Skip to main content

Non-Orphan Typeclass Instances

As of smithy4s version 0.18.x you are able to create custom typeclass instances inside the companion objects of classes in the code generated by smithy4s. This allows you to have instances that are found via implicit resolution without any need to special imports. Common examples where this will come in handy are for the cats.Show and cats.Eq typeclasses, but you can use this feature for any typeclass.

Here we will show an example using the cats.Hash typeclass.

Setup Typeclass in Smithy

Here we will use the smithy4s.meta#typeclass trait to define a hash trait that represents the cats.Hash typeclass.

use smithy4s.meta#typeclass

@typeclass(targetType: "cats.Hash", interpreter: "smithy4s.interopcats.SchemaVisitorHash")
structure hash {}

We are specifying cats.hash as the targetType since that is the typeclass which this trait represents. We are then specifying smithy4s.interopcats.SchemaVisitorHash as the classpath which points to a CachedSchemaCompiler for the cats.Hash typeclass.

Implement CachedSchemaCompiler

Smithy4s has a concept called CachedSchemaCompiler which is an abstraction which we use here to interpret a smithy4s.Schema to produce an instance of a typeclass. Here is what this will look like:

object SchemaVisitorHash extends CachedSchemaCompiler.Impl[Hash] {
protected type Aux[A] = Hash[A]
def fromSchema[A](
schema: Schema[A],
cache: Cache
): Hash[A] = {
schema.compile(new SchemaVisitorHash(cache))

Here we are delegating to the SchemaVisitorHash which is doing the heavy lifting of interpreting the smithy4s.Schema. The CachedSchemaCompiler.Impl provides a Cache which we utilize to make sure we are not recompiling the same schema more than once. For more details on implementing a SchemaVisitor, you can check out the full cats.Hash schema compiler and visitor here.

Use the hash typeclass trait

Now we are ready to use the hash trait we defined above.

structure MovieTheater {
name: String

This tells smithy4s to generate an instance of the Hash typeclass in the companion object of the MovieTheater type and to use the CachedSchemaCompiler defined above for the implementation. The generated code will look like:

case class MovieTheater(name: Option[String] = None)
object MovieTheater extends ShapeTag.Companion[MovieTheater] {
val id: ShapeId = ShapeId("smithy4s.example", "MovieTheater")

// ...

implicit val schema: Schema[MovieTheater] = // ...

implicit val movieTheaterHash: cats.Hash[MovieTheater] = SchemaVisitorHash.fromSchema(schema)