Spire's macros for zero-cost operator enrichment
Scala

README.md

Machinist

"Generic types and overloaded operators would let a user code up all of these, and in such a way that they would look in all ways just like types that are built in. They would let users grow the Java programming language in a smooth and clean way."

-- Guy Steele, "Growing a Language"

Overview

One of the places where type classes incur some unnecessary overhead is implicit enrichment. Generic types have very few methods that can be called directly, so Scala uses implicit conversions to enrich these types with useful operators and methods.

However, these conversions have a cost: they instantiate an implicit object which is only needed to immediately call another method. This indirection is usually not a big deal, but is prohibitive in the case of simple methods that may be called millions of times.

Machinist's ops macros provide a solution. These macros allow the same enrichment to occur without any allocations or additional indirection. These macros can work with most common type class encodings, and are easily extensible. They can also remap symbolic operators (e.g. **) to text names (e.g. pow).

Machinist started out as part of the Spire project.

For a more detailed description, you can read this article at typelevel.org.

Examples

Here's an example which defines a very minimal typeclass named Eq[A] with a single method called eqv. It is designed to support type-safe equals, similar to scalaz.Equal or spire.algebra.Eq, and it is specialized to avoid boxing primtive values like Int or Double.

import scala.{specialized => sp}

import machinist.DefaultOps

trait Eq[@sp A] {
  def eqv(lhs: A, rhs: A): Boolean
}

object Eq {
  implicit val intEq = new Eq[Int] {
    def eqv(lhs: Int, rhs: Int): Boolean = lhs == rhs
  }

  implicit class EqOps[A](x: A)(implicit ev: Eq[A]) {
    def ===(rhs: A): Boolean = macro DefaultOps.binop[A, Boolean]
  }
}

object Test {
  import Eq.EqOps

  def test(a: Int, b: Int)(implicit ev: Eq[Int]): Int =
    if (a === b) 999 else 0
}

Here are some intermediate representations for how the body of the test method will be compiled:

// our scala code
if (a === b) 999 else 0

// after implicit resolution
if (Eq.EqOps(a)(Eq.intEq).===(b)) 999 else 0

// after macro application
if (Eq.intEq.eqv(a, b)) 999 else 0

// after specialization
if (Eq.intEq.eqv$mcI$sp(a, b)) 999 else 0

There are a few things to notice:

  1. EqOps[A] does not need to be specialized. Since we will have removed any constructor calls by the time the typer phase is over, it will not introduce any boxing or interfere with specialization.

  2. We did not have to write very much boilerplate in EqOps beyond specifying which methods we want to provide implicit operators for. We did have to specify some type information though (in this case, the type of rhs (the "right-hand side" parameter) and the result type.

  3. machinist.DefaultOps automatically knew to connect the === operator with the eqv method, since it has a built-in mapping of symbolic operators to names. You can use your own mapping by extending machinist.Ops and implementing operatorNames.

Including Machinist in your project

Machinist supports Scala 2.10, 2.11, and 2.12. If you have an SBT project, add the following snippet to your build.sbt file:

resolvers += Resolver.sonatypeRepo("releases")

libraryDependencies += "org.typelevel" %% "machinist" % "0.6.1"

Shapes supported by Machinist

Machinist has macros for recognizing and rewriting the following shapes:

// unop
conversion(lhs)(ev).method()
  -> ev.method(lhs)

// unop0
conversion(lhs)(ev).method
  -> ev.method(lhs)

// unopWithEv
conversion(lhs).method(ev)
  -> ev.method(lhs)

// binop
conversion(lhs)(ev).method(rhs)
  -> ev.method(lhs, rhs)

// rbinop, for right-associative methods
conversion(rhs)(ev).method(lhs)
  -> ev.method(lhs, rhs)

// binopWithEv
conversion(lhs).method(rhs)(ev)
  -> ev.method(lhs, rhs)

// rbinopWithEv
conversion(rhs).method(lhs)(ev)
  -> ev.method(lhs, rsh)

Machinist also supports the following oddball cases (which may only be useful for Spire):

// binopWithLift
conversion(lhs)(ev0).method(rhs: Bar)(ev1)
  -> ev.method(lhs, ev1.fromBar(rhs))

// binopWithSelfLift
conversion(lhs)(ev).method(rhs: Bar)
  -> ev.method(lhs, ev.fromBar(rhs))

In both cases, if "method" is a symbolic operator, it may be rewritten to a new name if a match is found in operatorNames.

Details & Fiddliness

To see the names Machinist provides for symbolic operators, see the DefaultOperatorNames trait.

One caveat is that if you want to extend machinist.Ops yourself to create your own name mapping, you must do so in a separate project or sub-project from the one where you will be using the macros. Scala macros must be defined in a separate compilation run from where they are applied.

It's also possible that despite the wide variety of shapes provided by machinist.Ops your shape is not supported. Machinist only provides unary and binary operators, meaning that if your method takes 3+ parameters you will need to write your own macro.

It should be relatively easy to extend Ops to support these cases, but that work hasn't been done yet. Pull requests will be gladly accepted.

Copyright and License

All code is available to you under the MIT license, available at http://opensource.org/licenses/mit-license.php as well as in the COPYING file.

Copyright Erik Osheim and Tom Switzer 2011-2015.