Freast 
WIP
Freast is a Scalameta macro-based lib for easily building Free-monad based AST. It is a port of Freasy-monad to the newer Scalameta-based toolkit.
Learning Scalameta was a big motivation, and doing this port was similar to taking apart a wall clock and putting it into another sleeker case.
Goals
- Integrate well with IntelliJ's Scalameta support (auto-expansion and auto-complete)
- Make it easy to use Free Monads
- Compatible with Cats and Scalaz
SBT
In build.sbt
libraryDependencies += "com.beachape" %% "freast-cats" % "0.1.0-SNAPSHOT" // or freast-scalaz if you swing that way
// Additional ceremony for using Scalameta macro annotations
resolvers += Resolver.url(
"scalameta",
url("http://dl.bintray.com/scalameta/maven"))(Resolver.ivyStylePatterns)
// A dependency on macro paradise is required to both write and expand
// new-style macros. This is similar to how it works for old-style macro
// annotations and a dependency on macro paradise 2.x.
addCompilerPlugin(
"org.scalameta" % "paradise" % "4.0.0.142" cross CrossVersion.full)
scalacOptions += "-Xplugin-require:macroparadise"Example
import cats.Id
import cats.free.Free
@free
trait KVStore {
type KVStoreF[A] = Free[GrammarADT, A]
sealed trait GrammarADT[A]
def put[T](key: String, value: T): KVStoreF[Unit]
def get[T](key: String): KVStoreF[Option[T]]
def delete(key: String): KVStoreF[Unit]
def update[T](key: String, f: T => T): KVStoreF[Unit] =
for {
vMaybe <- get[T](key)
_ <- vMaybe.map(v => put[T](key, f(v))).getOrElse(Free.pure(()))
} yield ()
}
// Then
val impureInterpreter = new KVStore.Interp[Id] {
private var kvs = immutable.Map.empty[String, Any]
def get[T](key: String): Id[Option[T]] = {
kvs.get(key).map(_.asInstanceOf[T])
}
def put[T](key: String, value: T): Id[Unit] = {
kvs += key -> value
}
def delete(key: String): Id[Unit] = {
kvs -= key
}
}
import KVStore.ops._
it("should work") {
val program: KVStoreF[Option[Int]] =
for {
_ <- put("wild-cats", 2)
_ <- update[Int]("wild-cats", _ + 12)
_ <- put("tame-cats", 5)
n <- get[Int]("wild-cats")
} yield n
val r = impureInterpreter.run(program)
r shouldBe Some(14)
}The @free macro expands to this, which includes Inject for composing different ADT grammars.:
object KVStore {
import cats._
import cats.free._
import scala.language.higherKinds
sealed trait GrammarADT[A]
object GrammarADT {
case class Put[T](key: String, value: T) extends GrammarADT[Unit]
case class Get[T](key: String) extends GrammarADT[Option[T]]
case class Delete(key: String) extends GrammarADT[Unit]
}
object ops {
type KVStoreF[A] = Free[GrammarADT, A]
def put[T](key: String, value: T): KVStoreF[Unit] = injectOps.put[GrammarADT, T](key, value)
def get[T](key: String): KVStoreF[Option[T]] = injectOps.get[GrammarADT, T](key)
def delete(key: String): KVStoreF[Unit] = injectOps.delete[GrammarADT](key)
def update[T](key: String, f: T => T): KVStoreF[Unit] = injectOps.update[GrammarADT, T](key, f)
}
object injectOps {
def put[F[_], T](key: String, value: T)(implicit I: Inject[GrammarADT, F]): Free[F, Unit] = Free.liftF(I.inj(GrammarADT.Put(key, value)))
def get[F[_], T](key: String)(implicit I: Inject[GrammarADT, F]): Free[F, Option[T]] = Free.liftF(I.inj(GrammarADT.Get(key)))
def delete[F[_]](key: String)(implicit I: Inject[GrammarADT, F]): Free[F, Unit] = Free.liftF(I.inj(GrammarADT.Delete(key)))
def update[F[_], T](key: String, f: T => T)(implicit I: Inject[GrammarADT, F]): Free[F, Unit] = get[F, T](key).flatMap(vMaybe => vMaybe.map(v => put[F, T](key, f(v))).getOrElse(Free.pure(())).map(_ => ()))
}
class Injects[F[_]](implicit I: Inject[GrammarADT, F]) {
def put[T](key: String, value: T): Free[F, Unit] = injectOps.put[F, T](key, value)(I)
def get[T](key: String): Free[F, Option[T]] = injectOps.get[F, T](key)(I)
def delete(key: String): Free[F, Unit] = injectOps.delete[F](key)(I)
def update[T](key: String, f: T => T): Free[F, Unit] = injectOps.update[F, T](key, f)(I)
}
object Injects { implicit def injectOps[F[_]](implicit I: Inject[GrammarADT, F]): Injects[F] = new Injects[F] }
trait Interp[M[_]] {
import ops._
val interpreter = new ~>[GrammarADT, M] {
def apply[A](fa: GrammarADT[A]): M[A] = fa match {
case GrammarADT.Put(key, value) => put(key, value)
case GrammarADT.Get(key) => get(key)
case GrammarADT.Delete(key) => delete(key)
}
}
def run[A](op: KVStoreF[A])(implicit m: Monad[M]): M[A] = op.foldMap(interpreter)
def put[T](key: String, value: T): M[Unit]
def get[T](key: String): M[Option[T]]
def delete(key: String): M[Unit]
}
}