AGENDA
1. Intro and context
2. ASTs and related workflows
3. Scalameta
4. Scalafix
5. Live examples
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WHAT IS SCALAFIX
Scalafix is a rewrite and linting tool for Scala
— scalacenter.github.io/scalafix
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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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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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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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EXAMPLE 4 - NOINFER
val myList = List(Some(42), Right("foo"))
// ^
// |__
⚠
Product with Serializable
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LET'S TALK ABOUT
MAINTAINERS
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PURE EVIL
BREAK CODE WITHOUT
DOCUMENTING WHAT
BROKE
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MILD EVIL
BREAK CODE
DOCUMENTING WHAT
BROKE
(OR LINKING TO A SUPER-LONG ISSUE)
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GOOD EFFORT
BREAK CODE AND
DOCUMENT HOW TO FIX IT
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YOU ROCK!
BREAK CODE AFTER A
DEPRECATION CYCLE AND
EXPLAIN HOW TO FIX IT
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SUPERHERO
BREAK CODE AFTER A
DEPRECATION CYCLE AND
PROVIDE AN AUTOMATIC
REWRITE
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SCI-FI?
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MEANWHILE IN SWIFT
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MEANWHILE IN
JAVASCRIPT
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MEANWHILE IN JAVASCRIPT
3 paragraphs later
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The JS tooling landscape is living in the
future
— Ólafur Páll Geirsson, author of Scalafix
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IT'S TIME WE JUMP INTO THE
FUTURE
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HOW?
LET'S TAKE A STEP BACK
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A WORD ABOUT
ASTS
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ABSTRACT
SYNTAX
TREE
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WHY ABSTRACT?
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val answer = 42;
val answer = 42
val answer =
42
val
answer
=
42
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ASTS...
WHY SHOULD I
CARE?
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AST WORKFLOW 1: SCALAC
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AST WORKFLOW 2: MACROS
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AST WORKFLOW 3: SCALAFMT
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AST WORKFLOW 4: SCALAFIX
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AST WORKFLOW 4: SCALAFIX
Woops, copy-paste?
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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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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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"AST" WORKFLOW 4: SCALAFIX
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OK, LET'S DO
THIS!
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Introducing
SCALAMETA
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Scalameta is a modern
metaprogramming library for Scala
— scalameta.org
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METAPROGRAMMING?
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METAPROGRAMMING DEVTOOLS!
▸ code formatting
▸ code fixing
▸ code browsing
▸ and many more applications!
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SCALAMETA VADEMECUM
▸ Trees
▸ Tokens
▸ Parser
▸ Tree manipulation primitives
▸ Semantic API
▸ (pretty-printer)
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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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SCALAFIX
FINALLY!
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Rule
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Rule
A rule can be:
▸ checked
▸ fixed
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Patch
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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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RuleCtx
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RuleCtx
The toolbox for writing rules.
It provides the API for producing patches.
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SemanticdbIndex
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SemanticdbIndex
A index for looking up data in a scalameta's Semantic Database.
It contains all the available semantic information.