travisbrown / abstracted

Forget your methods

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Abstracted

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This is a small proof of concept that demonstrates how to implement a Scala macro that allows us to "forget" all of a value's methods and only use enrichment methods (which will usually be provided via a type class).

The idea behind abstracted was originally (as far as I know) suggested by Michael Pilquist in the cats room on Gitter, although the approach he suggests is different from the one I've used here.

Simple example

As an example, suppose we've got a Box type:

case class Box[A](val a: A) {
  def map[B](f: A => B): Box[B] = {
    println("Box's map")
    Box(f(a))
  }

  def flatMap[B](f: A => Box[B]): Box[B] = {
    println("Box's flatMap")
    f(a)
  }
}

And a monad instance for it:

import cats.Monad

implicit val boxMonad: Monad[Box] = new Monad[Box] {
  override def map[A, B](fa: Box[A])(f: A => B): Box[B] = {
    println("Box's functor's map")
    Box(f(fa.a))
  }

  def flatMap[A, B](fa: Box[A])(f: A => Box[B]): Box[B] = {
    println("Box's monad's flatMap")
    f(fa.a)
  }

  def pure[A](a: A): Box[A] = Box(a)
}

Now if we use Box in a for-comprehension, for example, the monad instance won't get used:

scala> import io.travisbrown.abstracted.demo._
import io.travisbrown.abstracted.demo._

scala> import cats.syntax.all._
import cats.syntax.all._

scala> for { foo <- Box("foo"); howMany <- Box(3) } yield foo * howMany
Box's flatMap
Box's map
res0: io.travisbrown.abstracted.demo.Box[String] = Box(foofoofoo)

Our abstracted macro allows us to change this:

scala> import io.travisbrown.abstracted._
import io.travisbrown.abstracted._

scala> for { foo <- Box("foo").abstracted; howMany <- Box(3) } yield foo * howMany
Box's monad's flatMap
Box's map
res1: io.travisbrown.abstracted.demo.Box[String] = Box(foofoofoo)

Finagle services

I decided to take a stab at implementing abstracted tonight because of a conversation about how Finagle services compose this afternoon. Finagle services are morally more or less Kleisli arrows over Twitter futures, but for whatever reason Service extends I => Future[O], which means that they have totally useless compose and andThen methods. In another project I provide category and profunctor instances for Service, but the compose and andThen enrichment methods provided by cats for things with Compose instances are blocked by the stupid methods that Service inherits from Function1.

For example, if we've got these services:

import cats.syntax.compose._
import com.twitter.util.Future
import com.twitter.finagle.Service
import io.catbird.finagle._

val is = Service.mk[Int, String](i => Future.value(i.toString))
val si = Service.mk[String, Int](s => Future(s.toInt))

We get an error when we try to compose them:

scala> val ss = si andThen is
<console>:22: error: type mismatch;
 found   : com.twitter.finagle.Service[Int,String]
 required: com.twitter.util.Future[Int] => ?
              si andThen is
                         ^

Our abstracted macro fixes this problem:

scala> import io.travisbrown.abstracted._
import io.travisbrown.abstracted._

scala> val ss = si.abstracted andThen is
ss: com.twitter.finagle.Service[String,String] = <function1>

How it works

The implementation is pretty straightforward. First we've got an implicit class that provides a def abstracted: Empty[A] method for any A, where our Empty type is a case class that wraps an A and provides access to the wrapped value, but doesn't have any other methods.

We also have a Converter[A, B] type that represents a conversion from Empty[A] to B (I ran into problems trying to use Empty[A] => B directly), and an implicit method that will apply the conversion automatically to any Empty[A] for any appropriately-typed Converter instance.

The interesting part is how we make Converter instances. We use the Scala macro system's fundep materialization, which allows us to determine in the body of the macro what the output type of the Converter will be. We look at the open implicits and find one that looks like the compiler is fishing for a WhateverOps enrichment class for our Empty[A]. We then ask for a view from A (our real type) to the target of that view. We read the return type off the view from A, and from there the implementation is pretty trivial.

Status

It seems like it works. The examples above can be run by opening up a REPL with sbt demo/console. If other people think it looks useful I guess it could end up in cats, although there's nothing cats-specific about the macro itself or the surrounding machinery.

License

abstracted is licensed under the Apache License, Version 2.0 (the "License"); you may not use this software except in compliance with the License.

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.