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Move fast and fix things

Move fast and fix things

An introduction to Scalafix, presented at Scala Exchange 2017

Gabriele Petronella

December 14, 2017
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  1. AGENDA 1. Intro and context 2. ASTs and related workflows

    3. Scalameta 4. Scalafix 5. Live examples
  2. WHAT IS SCALAFIX Scalafix is a rewrite and linting tool

    for Scala — scalacenter.github.io/scalafix
  3. EXAMPLE 1 - NOPROCEDURESYNTAX // before def main(args: Seq[String]) {

    println("Hello world!") } // after def main(args: Seq[String]): Unit = { println("Hello world!") }
  4. EXAMPLE 2 - NOAUTOTUPLING // before def someMethod(t: (Int, String))

    = ??? someMethod(1, "something") // after def someMethod(t: (Int, String)) = ??? someMethod((1, "something"))
  5. 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(_ + _)
  6. The JS tooling landscape is living in the future —

    Ólafur Páll Geirsson, author of Scalafix
  7. 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 )
  8. 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) }
  9. SCALAMETA VADEMECUM ▸ Trees ▸ Tokens ▸ Parser ▸ Tree

    manipulation primitives ▸ Semantic API ▸ (pretty-printer)
  10. 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;)
  11. 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.
  12. SemanticdbIndex A index for looking up data in a scalameta's

    Semantic Database. It contains all the available semantic information.