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Scala 3 Language Reference

Reflection

Language

Reflection enables inspection and construction of Typed Abstract Syntax Trees (Typed-AST). It may be used on quoted expressions (quoted.Expr) and quoted types (quoted.Type) from Macros or on full TASTy files.

If you are writing macros, please first read Macros. You may find all you need without using quote reflection.

API: From quotes and splices to TASTy reflect trees and back

With quoted.Expr and quoted.Type we can compute code but also analyze code by inspecting the ASTs. Macros provide the guarantee that the generation of code will be type-correct. Using quote reflection will break these guarantees and may fail at macro expansion time, hence additional explicit checks must be done.

To provide reflection capabilities in macros we need to add an implicit parameter of type scala.quoted.Quotes and import quotes.reflect.* from it in the scope where it is used.

import scala.quoted.*

inline def natConst(inline x: Int): Int = ${natConstImpl('{x})}

def natConstImpl(x: Expr[Int])(using Quotes): Expr[Int] =
  import quotes.reflect.*
  ...

Extractors

import quotes.reflect.* will provide all extractors and methods on quotes.reflect.Trees. For example the Literal(_) extractor used below.

def natConstImpl(x: Expr[Int])(using Quotes): Expr[Int] =
  import quotes.reflect.*
  val tree: Term = x.asTerm
  tree match
    case Inlined(_, _, Literal(IntConstant(n))) =>
      if n <= 0 then
        report.error("Parameter must be natural number")
        '{0}
      else
        tree.asExprOf[Int]
    case _ =>
      report.error("Parameter must be a known constant")
      '{0}

We can easily know which extractors are needed using Printer.TreeStructure.show, which returns the string representation the structure of the tree. Other printers can also be found in the Printer module.

tree.show(using Printer.TreeStructure)
// or
Printer.TreeStructure.show(tree)

The methods quotes.reflect.Term.{asExpr, asExprOf} provide a way to go back to a quoted.Expr. Note that asExpr returns a Expr[Any]. On the other hand asExprOf[T] returns a Expr[T], if the type does not conform to it an exception will be thrown at runtime.

Positions

The Position in the context provides an ofMacroExpansion value. It corresponds to the expansion site for macros. The macro authors can obtain various information about that expansion site. The example below shows how we can obtain position information such as the start line, the end line or even the source code at the expansion point.

def macroImpl()(quotes: Quotes): Expr[Unit] =
  import quotes.reflect.*
  val pos = Position.ofMacroExpansion

  val path = pos.sourceFile.jpath.toString
  val start = pos.start
  val end = pos.end
  val startLine = pos.startLine
  val endLine = pos.endLine
  val startColumn = pos.startColumn
  val endColumn = pos.endColumn
  val sourceCode = pos.sourceCode
  ...

Tree Utilities

quotes.reflect contains three facilities for tree traversal and transformation.

TreeAccumulator ties the knot of a traversal. By calling foldOver(x, tree)(owner) we can dive into the tree node and start accumulating values of type X (e.g., of type List[Symbol] if we want to collect symbols). The code below, for example, collects the val definitions in the tree.

def collectPatternVariables(tree: Tree)(using ctx: Context): List[Symbol] =
  val acc = new TreeAccumulator[List[Symbol]]:
    def foldTree(syms: List[Symbol], tree: Tree)(owner: Symbol): List[Symbol] = tree match
      case ValDef(_, _, rhs) =>
        val newSyms = tree.symbol :: syms
        foldTree(newSyms, body)(tree.symbol)
      case _ =>
        foldOverTree(syms, tree)(owner)
  acc(Nil, tree)

A TreeTraverser extends a TreeAccumulator and performs the same traversal but without returning any value. Finally, a TreeMap performs a transformation.

ValDef.let

quotes.reflect.ValDef also offers a method let that allows us to bind the rhs (right-hand side) to a val and use it in body. Additionally, lets binds the given terms to names and allows to use them in the body. Their type definitions are shown below:

def let(rhs: Term)(body: Ident => Term): Term = ...

def lets(terms: List[Term])(body: List[Term] => Term): Term = ...

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