Scala

• Developed at EPFL
• Scala 3
  • Both Scala-3 and Scala-2 have stuff which are not in each other
  • DOT (Dependent Object Types), a core calculus of depenedent path types, is the core of Scala 3
  • dotty: name of scala 3 language while it was being developed ʳ
• Targets JVM
• Can use java libraries
• sbt: build tool
• Tasty file: Typed Abstract Syntax Tree
  • https://docs.scala-lang.org/scala3/guides/tasty-overview.html
  • scalac -decompile foo.tasty
• Can write eDSLs in scala
  • https://typelevel.org/blog/2016/10/26/edsls-part-2.html
• Tuple2 is a class, Tuple is a trait
  • https://www.scala-lang.org/api/current/scala/Tuple2.html
  • https://www.scala-lang.org/api/current/scala/Tuple.html

General

• Comments: Like in Java
  • Single line: //
  • Multi-line: /* ... */

Type annotations

Apparently, explicit type annotations are discouraged??

• https://docs.scala-lang.org/style/types.html

Conventions

• Names use camelCase
• Acronyms considered normal words (eg: html instead of HTML)

https://docs.scala-lang.org/style/naming-conventions.html

Hello world

• 'main' function is marked as entry point with @main annotation
• String* indicates variable number of String arguments

// Filename: hi.scala
@main def hi(args: String*): Unit =
  println("Hello world!")

Compilation:

$ scalac --version
Scala compiler version 3.3.1 -- Copyright 2002-2023, LAMP/EPFL

# Generate class files
$ scalac hi.scala

$ ls
'hi$package$.class'  'hi$package.class'  'hi$package.tasty'   hi.class   hi.scala   hi.tasty

# Run
$ scala hi
Hello world!

See:

• https://docs.scala-lang.org/scala3/book/taste-hello-world.html
• https://docs.scala-lang.org/scala3/book/methods-main-methods.html
• https://docs.scala-lang.org/tutorials/scala-for-java-programmers.html#running-hello-world

case class

• Useful for pattern matching
• When two case class values are compared, their structure is compared (not whether they are the same object, unlike I guess normal classes).

https://docs.scala-lang.org/tour/case-classes.html

Path-dependent type

Example from here:

case class Board(width: Int, height: Int) {
  case class Coord(x: Int, y: Int) {
    require(0 <= x && x <= width &&
            0 <= y && y <= height)
  }
  val occupied = scala.collection.mutable.Set[Coord]()
}

@main def eg(args: String*): Unit =
  val b = Board(10, 20)
  val c1 = b.Coord(3, 5)
  b.occupied += c1  // Add new coordinate to set of occupied
  // No error

  val c2 = b.Coord(11, 20)
  // Run-time error! Not compile-time... ??

Running this would give error:

$ scalac pathdeptype.scala; scala eg
java.lang.IllegalArgumentException: requirement failed
        at scala.Predef$.require(Predef.scala:324)
        at Board$Coord.<init>(pathdeptype.scala:5)
        at Board$Coord$.apply(pathdeptype.scala:3)
        at pathdeptype$package$.eg(pathdeptype.scala:17)
        at eg.main(pathdeptype.scala:11)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
        at java.base/java.lang.reflect.Method.invoke(Method.java:580)
        at dotty.tools.runner.RichClassLoader$.run$extension$$anonfun$1(ScalaClassLoader.scala:36)
        at dotty.tools.runner.ScalaClassLoader$.asContext(ScalaClassLoader.scala:80)
        at dotty.tools.runner.RichClassLoader$.dotty$tools$runner$RichClassLoader$$$asContext$extension(ScalaClassLoader.scala:18)
        at dotty.tools.runner.RichClassLoader$.run$extension(ScalaClassLoader.scala:36)
        at dotty.tools.runner.CommonRunner.run(ObjectRunner.scala:23)
        at dotty.tools.runner.CommonRunner.run$(ObjectRunner.scala:13)
        at dotty.tools.runner.ObjectRunner$.run(ObjectRunner.scala:48)
        at dotty.tools.runner.CommonRunner.runAndCatch(ObjectRunner.scala:30)
        at dotty.tools.runner.CommonRunner.runAndCatch$(ObjectRunner.scala:13)
        at dotty.tools.runner.ObjectRunner$.runAndCatch(ObjectRunner.scala:48)
        at dotty.tools.MainGenericRunner$.run$1(MainGenericRunner.scala:215)
        at dotty.tools.MainGenericRunner$.process(MainGenericRunner.scala:270)
        at dotty.tools.MainGenericRunner$.main(MainGenericRunner.scala:281)
        at dotty.tools.MainGenericRunner.main(MainGenericRunner.scala)

Type of Coord depends on the Board instance which instantiated the Coord.
ie, Coord type depends on Board value.

The name is 'path-dependent type' because type depends 'selector path' through objects. ʳ

See:

• https://docs.scala-lang.org/scala3/book/types-dependent-function.html
• https://docs.scala-lang.org/scala3/reference/new-types/dependent-function-types.html
• https://stackoverflow.com/questions/12935731/any-reason-why-scala-does-not-explicitly-support-dependent-types
• https://dev.to/markehammons/a-quick-explanation-of-path-dependent-types-4pki
• https://www.baeldung.com/scala/path-dependent-types

match .. case

Example from here:

val suffix = (age % 100) match
  case 11 | 12 | 13 => "th"
  case _ => (age % 10) match
    case 1 => "st"
    case 2 => "nd"
    case 3 => "rd"
    case _ => "th"

Predefs: require, assert, etc

Useful to represent invariants that is difficult to represent using types.

• require: use to check if caller has adhered to require conditions
  • If a require fails, pre-condition necessary to use a definition was not satisfied
  • Think of it as a 400 http bad request response
• assert: can be used to assert something a function should be doing.
  • If an assertion fails, function definition itself is wrong
  • Think of it as a 500 http response

More: ensuring, assume, implicitly

See:

• https://www.scala-lang.org/api/current/scala/Predef$.html
• https://stackoverflow.com/questions/26140757/what-to-choose-between-require-and-assert-in-scala
• https://users.scala-lang.org/t/require-assume-and-ensuring/5552

More

• <static>:
  • For auto-generated code ??
  • Not available for user
  • Modifier indicating a class method is static
• final
• Command line argument parsing: scala.util.CommandLineParser ??

Emacs

• https://ag91.github.io/blog/2020/10/16/my-emacs-setup-for-scala-development/
• https://scalameta.org/metals/docs/editors/emacs/