Move fast and fix things

An introduction to Scalafix, presented at Scala World 2017

Transcript

1. Move fast and BREAK FIX things
2. ME, HI!
3. STUFF I DO (BY DAY)
4. STUFF I DO (BY NIGHT) https://astexplorer.net/ https://github.com/gabro/vscode-scalafmt https://github.com/scalacenter/scalafix
5. AGENDA 1. Intro and context 2. ASTs and related workflows 3. Scalameta 4. Scalafix 5. Live examples
6. WHAT IS SCALAFIX Scalafix is a rewrite and linting tool for Scala — scalacenter.github.io/scalafix
7. EXAMPLE 1 - NOPROCEDURESYNTAX // before def main(args: Seq[String]) { println("Hello world!") } // after def main(args: Seq[String]): Unit = { println("Hello world!") }
8. EXAMPLE 2 - NOAUTOTUPLING // before def someMethod(t: (Int, String)) = ??? someMethod(1, "something") // after def someMethod(t: (Int, String)) = ??? someMethod((1, "something"))
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(_ + _)
10. EXAMPLE 4 - NOINFER val myList = List(Some(42), Right("foo")) // ^ // |__ ⚠ Product with Serializable
11. LET'S TALK ABOUT MAINTAINERS
12. PURE EVIL BREAK CODE WITHOUT DOCUMENTING WHAT BROKE
13. MILD EVIL BREAK CODE DOCUMENTING WHAT BROKE (OR LINKING TO A SUPER-LONG ISSUE)
14. GOOD EFFORT BREAK CODE AND DOCUMENT HOW TO FIX IT
15. YOU ROCK! BREAK CODE AFTER A DEPRECATION CYCLE AND EXPLAIN HOW TO FIX IT
16. SUPERHERO BREAK CODE AFTER A DEPRECATION CYCLE AND PROVIDE AN AUTOMATIC REWRITE
17. SCI-FI?
18. MEANWHILE IN SWIFT
19. MEANWHILE IN JAVASCRIPT
20. MEANWHILE IN JAVASCRIPT 3 paragraphs later
21. The JS tooling landscape is living in the future — Ólafur Páll Geirsson, author of Scalafix
22. IT'S TIME WE JUMP INTO THE FUTURE
23. HOW? LET'S TAKE A STEP BACK
24. A QUICK PRIMER TO THE AST
25. LET'S START FROM THE ANSWER val answer = 42
26. TOKENS val answer = 42 // | | | | | | | | // \_'val'_/ \_Name_/ \_'='_/ \_Literal_/
27. GRAMMAR val answer = 42 // | | | | | | | | // | | \__Term.Name__/ | \_Lit.Int_/ | // | | | | // | \_____Pat.Var.Term__/ | // | | // \_____________________Defn.Val____________________/
28. SO... AST?
29. ABSTRACT SYNTAX TREE
30. WHY ABSTRACT?
31. val answer = 42; val answer = 42 val answer = 42 val answer = 42
33. ASTS... WHY SHOULD I CARE?
34. AST WORKFLOW 1: SCALAC
35. AST WORKFLOW 2: MACROS
36. AST WORKFLOW 3: SCALAFMT
37. AST WORKFLOW 4: SCALAFIX
38. AST WORKFLOW 4: SCALAFIX Woops, copy-paste?
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 )
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) }
42. "AST" WORKFLOW 4: SCALAFIX
43. OK, LET'S DO THIS!
45. Introducing SCALAMETA
46. Scalameta is a modern metaprogramming library for Scala — scalameta.org
47. METAPROGRAMMING?
48. METAPROGRAMMING DEVTOOLS! ▸ code formatting ▸ code fixing ▸ code browsing ▸ and many more applications!
49. SCALAMETA VADEMECUM ▸ Trees ▸ Tokens ▸ Parser ▸ Tree manipulation primitives ▸ Semantic API ▸ (pretty-printer)
50. TOKENS The "atoms"
51. TREES The fundamental data structure
52. PARSER ! ➡ # several dialects
53. TREE MANIPULATION PRIMITIVES ▸ transform ▸ traverse ▸ collect
54. SEMANTIC API: DATABASE ▸ Names ▸ Symbols ▸ Messages ▸ Synthetics
55. SEMANTIC API: NAMES package scalaworld.semantic Names: [8..18): scalaworld => _root_.scalaworld. [19..27): semantic => _root_.scalaworld.semantic.
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
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
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)
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;)
61. SCALAFIX FINALLY!
62. Rule Rewrite
63. Rule A rule can be: ▸ checked ▸ fixed
64. Patch
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.
66. LintMessage LintCategory
67. LintMessage / LintCategory A LintMessage is a warning/error that gets displayed to the user. Each LintMessage belongs to LintCategory (e.g. "MissingExplicitType" )
68. RuleCtx
69. RuleCtx The toolbox for writing rules. It provides the API for producing patches.
70. SemanticdbIndex
71. SemanticdbIndex A index for looking up data in a scalameta's Semantic Database. It contains all the available semantic information.
73. LIVE
74. WHAT'S NEXT?
75. SHORT-TERM ▸ more linter rules ▸ rule "bundles" ▸ more robust expansion of inferred types / implicits
76. MEDIUM TERM ▸ editor integrations ▸ stable API ▸ docs
78. Questions? @gabro27 @buildoHQ @ScalaItaly