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implicit-dependent-type

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implicit-dependent-type is a Scala compiler plugin that resolves dependent types from implicit type classes, especially useful when working with shapeless or other type-level programming libraries.

Setup

addCompilerPlugin("com.thoughtworks.implicit-dependent-type" %% "implicit-dependent-type" % "latest.release")

addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full)

libraryDependencies += "com.chuusai" %% "shapeless" % "latest.release"

Foo[Bar]##Baz syntax

This plugin provides a syntactic sugar that substitutes all Foo[Bar]##Baz with shapeless.the.`Foo[Bar]`.Baz, which inlines resolved implicit type classes into type declaration positions.

import shapeless._

final case class Foo(bar: Int, baz: String)

val hlistForFoo: Generic[Foo]##Repr = 1 :: "xxx" :: HNil
val foo: Foo = Generic[Foo].from(hlistForFoo)

The above example equals to the following code:

import shapeless._

final case class Foo(bar: Int, baz: String)

val g = Generic[Foo]
val hlistForFoo: g.Repr = 1 :: "xxx" :: HNil
val foo: Foo = Generic[Foo].from(hlistForFoo)

As you see, without this plugin, Generic[Foo] is not a stable value, thus it is unable to be placed at a type position. You will have to assign it to a temporary variable g.

This plugin resolves this problem.

Foo @Bar syntax (since 2.0)

Another syntactic sugar provided by this plugin is Foo @Bar, which will be converted to shapeless.the.`Bar[Foo]`.`@`.

For example:

trait GetElement[A] {
  type `@`
}

implicit def getArrayElement[Element] = new GetElement[Array[Element]] {
  override type `@` = Element
}

implicit def getSeqElement[Element] = new GetElement[Seq[Element]] {
  override type `@` = Element
}

val i: Array[Int] @GetElement = 1 // OK
val s: Seq[String] @GetElement = "text" // OK

val d: Option[Double] @GetElement = 0.5 // Does not compile because no implicit GetElement[Option[Double]] found

In the above example, @GetElement acts as a type level function, calculating the element type of the given type.

Note that the Foo @Bar syntax only applied if Bar starts with an upper case character. Thus, this plugin does not affect built-in annotations like @specialize, @cps or @suspendable because they start with a lower case character.