Fixes to the assertImpl example

Make sure `assertImpl` prints the tested expression rather than its result.

Author: odersky

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

@@ -32,8 +32,7 @@ prints it again in an error message if it evaluates to `false`.
       ~ assertImpl(’(expr))
 
     def assertImpl(expr: Expr[Boolean]) =
-      ’{ if !(~expr) then throw new AssertionError(s"failed assertion: ${~expr.toString}") }
-
+      ’{ if !(~expr) then throw new AssertionError(s"failed assertion: ${~showExpr(expr)}") }
 
 If `e` is an expression, then `’(e)` or `’{e}` represent the typed
 abstract syntax tree representing `e`. If `T` is a type, then `’[T]`
@@ -531,15 +530,12 @@ In the end, `Liftable` resembles very much a serialization
 framework. Like the latter it can be derived systematically for all
 collections, case classes and enums.
 
-In fact, the initial example of assertions also uses a lifting conversion under the hood.
-Recall the failure clause:
+Using lifting, we can now give the missing definition of `showExpr` in the introductory example:
 
-    throw new AssertionError(s"failed assertion: ${~expr.toString}") }
+    def showExpr[T](expr: Expr[T]): Expr[String] = expr.toString
 
-Here, `expr.toString` yields `expr`'s representation in String form. That string
-is lifted to an `Expr[String]` since the required type of a splice argument is an `Expr`.
-The lifted result is then spliced in into the `AssertionError` argument, giving
-back again the original string representation of `expr`.
+That is, the `showExpr` method converts its `Expr` argument to a string, and lifts
+the result back to an `Expr[String]` using the implicit `toExpr` conversion.
 
 ## Implementation
 

library/src/scala/quoted/Liftable.scala

@@ -15,4 +15,5 @@ abstract class Liftable[T] {
 object Liftable {
   implicit def IntIsLiftable: Liftable[Int] = ???
   implicit def BooleanIsLiftable: Liftable[Boolean] = ???
+  implicit def StringIsLiftable: Liftable[String] = ???
 }

tests/pos/quoted.scala

@@ -8,7 +8,9 @@ class Test {
       ~ assertImpl('(expr))
 
     def assertImpl(expr: Expr[Boolean]) =
-      '{ if !(~expr) then throw new AssertionError(s"failed assertion: ${~expr}") }
+      '{ if !(~expr) then throw new AssertionError(s"failed assertion: ${~showExpr(expr)}") }
+
+    def showExpr[T](expr: Expr[T]): Expr[String] = expr.toString
 
     inline def power(inline n: Int, x: Double) = ~powerCode(n, '(x))