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Fix example code from How to write a type class derived method using macros #10382

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Nov 19, 2020
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Fix example code from How to write a type class derived method using macros #10382

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Update derivation-macro.md
ghostdogpr Nov 19, 2020
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nicolasstucki Nov 19, 2020
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nicolasstucki Nov 19, 2020
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44 changes: 22 additions & 22 deletions docs/docs/reference/contextual/derivation-macro.md
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Original file line number Diff line number Diff line change
Expand Up @@ -44,11 +44,11 @@ from the signature. The body of the `derived` method is shown below:
given derived[T: Type](using qctx: QuoteContext) as Expr[Eq[T]] = {
import qctx.reflect._

val ev: Expr[Mirror.Of[T]] = Expr.summon(using '[Mirror.Of[T]]).get
val ev: Expr[Mirror.Of[T]] = Expr.summon(using Type.of[Mirror.Of[T]]).get

ev match {
case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = $elementTypes }} =>
val elemInstances = summonAll(elementTypes)
case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = elementTypes }} =>
val elemInstances = summonAll[elementTypes]
val eqProductBody: (Expr[T], Expr[T]) => Expr[Boolean] = (x, y) => {
elemInstances.zipWithIndex.foldLeft(Expr(true: Boolean)) {
case (acc, (elem, index)) =>
Expand Down Expand Up @@ -84,17 +84,17 @@ Instead we extract the tuple-type for element types using pattern matching over
quotes and more specifically of the refined type:

```scala
case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = $elementTypes } } => ...
case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = elementTypes }} => ...
```

The implementation of `summonAll` as a macro can be show below assuming that we
have the given instances for our primitive types:

```scala
def summonAll[T](t: Type[T])(using qctx: QuoteContext): List[Expr[Eq[_]]] = t match {
case '[String *: $tpes] => '{ summon[Eq[String]] } :: summonAll(tpes)
case '[Int *: $tpes] => '{ summon[Eq[Int]] } :: summonAll(tpes)
case '[$tpe *: $tpes] => derived(using tpe, qctx) :: summonAll(tpes)
def summonAll[T: Type](using qctx: QuoteContext): List[Expr[Eq[_]]] = t match {
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@nicolasstucki Does this actually compile? I don't see where t is defined here. Also if we update the doc we should keep the test case synchronized: https://github.com/lampepfl/dotty/blob/master/tests/run-macros/i8007/Macro_3.scala

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I missed that. I will fix it.

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Fix in #10395

case '[String *: tpes] => '{ summon[Eq[String]] } :: summonAll[tpes]
case '[Int *: tpes] => '{ summon[Eq[Int]] } :: summonAll[tpes]
case '[tpe *: tpes] => derived[tpe] :: summonAll[tpes]
case '[EmptyTuple] => Nil
}
```
Expand All @@ -120,8 +120,8 @@ Alternatively and what is shown below is that we can call the `eqv` method
directly. The `eqGen` can trigger the derivation.

```scala
extension [T](x: =>T)
inline def === (y: =>T)(using eq: Eq[T]): Boolean = eq.eqv(x, y)
extension [T](inline x: T)
inline def === (inline y: T)(using eq: Eq[T]): Boolean = eq.eqv(x, y)

implicit inline def eqGen[T]: Eq[T] = ${ Eq.derived[T] }
```
Expand All @@ -144,7 +144,7 @@ The full code is shown below:
```scala
import scala.deriving._
import scala.quoted._
import scala.quoted.matching._


trait Eq[T] {
def eqv(x: T, y: T): Boolean
Expand All @@ -169,21 +169,21 @@ object Eq {
def eqv(x: T, y: T): Boolean = body(x, y)
}

def summonAll[T](t: Type[T])(using qctx: QuoteContext): List[Expr[Eq[_]]] = t match {
case '[String *: $tpes] => '{ summon[Eq[String]] } :: summonAll(tpes)
case '[Int *: $tpes] => '{ summon[Eq[Int]] } :: summonAll(tpes)
case '[$tpe *: $tpes] => derived(using tpe, qctx) :: summonAll(tpes)
def summonAll[T: Type](using qctx: QuoteContext): List[Expr[Eq[_]]] = t match {
case '[String *: tpes] => '{ summon[Eq[String]] } :: summonAll[tpes]
case '[Int *: tpes] => '{ summon[Eq[Int]] } :: summonAll[tpes]
case '[tpe *: tpes] => derived[tpe] :: summonAll[tpes]
case '[EmptyTuple] => Nil
}

given derived[T: Type](using qctx: QuoteContext) as Expr[Eq[T]] = {
import qctx.reflect._

val ev: Expr[Mirror.Of[T]] = Expr.summon(using '[Mirror.Of[T]]).get
val ev: Expr[Mirror.Of[T]] = Expr.summon(using Type.of[Mirror.Of[T]]).get

ev match {
case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = $elementTypes }} =>
val elemInstances = summonAll(elementTypes)
case '{ $m: Mirror.ProductOf[T] { type MirroredElemTypes = elementTypes }} =>
val elemInstances = summonAll[elementTypes]
val eqProductBody: (Expr[T], Expr[T]) => Expr[Boolean] = (x, y) => {
elemInstances.zipWithIndex.foldLeft(Expr(true: Boolean)) {
case (acc, (elem, index)) =>
Expand All @@ -197,8 +197,8 @@ object Eq {
eqProduct((x: T, y: T) => ${eqProductBody('x, 'y)})
}

case '{ $m: Mirror.SumOf[T] { type MirroredElemTypes = $elementTypes }} =>
val elemInstances = summonAll(elementTypes)
case '{ $m: Mirror.SumOf[T] { type MirroredElemTypes = elementTypes }} =>
val elemInstances = summonAll[elementTypes]
val eqSumBody: (Expr[T], Expr[T]) => Expr[Boolean] = (x, y) => {
val ordx = '{ $m.ordinal($x) }
val ordy = '{ $m.ordinal($y) }
Expand All @@ -217,8 +217,8 @@ object Eq {
}

object Macro3 {
extension [T](x: =>T)
inline def === (y: =>T)(using eq: Eq[T]): Boolean = eq.eqv(x, y)
extension [T](inline x: T)
inline def === (inline y: T)(using eq: Eq[T]): Boolean = eq.eqv(x, y)

implicit inline def eqGen[T]: Eq[T] = ${ Eq.derived[T] }
}
Expand Down