Meta-program and/or shapeless all the things!

Chris Birchall

London Scala User Group

6th Dec 2016

Programming

Program

Data

Data

Side effects

Metaprogramming

Program

Program

Side effects

Program

Scala macro

• Function that runs at compile time

• Input and output are trees (ASTs)

foo.bar()

Apply

Select

Ident

TermName("foo")

TermName("bar")

List()

=

Hello World

Let's write a macro that doesn't do anything!

Level up

Pattern matching and code generation using quasiquotes

Typechecked Mustache

At compile time:

1. Load a Mustache template
2. Parse it to find all the {{placeholders}}
3. Generate a corresponding case class

Hello {{firstName}}!
The weather is {{weather}} today.

case class Context(firstName: String, weather: String)

scala.meta

Syntactic API

• Tokens, trees
• Quasiquotes
• Available now

Semantic API

• Symbols, name resolution,  typechecking?, ...
• Work in progress
  • See Eugene Burmako's scalax talk

meta paradise

inline/meta

• inline keyword replaces @inline annotation
• meta demarcates code that executes at compile time
• See SIP-28 & 29 for the gory details

meta paradise

Let's port the typechecked Mustache example

IntelliJ support

tutorial

http://scalameta.org/tutorial/

Spectrum of Scala developer happiness

joy

misery

Spectrum of Scala developer happiness

joy

misery

write some boilerplate code

Spectrum of Scala developer happiness

joy

misery

write some boilerplate code

use runtime reflection

Spectrum of Scala developer happiness

joy

misery

write some boilerplate code

use runtime reflection

generate source code

Spectrum of Scala developer happiness

joy

misery

write some boilerplate code

use runtime reflection

generate source code

write a macro

Spectrum of Scala developer happiness

joy

misery

write some boilerplate code

use runtime reflection

generate source code

write a macro

let Miles write the macro for you

shapeless

case class Person(name: String, age: Int)

String :: Int :: HNil

("name", String) :: ("age", Int) :: HNil

Generic

LabelledGeneric

Convert case class to Map

goal

case class Foo(wow: String, yeah: Int)

val instance = Foo("hello", 123)

val map: Map[String, Any] = caseClassToMap(instance)

println(map) // Map("wow" -> "hello", "yeah" -> 123)

Convert case class to Map

with a macro

import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context

def caseClassToMap[A](a: A): Map[String, Any] =
  macro impl[A]

def impl[A: c.WeakTypeTag](c: Context)(a: c.Tree): c.Tree = {
  import c.universe._
  val A = weakTypeOf[A]
  if (!(A.typeSymbol.isClass && A.typeSymbol.asClass.isCaseClass))
    c.abort(c.enclosingPosition, "Sorry, case classes only")
  val primaryCtor = A.typeSymbol.asClass.primaryConstructor.asMethod
  val params = primaryCtor.paramLists.flatten
  val kvPairs = params.map { k =>
    q"${k.name.toString} -> $a.${k.name.toTermName}"
  }
  q"""_root_.scala.collection.immutable.Map($kvPairs: _*)"""
}

Convert case class to Map

with shapeless

import shapeless._
import shapeless.ops.hlist.ToList
import shapeless.ops.record.Fields

def caseClassToMap[A, L <: HList, F <: HList](a: A)
  (implicit 
    generic: LabelledGeneric.Aux[A, L],
    fields: Fields.Aux[L, F],
    toList: ToList[F, (Symbol, Any)]
  ): Map[String, Any] = {
  val labelledGen = generic.to(a)
  val fieldsHlist = fields(labelledGen)
  toList(fieldsHlist)
    .map { case (symbol, value) => (symbol.name, value) }
    .toMap
}

Convert one case class to another

goal

case class Input(foo: Int, baz: String)
case class Output(foo: Int, bar: Double, baz: String)

val input = Input(123, "wow")

val output = transform[Output](input, "bar" -> 4.56)

println(output) // Output(123, 4.56, "wow")

Convert one case class to another

with a macro

def caseClassToCaseClass[A, B](a: A, extraParams: (String, Any)*): B =
  macro Bundle.impl[A, B]

class Bundle(val c: Context) {
  import c.universe._

  private case class TreeWithActualType(tree: Tree, actualType: Type)

  private def fail(msg: String) =
    c.abort(c.enclosingPosition, msg)

  def impl[A: c.WeakTypeTag, B: c.WeakTypeTag](a: c.Tree, extraParams: c.Expr[Tuple2[String, Any]]*): c.Tree = {
    val A = weakTypeOf[A]
    val B = weakTypeOf[B]

    if (!(A.typeSymbol.isClass && A.typeSymbol.asClass.isCaseClass))
      fail("Sorry, case classes only")
    if (!(B.typeSymbol.isClass && B.typeSymbol.asClass.isCaseClass))
      fail("Sorry, case classes only")

    val inputPrimaryCtor = A.typeSymbol.asClass.primaryConstructor.asMethod
    val inputParams = inputPrimaryCtor.paramLists.flatten
    val inputParamsMap: Map[TermName, TreeWithActualType] = inputParams.map { k =>
      val termName = k.name.toTermName
      termName -> TreeWithActualType(q"$a.$termName", k.info)
    }.toMap

    val extraParamsMap: Map[TermName, TreeWithActualType] = extraParams.map { expr =>
      val (key, value) = expr.tree match {
        case q"scala.Predef.ArrowAssoc[$_]($k).->[$_]($v)" => (k, v)
        case q"($k, $v)" => (k, v)
        case other => fail("You must pass extra params as either key -> value or (key, value)")
      }
      val q"${keyAsString: String}" = key
      val keyName = TermName(keyAsString)
      val actualValueType = expr.actualType.typeArgs(1)
      keyName -> TreeWithActualType(value.asInstanceOf[Tree], actualValueType)
    }.toMap

    val allParams: Map[TermName, TreeWithActualType] = inputParamsMap ++ extraParamsMap

    val outputPrimaryCtor = B.typeSymbol.asClass.primaryConstructor.asMethod
    val paramLists: List[List[Tree]] = 
      for (ps <- outputPrimaryCtor.paramLists) yield {
        for (p <- ps) yield {
          val termName = p.name.toTermName
          allParams.get(termName) match {
            case Some(t) if t.actualType weak_<:< p.typeSignature => t.tree
            case Some(t) => fail(s"Parameter ${termName.toString} has wrong type. Expected ${p.typeSignature} but got ${t.actualType}")
            case None => fail(s"Missing parameter of type ${termName.toString}")
          }
        }
      }

    q"new ${B.typeSymbol}(...$paramLists)"
  }
}

Convert one case class to another

with shapeless

import shapeless._
import shapeless.ops.hlist._

trait Transform[A, B, E <: HList] {
  def apply(a: A, extraFields: E): B
}

object Transform {
  implicit def genericTransform[
    A,
    B,
    ARepr         <: HList,
    BRepr         <: HList,
    CommonFields  <: HList,
    ExtraFields   <: HList,
    Unaligned     <: HList
  ](implicit
    aGen    : LabelledGeneric.Aux[A, ARepr],
    bGen    : LabelledGeneric.Aux[B, BRepr],
    inter   : Intersection.Aux[ARepr, BRepr, CommonFields],
    diff    : Diff.Aux[BRepr, CommonFields, ExtraFields],
    prepend : Prepend.Aux[ExtraFields, CommonFields, Unaligned],
    align   : Align[Unaligned, BRepr]
  ): Transform[A, B, ExtraFields] =
    new Transform[A, B, ExtraFields] {
      def apply(a: A, extra: ExtraFields): B = {
        val aRepr     = aGen.to(a)
        val common    = inter(aRepr)
        val unaligned = prepend(extra, common)
        val bRepr     = align(unaligned)
        bGen.from(bRepr)
      }
    }
}

learning shapeless

https://github.com/underscoreio/shapeless-guide

Also see Dave's talk at LSUG last month

Summary

• Macros are fun and useful
• Do the scala.meta tutorial
• Read the shapeless book

Code: https://github.com/cb372/meta-prog-shapeless-talk