Coding in Shapeless

O mnie

  • Scala dev @ VirtusLab
  • Docker fan

Agenda

  • O Shapeless
  • Wykorzystanie Shapeless
  • Lensy
  • HList
  • HMapy
  • Polimorficzne funkcje
  • Generic[T]
  • Testy type level
  • Typ OR
  • Podsumowanie

Czym jest Shapeless?

Wykorzystanie Shapeless

  • Breeze
  • Scodec
  • Specs
  • Anorm
  • Spray

Lensy

HList

HMap

Polimorficzne funkcje

Motywacja


// Foo.scala
class Foo {
  def foo[T](a: T): Set[T] = Set(a)

  val list = List(1,2,3)

  list.map(foo)
}

Kompilator

scalac -Xshow-phases

    phase name  id  description
    ----------  --  -----------
        parser   1  parse source into ASTs, perform simple desugaring
         namer   2  resolve names, attach symbols to named trees
packageobjects   3  load package objects
         typer   4  the meat and potatoes: type the trees
        patmat   5  translate match expressions
superaccessors   6  add super accessors in traits and nested classes
    extmethods   7  add extension methods for inline classes
       pickler   8  serialize symbol tables
     refchecks   9  reference/override checking, translate nested objects
       uncurry  10  uncurry, translate function values to anonymous classes
     tailcalls  11  replace tail calls by jumps
    specialize  12  @specialized-driven class and method specialization
 explicitouter  13  this refs to outer pointers
       erasure  14  erase types, add interfaces for traits
   posterasure  15  clean up erased inline classes
      lazyvals  16  allocate bitmaps, translate lazy vals into lazified defs
    lambdalift  17  move nested functions to top level
  constructors  18  move field definitions into constructors
       flatten  19  eliminate inner classes
         mixin  20  mixin composition
       cleanup  21  platform-specific cleanups, generate reflective calls
    delambdafy  22  remove lambdas
         icode  23  generate portable intermediate code
           jvm  24  generate JVM bytecode
      terminal  25  the last phase during a compilation run

Bytecode

javap -c Foo.scala

39: new           #62                 // class Foo$$anonfun$1
42: dup
43: aload_0
44: invokespecial #65                 // Method Foo$$anonfun$1."":(LFoo;)V
47: getstatic     #51                 // Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$;
50: invokevirtual #69                 // Method scala/collection/immutable/List$.canBuildFrom:()Lscala/collection/generic/CanBuildFrom;
53: invokevirtual #75                 // Method scala/collection/immutable/List.map:(Lscala/Function1;Lscala/collection/generic/CanBuildFrom;)Ljava/lang/Object;
56: pop
57: return

typer

scalac -Xprint:typer Foo.scala

[[syntax trees at end of typer]] // Foo.scala
package  {
  class Foo extends scala.AnyRef {
    def (): Foo = {
      Foo.super.();
      ()
    };
    def foo[T](a: T): Set[T] = scala.this.Predef.Set.apply[T](a);
    private[this] val list: List[Int] = immutable.this.List.apply[Int](1, 2, 3);
      def list: List[Int] = Foo.this.list;
    Foo.this.list.map[Set[Int], List[Set[Int]]]({
      ((a: Int) => Foo.this.foo[Int](a))
    })(immutable.this.List.canBuildFrom[Set[Int]])
  }
}

Generic[T]


case class User(name: String, surname: String, email: String)

val genericUser = Generic[User]
val userInstance = User("Jan", "Kowalski", "jan.kowalski@foo.pl")

val userRepr = genericUser.to(userInstance)
val anotherUser = genericUser.from("Stefan" :: "Stefanowski" :: "stefcik@buziaczek.pl" :: HNil)

typed[String :: String :: String :: HNil](userRepr)
anotherUser shouldBe User("Stefan", "Stefanowski", "stefcik@buziaczek.pl")

Testy type level


import shapeless._
import test._

case class User(name: String, surname: String, email: String)

typed[Int](1)

val user = User("Jan", "Kowalski", "jan.kowalski@foo.pl")
typed[User](user)

// typed[User](1) <- won't compile


import shapeless._
import test._

case class User(name: String, surname: String, email: String)

val user = User("Jan", "Kowalski", "jan.kowalski@foo.pl")

illTyped("""user: Int""") // <- trap!

typed[User](user)

// typed[User](1) <- won't compile

Typ OR