Merge pull request #4129 from dotty-staging/remove-implicit-conversion-toExpr

Remove implicit conversion from liftable types to Expr[T]

Author: nicolasstucki

docs/docs/reference/principled-meta-programming.md

@@ -507,7 +507,7 @@ The compiler takes an environment that maps variable names to Scala `Expr`s.
 
     def compile(e: Exp, env: Map[String, Expr[Int]]): Expr[Int] = e match {
       case Num(n) =>
-        n
+        n.toExpr
       case Plus(e1, e2) =>
         ’(~compile(e1, env) + ~compile(e2, env))
       case Var(x) =>
@@ -520,24 +520,25 @@ Running `compile(letExp, Map())` would yield the following Scala code:
 
     ’{ val y = 3; (2 + y) + 4 }
 
-The body of the first clause, `case Num(n) => n`, looks suspicious. `n`
-is declared as an `Int`, yet the result of `compile` is declared to be
-`Expr[Int]`. Shouldn’t `n` be quoted? In fact this would not
+The body of the first clause, `case Num(n) => n.toExpr`, looks suspicious. `n`
+is declared as an `Int`, yet it is conveted to an `Expr[Int]` with `toExpr`.
+Shouldn’t `n` be quoted? In fact this would not
 work since replacing `n` by `’n` in the clause would not be phase
 correct.
 
-What happens instead "under the hood" is an implicit conversion: `n`
-is expanded to `scala.quoted.Expr.toExpr(n)`. The `toExpr` conversion
-is defined in the companion object of class `Expr` as follows:
+What happens instead "under the hood" is an extension method `toExpr` is added: `n.toExpr`
+is expanded to `new scala.quoted.LiftExprOps(n).toExpr`. The `toExpr` extension
+is defined in the companion object of class `Liftable` as follows:
 
-    object Expr {
-      implicit def toExpr[T](x: T)(implicit ev: Liftable[T]): Expr[T] =
-        ev.toExpr(x)
+    package object quoted {
+      implicit class LiftExprOps[T](val x: T) extends AnyVal {
+        def toExpr(implicit ev: Liftable[T]): Expr[T] = ev.toExpr(x)
+      }
     }
 
-The conversion says that values of types implementing the `Liftable`
-type class can be converted ("lifted") automatically to `Expr`
-values. Dotty comes with instance definitions of `Liftable` for
+The extension says that values of types implementing the `Liftable` type class can be
+converted ("lifted") to `Expr` values using `toExpr` when `scala.quoted._` is imported.
+Dotty comes with instance definitions of `Liftable` for
 several types including `Boolean`, `String`, and all primitive number
 types. For example, `Int` values can be converted to `Expr[Int]`
 values by wrapping the value in a `Literal` tree node. This makes use
@@ -570,7 +571,7 @@ a `List` is liftable if its element type is:
 
     implicit def ListIsLiftable[T: Liftable]: Liftable[List[T]] = new {
       def toExpr(xs: List[T]): Expr[List[T]] = xs match {
-        case x :: xs1 => ’(~implicitly[Liftable[T]].toExpr(x) :: ~toExpr(xs1))
+        case x :: xs1 => ’(~x.toExpr :: ~toExpr(xs1))
         case Nil => ’(Nil: List[T])
       }
     }

library/src/scala/quoted/Expr.scala

@@ -10,8 +10,6 @@ sealed abstract class Expr[T] {
 }
 
 object Expr {
-  implicit def toExpr[T](x: T)(implicit ev: Liftable[T]): Expr[T] =
-    ev.toExpr(x)
 
   implicit class AsFunction[T, U](private val f: Expr[T => U]) extends AnyVal {
     def apply(x: Expr[T]): Expr[U] = new Exprs.FunctionAppliedTo[T, U](f, x)
@@ -25,7 +23,7 @@ object Expr {
 object Exprs {
   /** An Expr backed by a pickled TASTY tree */
   final class TastyExpr[T](val tasty: Pickled, val args: Seq[Any]) extends Expr[T] {
-    override def toString(): String = s"Expr(<pickled>)"
+    override def toString: String = s"Expr(<pickled>)"
   }
 
   /** An Expr backed by a value.

library/src/scala/quoted/Liftable.scala

@@ -6,7 +6,7 @@ import scala.quoted.Exprs.ValueExpr
  *  without going through an explicit `'(...)` operation.
  */
 abstract class Liftable[T] {
-  implicit def toExpr(x: T): Expr[T]
+  def toExpr(x: T): Expr[T]
 }
 
 /** Some liftable base types. To be completed with at least all types

library/src/scala/quoted/Type.scala

@@ -39,4 +39,4 @@ object Types {
   final class TreeType[Tree](val tree: Tree) extends quoted.Type[Any] {
     override def toString: String = s"Type(<raw>)"
   }
-}
+}

tests/pos/i3898/quoted_1.scala

@@ -1,5 +1,5 @@
 import scala.quoted._
 object Macro {
   inline def ff(args: Any*): String = ~impl('(args))
-  def impl(args: Expr[Seq[Any]]): Expr[String] = ""
+  def impl(args: Expr[Seq[Any]]): Expr[String] = '("")
 }

tests/pos/i3898b/quoted_1.scala

@@ -1,5 +1,5 @@
 import scala.quoted._
 object Macro {
   inline def ff(x: Int, inline y: Int): String = ~impl('(x))
-  def impl(x: Expr[Int]): Expr[String] = ""
+  def impl(x: Expr[Int]): Expr[String] = '("")
 }

tests/pos/i3898c/quoted_1.scala

@@ -1,5 +1,5 @@
 import scala.quoted._
 object Macro {
   inline def ff(x: Int, inline y: Int): String = ~impl('(x))
-  def impl(x: Expr[Int]): Expr[String] = ""
+  def impl(x: Expr[Int]): Expr[String] = '("")
 }

tests/pos/i3916/Macro_1.scala

@@ -14,7 +14,7 @@ object FInterpolation {
   }
 
   def fInterpolation(sc: StringContext, args: Seq[Expr[Any]]): Expr[String] = {
-    val str: Expr[String] = sc.parts.mkString("")
+    val str: Expr[String] = sc.parts.mkString("").toExpr
     val args1: Expr[Seq[Any]] = liftSeq(args)
     '{  (~str).format(~args1: _*) }
   }

tests/pos/i4023b/Macro_1.scala

@@ -1,5 +1,5 @@
 import scala.quoted._
 object Macro {
   inline def ff[T](implicit t: Type[T]): Int = ~impl[T]
-  def impl[T]: Expr[Int] = 4
+  def impl[T]: Expr[Int] = '(4)
 }

tests/pos/quote-0.scala

@@ -1,4 +1,5 @@
 import scala.quoted._
+
 import dotty.tools.dotc.quoted.Toolbox._
 
 class Test {
@@ -11,7 +12,7 @@ class Test {
     def assertImpl(expr: Expr[Boolean]) =
       '{ if !(~expr) then throw new AssertionError(s"failed assertion: ${~showExpr(expr)}") }
 
-    def showExpr[T](expr: Expr[T]): Expr[String] = expr.toString
+    def showExpr[T](expr: Expr[T]): Expr[String] = expr.toString.toExpr
 
     inline def power(inline n: Int, x: Double) = ~powerCode(n, '(x))