Migration guide: shapeless 2.0.0 to 2.1.0

shapeless-2.1.0 is largely backwards source compatible with shapeless-2.0.0, however you might need to make some minor changes as documented below. You might also find the complete set of differences in shapeless's examples and tests between 2.0.0 and 2.1.0 a useful guide to what's involved in updating. They can be found in this commit.

Contents

• [TypeClass changes]

TypeClass changes

The type class deriving infrastructure included in shapeless-2.0.0 has been significantly reworked in shapeless-2.1.0. It is now implemented entierly in terms of the Generic and Lazy type class primitives. Some applications which required more flexibility that TypeClass could provide can now be implemented directly in terms of those primitives and general Scala implicit-based type level computation as seen throughout shapeless.

Applications already using the shapeless-2.0.0 TypeClass infrastructure will need the following minor modifications,

• Removal of auto._ imports.

Previously type class instances were only derived automatically in scopes where a wildcard import from the auto member of the type classes companion object. This mechanism didn't transfer over well to the new implementation and in any case experience showed it to be overly verbose while adding little value.

• Implicit TypeClass member replace by typeClass object in type class companion.

Where previously the type class derivation rules where introduced via an implicit definition of the form,

object Show {
  implicit val showInstance: ProductTypeClass[Show] = new LabelledTypeClass[Show] {
    def emptyProduct = ...
    def product ...

    def emptyCoproduct = ...
    def coproduct ...

    def project ...
  }
}

they are now introduced via a nested object named typeClass,

object Show {
  object typeClass extends LabelledTypeClass[Show] {
    def emptyProduct = ...
    def product ...

    def emptyCoproduct = ...
    def coproduct ...

    def project ...
  }
}