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![SemanticDB
Symbols:
com/alessandromarrella/presen
tation/IOExample. => final
object IOExample extends
AnyRef { +1 decls }
com/alessandromarrella/presen
tation/IOExample.x. => val
method x: IO[Unit]
package com.alessandromarrella.presentation
import cats.effect.IO
object IOExample {
val x = IO(println("Hello world"))
}](https://image.slidesharecdn.com/introductiontometa-programminginscala-190123095028/75/Introduction-to-meta-programming-in-scala-18-2048.jpg)
![SemanticDB
Symbols:
com/alessandromarrella/presen
tation/IOExample. => final
object IOExample extends
AnyRef { +1 decls }
com/alessandromarrella/presen
tation/IOExample.x. => val
method x: IO[Unit]
package com.alessandromarrella.presentation
import cats.effect.IO
object IOExample {
val x = IO(println("Hello world"))
}](https://crownmelresort.com/image.slidesharecdn.com/introductiontometa-programminginscala-190123095028/75/Introduction-to-meta-programming-in-scala-18-2048.jpg)
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Introduction to meta-programming in scala
- 1. Introduction to meta-programming inScala Alessandro Marrella
- 2. About me ● SoftwareEngineer ● Working with Scala professionally for 3 years ● ♡ automating stuff ● ♡ functional programming ● github.com/amarrella ● linkedin.com/in/alessandromarrella
- 3. CREDITS ● Ólafur PállGeirsson (@olafurpg) ● Scala Center / EPFL (scala.epfl.ch) ● Twitter + many other contributors in the Scala community
- 4. Meta-programming ● Treating otherprograms as Data ● Ability to ○ Read ○ Generate other programs WHY? ● Reduce lines of code & dev time ● Reduce boilerplate, errors and boring stuff ○ Transform ○ Analyze
- 5. Scalameta ● Library toread, analyze, transform and generate Scala programs ○ Syntactic API: parse, transform & prettyprint ○ Semantic API: understand symbols and types ● Ecosystem ○ Scalameta (Trees & SemanticDB) ○ Scalafmt: code formatting ○ Scalafix: code linting & rewriting ○ Metals: language server & more( scalameta.org/docs/misc/built-with-scalameta.html )
- 6.
- 7.
- 8.
- 9. Syntax trees val x:Int = 1+1 Lit.Int(1)Lit.Int(1) Term.ApplyInfix Term.Name("+") Defn.Val Patn.Var Term.Name(“x”) Type.Name
- 10. Syntax trees object Main{ val x: Int = 1+1 } Lit.Int(1)Lit.Int(1) Term.ApplyInfix Term.Name("+") Defn.Val Patn.Var Term.Name(“x”) TemplateTerm.Name(“Main”) Defn.Object Type.Name
- 11. Syntax trees: fromtree to code object Main { } Lit.Int(1)Lit.Int(1) Term.ApplyInfix Term.Name("+") Defn.Val Patn.Var Term.Name(“x”) TemplateTerm.Name(“Main”) Defn.Object Type.Name
- 12. Syntax trees: fromtree to code object Main { val x: Int } Lit.Int(1)Lit.Int(1) Term.ApplyInfix Term.Name("+") Defn.Val Patn.Var Term.Name(“x”) TemplateTerm.Name(“Main”) Defn.Object Type.Name
- 13. Syntax trees: fromtree to code object Main { val x: Int = 1 + 1 } Lit.Int(1)Lit.Int(1) Term.ApplyInfix Term.Name("+") Defn.Val Patn.Var Term.Name(“x”) TemplateTerm.Name(“Main”) Defn.Object Type.Name
- 14. Scalameta ● Scalameta Treesare lossless ● Scalafmt: uses trees to figure out how to format ● Scalafix: uses trees to apply syntactic rules (more later) ● Explicit syntax or quasiquotes q”object $name { val $val: $t = $expr }” https://scalameta.org/docs/trees/guide.html
- 15. SemanticDB ● Compiles scala programsto semantic information ● Decouples producing and consuming semantic information
- 16. SemanticDB package com.alessandromarrella.presentation import cats.effect.IO objectIOExample { val x = IO(println("Hello world")) }
- 17. SemanticDB Summary: Schema =>SemanticDB v4 Uri => src/main/scala/com/alessandro marrella/presentation/IOExamp le.scala Text => empty Language => Scala Symbols => 2 entries Occurrences => 11 entries package com.alessandromarrella.presentation import cats.effect.IO object IOExample { val x = IO(println("Hello world")) }
- 18. SemanticDB Symbols: com/alessandromarrella/presen tation/IOExample. => final objectIOExample extends AnyRef { +1 decls } com/alessandromarrella/presen tation/IOExample.x. => val method x: IO[Unit] package com.alessandromarrella.presentation import cats.effect.IO object IOExample { val x = IO(println("Hello world")) }
- 19. SemanticDB Occurrences: [0:8..0:11) => com/ [0:12..0:30)=> com/alessandromarrella/ [0:31..0:43) => com/alessandromarrella/presen tation/ package com.alessandromarrella.presentation import cats.effect.IO object IOExample { val x = IO(println("Hello world")) } position
- 20. SemanticDB Occurrences: [2:7..2:11) => cats/ [2:12..2:18)=> cats/effect/ [2:19..2:21) => cats/effect/IO# [2:19..2:21) => cats/effect/IO. package com.alessandromarrella.presentation import cats.effect.IO object IOExample { val x = IO(println("Hello world")) } <- class <- object
- 21. SemanticDB Occurrences: [4:7..4:16) <= com/alessandromarrella/presen tation/IOExample. [6:8..6:9)<= com/alessandromarrella/presen tation/IOExample.x. [6:12..6:14) => cats/effect/IO. [6:15..6:22) => scala/Predef.println(+1). package com.alessandromarrella.presentation import cats.effect.IO object IOExample { val x = IO(println("Hello world")) } It “knows” Method overload
- 22. SemanticDB ● Compiles theprogram to a SemanticDB file ● Symbol information ● Disambiguation ● Scalafix can use it for semantic rules (more later) ● Metals uses it for code navigation https://scalameta.org/docs/semanticdb/guide.html
- 23. Scalafix ● Refactoring andlinting tool ● Syntactic and semantic rules ● As a user: ○ Apply rules ○ Integrate with your CI ● As a dev: ○ Create & publish rules
- 24. Applying rules Add scalafixdependency project/plugins.sbt addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.1") Run the rule (e.g. http4s 0.18 to 0.20 upgrade) $ sbt ";scalafixEnable; scalafix github:http4s/http4s/v0_20"
- 25. Creating new rules Createa new project with the Giter8 template $ sbt new scalacenter/scalafix.g8 You get a new directory with: build.sbt readme.md input output rules test
- 26. Example: No returnUnit rule def foo(x: Int): Unit What does calling foo(42) do?
- 27. Example: No returnUnit rule def foo(x: Int): Unit What does calling foo(42) do? ● Nothing ● Performs a side effect
- 28. Example: No returnUnit rule def foo(x: Int): Unit What does calling foo(42) do? ● Nothing ● Performs a side effect We won’t consider: ● Throwing exceptions ● Returning null We don’t want the user to be able to return Unit.
- 29. Example: No returnUnit rule Demo https://github.com/amarrella/noreturnunit
- 30.