Add let abstractions with customizable names

Author: nicolasstucki

docs/docs/reference/metaprogramming/macros.md

@@ -583,19 +583,18 @@ val y: scala.Double = y.*(y)
 y
 ```
 Even though there is no hygiene issue it may be hard to undestand the code. To overcome this inconvenience
-each `y` can be assigned a meaningful name using the `scala.quoted.show.showName` annotation.
-For example `'{ @showName(${Expr("y" + i)}) val y = $x * $x; ... }` will assign to each `y` a name
-`a{i}` where `{i}` is a known String, if `i == 3` then it would be named `a3`.
+each `y` can be assigned a meaningful name using the `scala.quoted.util.let`.
+For example `let("y" + i)('{ $x * $x })(yi => ...)` will assign to each `y` a name
+`y{i}` where `{i}` is a known String, if `i == 3` then it would be named `y3`.
 
 The `powerCode` can be defined as follows using `showName`
 ```scala
-def powerCodeD(n: Long, x: Expr[Double]))(given QuoteContext): Expr[Double] = '{
-  val a1 = $x
-  ${ powerCodeD(n, 2, 'x1) }
-}
+def powerCodeD(n: Long, x: Expr[Double]))(given QuoteContext): Expr[Double] =
+  let("y1")(x)(y1 => powerCodeD(n, 2, y1))
+
 def powerCodeD(n: Long, i: Int, x: Expr[Double])(given QuoteContext): Expr[Double] =
   if (n == 0) '{1.0}
-  else if (n % 2 == 0) '{ @showName(${Expr("a" + i)}) val y = $x * $x; ${powerCodeD(n / 2, idx * 2, 'y)} }
+  else if (n % 2 == 0) let("y" + i)('{ $x * $x })(ai => powerCodeD(n / 2, idx * 2, yi) }
   else '{ $x * ${powerCodeD(n - 1, idx, x)} }
 ```
 then
@@ -604,11 +603,11 @@ powerCodeD(16, '{7}).show
 ```
 will show
 ```scala
-val a1: scala.Double = 7
-val a2: scala.Double = a1.*(a1)
-val a4: scala.Double = a2.*(a2)
-val a8: scala.Double = a4.*(a4)
-val a16: scala.Double = a8.*(a8)
+val y1: scala.Double = 7
+val y2: scala.Double = y1.*(y1)
+val y4: scala.Double = y2.*(y2)
+val y8: scala.Double = y4.*(y4)
+val y16: scala.Double = y8.*(y8)
 a16
 ```
 

library/src-bootstrapped/scala/quoted/util/package.scala

@@ -0,0 +1,23 @@
+package scala.quoted
+
+import scala.internal.quoted.showName
+
+package object util {
+
+  /** Genrate the code `'{ val `<name>` = $expr; ${ body('<name>) } }` with the given name */
+  def let[T: Type, U: Type](name: String)(expr: Expr[T])(body: Expr[T] => Expr[U])(given QuoteContext): Expr[U] = '{
+     @showName(${Expr(name)}) val x = $expr
+     ${ body('x) }
+  }
+
+  /** Genrate the code `'{ val x$<i> = $expr; ${ body('x$<i>) } }` for a fresh i */
+  def let[T: Type, U: Type](expr: Expr[T])(body: Expr[T] => Expr[U])(given QuoteContext): Expr[U] =
+    let("x$" + nextIdx)(expr)(body)
+
+  private[util] var lastIdx = 0
+  private def nextIdx: Int = {
+    lastIdx += 1
+    lastIdx
+  }
+
+}

tests/run-staging/quoted-let-name-1.check

@@ -0,0 +1,13 @@
+((x1: scala.Double) => x1.*({
+   val x$1: scala.Double = x1.*(x1)
+   val x$2: scala.Double = x$1.*(x$1)
+  x$2.*({
+     val x$3: scala.Double = x$2.*(x$2)
+    x$3.*({
+       val x$4: scala.Double = x$3.*(x$3)
+       val x$5: scala.Double = x$4.*(x$4)
+       val x$6: scala.Double = x$5.*(x$5)
+      x$6
+    })
+  })
+}))

tests/run-staging/quoted-let-name-1.scala

@@ -0,0 +1,21 @@
+import scala.quoted._
+import scala.quoted.util.let
+import scala.quoted.staging._
+import scala.reflect.ClassTag
+
+object Test {
+  given Toolbox = Toolbox.make(getClass.getClassLoader)
+  def main(args: Array[String]): Unit = withQuoteContext {
+    println(powerCode(77).show)
+  }
+
+  def powerCode(n: Long)(given QuoteContext): Expr[Double => Double] =
+    '{ x1 => ${powerCode(n, 2, 'x1)} }
+
+  def powerCode(n: Long, idx: Int, x: Expr[Double])(given QuoteContext): Expr[Double] =
+    if (n == 0) '{1.0}
+    else if (n == 1) x
+    else if (n % 2 == 0) let('{ $x * $x })(y => powerCode(n / 2, idx * 2, y))
+    else '{ $x * ${powerCode(n - 1, idx, x)} }
+
+}

tests/run-staging/quoted-let-name-2.check

@@ -0,0 +1,13 @@
+((x1: scala.Double) => x1.*({
+   val x2: scala.Double = x1.*(x1)
+   val x4: scala.Double = x2.*(x2)
+  x4.*({
+     val x8: scala.Double = x4.*(x4)
+    x8.*({
+       val x16: scala.Double = x8.*(x8)
+       val x32: scala.Double = x16.*(x16)
+       val x64: scala.Double = x32.*(x32)
+      x64
+    })
+  })
+}))

tests/run-staging/quoted-let-name-2.scala

@@ -0,0 +1,21 @@
+import scala.quoted._
+import scala.quoted.util.let
+import scala.quoted.staging._
+import scala.reflect.ClassTag
+
+object Test {
+  given Toolbox = Toolbox.make(getClass.getClassLoader)
+  def main(args: Array[String]): Unit = withQuoteContext {
+    println(powerCode(77).show)
+  }
+
+  def powerCode(n: Long)(given QuoteContext): Expr[Double => Double] =
+    '{ x1 => ${powerCode(n, 2, 'x1)} }
+
+  def powerCode(n: Long, idx: Int, x: Expr[Double])(given QuoteContext): Expr[Double] =
+    if (n == 0) '{1.0}
+    else if (n == 1) x
+    else if (n % 2 == 0) let(s"x$idx")('{ $x * $x })(y => powerCode(n / 2, idx * 2, y))
+    else '{ $x * ${powerCode(n - 1, idx, x)} }
+
+}