Macros in Scala 3

Best Practices

This doc page is specific to Scala 3, and may cover new concepts not available in Scala 2. Unless otherwise stated, all the code examples in this page assume you are using Scala 3.


Be careful when inlining for performance

To take the most advantage of the JVM JIT optimisations, you want to avoid generating large methods.


Coming soon

Quoted code

Keep quotes readable

  • Try to avoid ${...} with arbitrary expressions inside
    • Use $someExpr
    • Use ${ someExprFrom('localExpr) }

To illustrate, consider the following example:

val sc: StringContext = ...
'{ StringContext(${Varargs(}: _*) }

Instead, we can write the following:

val sc: StringContext = ...
val partExprs =
val partsExpr = Varargs(partExprs)
'{ StringContext($partsExpr: _*) }

The contents of the quote are much more clear in the second example.

Avoid nested contexts

Consider the following code:

val y: Expr[Int] = ...
def body(x: Expr[Int])(using quotes.Nested) =  '{ $x + $y }
'{ (x: Int) => ${ body('x) } }

Instead, use a normal context and pass all needed expressions. This has also the advantage of allowing the function to not be defined locally.

def body(x: Expr[Int], y: Expr[Int])(using Quotes) =
  '{ $x + $y }

val y: Expr[Int] = ...
'{ (x: Int) => ${ body('x, y) } }

Quotes Reflect

For this section, consider the following setup:

object Box:
  sealed trait Base
  case class Leaf(x: Int) extends Base

// Quotes in contextual scope
val boxTpe : TypeRepr = TypeRepr.of[Box.type]
val baseTpe: TypeRepr = TypeRepr.of[Box.Base]
val baseSym: Symbol   = baseTpe.typeSymbol
val leafTpe: TypeRepr = TypeRepr.of[Box.Leaf]
val leafSym: Symbol   = leafTpe.typeSymbol

Avoid Symbol.tree

On an object sym: Symbol, sym.tree returns the Tree associated with the symbol. Be careful when using this method, as the tree for a symbol might not be defined. When the code associated with a symbol is defined at a different time than this access, if the -Yretain-trees compilation option is not used, then the tree of the symbol will not be available. Symbols originating from Java code do not have an associated tree.

Obtaining a TypeRepr from a Symbol

In the previous heading, we saw that Symbol.tree should be avoided and that therefore you should not use sym.tree.tpe on sym: Symbol. Thus, to obtain the TypeRepr corresponding to a Symbol, it is recommended to use tpe.memberType on tpe: TypeRepr objects.

We can obtain the TypeRepr of Leaf in two ways:

  1. TypeRepr.of[Box.Leaf]
  2. boxTpe.memberType(leafSym) (In other words, we request the TypeRepr of the member of Box whose symbol is equal to the symbol of leafSym.)

While the two approaches are equivalent, the first is only possible if you already know that you are looking for the type Box.Leaf. The second approach allows you to explore an unknown API.

Use Symbols to compare definitions

Read more about Symbols here.

Symbols allow you to compare definitions using ==:

leafSym == baseSym.children.head // Is true

However, == on TypeReprs does not produce the same result:

boxTpe.memberType(baseSym.children.head) == leafTpe // Is false

Obtaining a Symbol for a type

There is a handy shortcut to get the symbol for the definition of T. Instead of


you can use


Pattern match your way into the API

Pattern matching is a very ergonomic approach to the API. Always have a look at the unapply method defined in *Module objects.

Search the contextual scope in your macros

You can search for given instances using

For example:

def summonOrFail[T: Type]: Expr[T] =
  val tpe = TypeRepr.of[T] match
    case success: ImplicitSearchSuccess =>
      val implicitTerm = success.tree
    case failure: ImplicitSearchFailure =>"Could not find an implicit for " +[T])

If you are writing a macro and prefer to handle Exprs, Expr.summon is a convenient wrapper around

def summonOrFail[T: Type]: Expr[T] =
  Expr.summon[T] match
    case Some(imp) => imp
    case None =>"Could not find an implicit for " +[T])

Contributors to this page: