Move fast and fix things

An introduction to Scalafix, presented at Scala World 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)
   https://astexplorer.net/
   https://github.com/gabro/vscode-scalafmt
   https://github.com/scalacenter/scalafix
   

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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 QUICK PRIMER
    TO THE AST
    

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25. LET'S START FROM THE ANSWER
    val answer = 42
    

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26. TOKENS
    val answer = 42
    // | | | | | | | |
    // \_'val'_/ \_Name_/ \_'='_/ \_Literal_/
    

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27. GRAMMAR
    val answer = 42
    // | | | | | | | |
    // | | \__Term.Name__/ | \_Lit.Int_/ |
    // | | | |
    // | \_____Pat.Var.Term__/ |
    // | |
    // \_____________________Defn.Val____________________/
    

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28. SO... AST?
    

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29. ABSTRACT
    SYNTAX
    TREE
    

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30. WHY ABSTRACT?
    

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31. val answer = 42;
    val answer = 42
    val answer =
    42
    val
    answer
    =
    42
    

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33. ASTS...
    WHY SHOULD I
    CARE?
    

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34. AST WORKFLOW 1: SCALAC
    

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35. AST WORKFLOW 2: MACROS
    

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36. AST WORKFLOW 3: SCALAFMT
    

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37. AST WORKFLOW 4: SCALAFIX
    

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

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39. 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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41. 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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42. "AST" WORKFLOW 4: SCALAFIX
    

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43. OK, LET'S DO
    THIS!
    

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45. Introducing
    SCALAMETA
    

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

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47. METAPROGRAMMING?
    

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

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

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50. TOKENS
    The "atoms"
    

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51. TREES
    The fundamental data structure
    

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52. PARSER
    ! ➡ #
    several dialects
    

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53. TREE MANIPULATION PRIMITIVES
    ▸ transform
    ▸ traverse
    ▸ collect
    

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54. SEMANTIC API: DATABASE
    ▸ Names
    ▸ Symbols
    ▸ Messages
    ▸ Synthetics
    

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55. SEMANTIC API: NAMES
    package scalaworld.semantic
    Names:
    [8..18): scalaworld => _root_.scalaworld.
    [19..27): semantic => _root_.scalaworld.semantic.
    

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56. SEMANTIC API: SYMBOLS
    List(1, 2, 3).head
    Symbols:
    _root_.scala.collection.immutable.List.
    => final object List
    _root_.scala.collection.IterableLike#head()Ljava/lang/Object;.
    => def head: A
    

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57. SEMANTIC API: MESSAGES
    object A {
    1 + 1
    }
    Messages:
    [140..145): [warning] a pure expression does nothing in
    statement position; you may be omitting necessary
    parentheses
    

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58. SEMANTIC API: SYNTHETICS (SUGARS)
    List(1, 2, 3)
    Synthetics:
    [2..7): apply => _root_.scala.collection.immutable.List.apply
    (Lscala/collection/Seq;)
    [8..11): Int => _root_.scala.Int#
    List.apply[Int](1, 2, 3)
    

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59. RECAP
    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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61. SCALAFIX
    FINALLY!
    

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62. Rule
    Rewrite
    

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

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64. Patch
    

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65. 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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66. LintMessage
    LintCategory
    

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67. LintMessage / LintCategory
    A LintMessage is a warning/error that gets displayed to the user.
    Each LintMessage belongs to LintCategory
    (e.g. "MissingExplicitType" )
    

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68. RuleCtx
    

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

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70. SemanticdbIndex
    

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

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73. LIVE
    

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74. WHAT'S NEXT?
    

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75. SHORT-TERM
    ▸ more linter rules
    ▸ rule "bundles"
    ▸ more robust expansion of inferred types / implicits
    

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76. MEDIUM TERM
    ▸ editor integrations
    ▸ stable API
    ▸ docs
    

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78. Questions?
    @gabro27
    @buildoHQ
    @ScalaItaly
    

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