © Normation 2010 - 10
  • 11. 5. Don't import name in namespace © Normation 2010 - 11
  • 12. 6. Syntactic sugar for existential type Existential type ? http://stackoverflow.com/questions/1031042/scalas-existential-types Java: – Type erasure and raw types – “?” generic wildcard Scala © Normation 2010 - 12
  • 13. 6. Syntactic sugar for existential type And in pattern matching © Normation 2010 - 13
  • 14. And some more _ with an other char 1) Accessors in tuples • ("foo", "bar")._2  // getter for “bar” 2) _* In repeated parameter (Java 1.5 varargs) • String.format("one: %s two: %s", "1" , "2") • String.format("one: %s two: %s", Seq("1","2"):_*) In XML pattern matching • See Scala XML book http://www.scala­lang.org/node/131  In pattern matching, as sequence wildcard • Search for UnapplySeq documentation 3) In identifier name ending with an operator • ex: “query?” not valide name, use “query_?” 4) In setter name • _= http://stackoverflow.com/questions/1392862/scala-overriding-a-non-abstract-def-with-a-var/1393389#1393389 © Normation 2010 - 14
  • 15. Q&A Thanks ! © Normation 2010 15