lloydmeta / kontext

Like `implicitly` but for _all_ context-bound typeclass instances, automatically.

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implicitly but for all context-bound typeclass instances, automatically.

Useful when you have methods that are bound by lots of typeclasses and you don't want to maintain a long list of implicit parameters. Possibly useful when you want to do Tagless Final style EDSLs.

Written in scala.meta for future compatibility and other nice things (e.g. free IDE support, like in IntelliJ).

Why

Turns this

def cachedFib[F[_]](i: Int)(implicit Monad: Monad[F], KVStore: KVStore[F], Maths: Maths[F], Logger: Logger[F]): F[Int] = {
  for {
    cached <- KVStore.get[Int]("key")
    _      <- Logger.info("on noes!")
  // etc
}

into

@boundly
def cachedFib[F[_]: Monad: KVStore: Maths: Logger](i: Int): F[Int] = {
  for {
    cached <- KVStore.get[Int]("key")
    _      <- Logger.info("on noes!")
  // etc
}

Not having to explicitly spell out names for your typeclass instances means less unneeded thinking (your typeclasses already have good enough names as is, right?) and no repetition of [F] means your code is more to the point. As a bonus, the typeclass constraints are still located in the front of your method signature, which acts as a visual cue as to what is needed to call the method (much like in Haskell).

Examples

import kontext._

trait Maths[G[_]] {
  def int(i: Int): G[Int]
  def add(l: G[Int], r: G[Int]): G[Int]
}

// Simple usage
@boundly
def add[F[_]: Maths](x: Int, y: Int) = Maths.add(Maths.int(x), Maths.int(y))

// Example of renaming
@boundly('Maths -> 'M)
def addAliased[F[_]: Maths](x: Int, y: Int) = M.add(M.int(x), M.int(y))

// Write an interpreter
type Id[A] = A
implicit val interpreter = new Maths[Id] {
  def int(i: Int)                 = i
  def add(l: Id[Int], r: Id[Int]) = l + r
}

// Use
add[Id](3, 10)
// res0: Int = 13

addAliased[Id](3, 10)
// res1: Int = 13

For a more realistic usage scenario, check out examples/FibApp, where 3 Tagless Final DSLs are mixed together.

Sbt

Maven Central

libraryDependencies += "com.beachape" %% "kontext" % s"$latest_version"


// 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" % "3.0.0-M8" cross CrossVersion.full)

scalacOptions += "-Xplugin-require:macroparadise"

How it works

The @boundly annotation simply injects values named after your typeclasses (and pointing to your typeclass instances) into the top of the method body.

@boundly
def add[F[_]: Maths](x: Int, y: Int) = Maths.add(Maths.int(x), Maths.int(y))

// turns into 
def add[F[_]: Maths](x: Int, y: Int) = {
  val Maths = implicitly[Maths[F]]
  Maths.add(Maths.int(x), Maths.int(y))
}