Move fast and fix things

An introduction to Scalafix, presented at Scala Exchange 2017

Transcript

1. Move fast and
   BREAK FIX
   things
   

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2. ME, HI!
   

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3. STUFF I DO (BY DAY)
   

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4. STUFF I DO (BY NIGHT)
   Scala and JS open-source
   

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5. AGENDA
   1. Intro and context
   2. ASTs and related workflows
   3. Scalameta
   4. Scalafix
   5. Live examples
   

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6. WHAT IS SCALAFIX
   Scalafix is a rewrite and linting tool for Scala
   — scalacenter.github.io/scalafix
   

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7. EXAMPLE 1 - NOPROCEDURESYNTAX
   // before
   def main(args: Seq[String]) {
   println("Hello world!")
   }
   // after
   def main(args: Seq[String]): Unit = {
   println("Hello world!")
   }
   

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8. EXAMPLE 2 - NOAUTOTUPLING
   // before
   def someMethod(t: (Int, String)) = ???
   someMethod(1, "something")
   // after
   def someMethod(t: (Int, String)) = ???
   someMethod((1, "something"))
   

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9. EXAMPLE 3 - REMOVECARTESIANBUILDER
   // before
   import cats.syntax.cartesian._
   val o1: Option[Int] = Some(42)
   val o2: Option[String] = Some("hello")
   o1 |@| o2 map (_ + _)
   // after
   import cats.syntax.apply._
   val o1: Option[Int] = Some(42)
   val o2: Option[String] = Some("hello")
   (o1, o2).mapN(_ + _)
   

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10. EXAMPLE 4 - NOINFER
    val myList = List(Some(42), Right("foo"))
    // ^
    // |__
    ⚠
    Product with Serializable
    

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11. LET'S TALK ABOUT
    MAINTAINERS
    

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12. PURE EVIL
    BREAK CODE WITHOUT
    DOCUMENTING WHAT
    BROKE
    

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13. MILD EVIL
    BREAK CODE
    DOCUMENTING WHAT
    BROKE
    (OR LINKING TO A SUPER-LONG ISSUE)
    

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14. GOOD EFFORT
    BREAK CODE AND
    DOCUMENT HOW TO FIX IT
    

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15. YOU ROCK!
    BREAK CODE AFTER A
    DEPRECATION CYCLE AND
    EXPLAIN HOW TO FIX IT
    

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16. SUPERHERO
    BREAK CODE AFTER A
    DEPRECATION CYCLE AND
    PROVIDE AN AUTOMATIC
    REWRITE
    

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17. SCI-FI?
    

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18. MEANWHILE IN SWIFT
    

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19. MEANWHILE IN
    JAVASCRIPT
    

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20. MEANWHILE IN JAVASCRIPT
    3 paragraphs later
    

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21. The JS tooling landscape is living in the
    future
    — Ólafur Páll Geirsson, author of Scalafix
    

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22. IT'S TIME WE JUMP INTO THE
    FUTURE
    

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23. HOW?
    LET'S TAKE A STEP BACK
    

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24. A WORD ABOUT
    ASTS
    

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25. ABSTRACT
    SYNTAX
    TREE
    

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26. WHY ABSTRACT?
    

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27. val answer = 42;
    val answer = 42
    val answer =
    42
    val
    answer
    =
    42
    

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28. ASTS...
    WHY SHOULD I
    CARE?
    

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29. AST WORKFLOW 1: SCALAC
    

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30. AST WORKFLOW 2: MACROS
    

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31. AST WORKFLOW 3: SCALAFMT
    

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32. AST WORKFLOW 4: SCALAFIX
    

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33. AST WORKFLOW 4: SCALAFIX
    Woops, copy-paste?
    

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34. WHAT SCALAFIX COULD DO:
    // before
    def someMethod(string: String) { // I like trees
    println(string.trim.split("/").lastOption)
    }
    // after
    def someMethod(string: String): Unit =
    println(
    string
    .trim
    .split("/")
    .lastOption
    )
    

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36. WHAT SCALAFIX DOES INSTEAD:
    // before
    def someMethod(string: String) { // I like trees
    println(string.trim.split("/").lastOption)
    }
    // after
    def someMethod(string: String): Unit = { // I like trees
    println(string.trim.split("/").lastOption)
    }
    

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37. "AST" WORKFLOW 4: SCALAFIX
    

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38. OK, LET'S DO
    THIS!
    

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40. Introducing
    SCALAMETA
    

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41. Scalameta is a modern
    metaprogramming library for Scala
    — scalameta.org
    

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42. METAPROGRAMMING?
    

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43. METAPROGRAMMING DEVTOOLS!
    ▸ code formatting
    ▸ code fixing
    ▸ code browsing
    ▸ and many more applications!
    

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44. SCALAMETA VADEMECUM
    ▸ Trees
    ▸ Tokens
    ▸ Parser
    ▸ Tree manipulation primitives
    ▸ Semantic API
    ▸ (pretty-printer)
    

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45. SEMANTIC INFORMATION
    List(1, 2, 3)
    // Symbol
    _root_.scala.collection.immutable.List.
    // Denotation
    final object List
    // Synthetic
    [2..7): apply => _root_.scala.collection.immutable.List.apply
    (Lscala/collection/Seq;)
    

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47. SCALAFIX
    FINALLY!
    

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48. Rule
    

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49. Rule
    A rule can be:
    ▸ checked
    ▸ fixed
    

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50. Patch
    

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51. Patch
    A Scalafix Patch is a patch in the diff sense of patch.
    It boils down to a list of -/+ to apply to the source code.
    

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52. RuleCtx
    

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53. RuleCtx
    The toolbox for writing rules.
    It provides the API for producing patches.
    

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54. SemanticdbIndex
    

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55. SemanticdbIndex
    A index for looking up data in a scalameta's Semantic Database.
    It contains all the available semantic information.
    

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57. LIVE
    

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58. WHAT'S NEXT?
    

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59. MOAR LINTING!
    

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60. EDITOR SUPPORT
    

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61. DEMO
    

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63. Questions?
    @gabro27
    @buildoHQ
    @ScalaItaly
    

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