What are macros?#
With a macro, we can treat programs as values, which allows us to analyze and generate them at compile time.
A simple macro to print execution time#
Let’s start with a simple example. We’ll create a function that times a given operation and prints the time it took to run the function.
Macro Definition - Macro.scala#
When creating macros, you need to make sure they exist in a separate file. We’ll create a Macro.scala file and add this code to it:
import scala.quoted.*
object Macro:
inline def timed[T](inline expr: T): T = ${ timedImpl('expr) }
private def timedImpl[T: Type](expr: Expr[T])(using Quotes): Expr[T] =
'{
val start = System.currentTimeMillis()
try $expr
finally {
val end = System.currentTimeMillis()
val exprAsString = ${ Expr(expr.show) }
println(s"Evaluating $exprAsString took: ${end - start}ms")
}
}
private def exprAsCompactString[T: Type](expr: Expr[T])(using Quotes): String =
import quotes.reflect.*
expr.asTerm match {
case Inlined(_, _, Apply(method, params)) => s"${method.symbol.name}(${params.map(_.show).mkString(",")})"
case _ => expr.show
}
Macro Usage - App.scala#
//> using scala "3.2.2"
import Macro.*
def f(n: Int) = {
Thread.sleep(500L * n)
20
}
@main
def main =
timed {
println("Start2")
Thread.sleep(1000L)
println("End")
}
timed(f(1))
timed(f(2))
Run scala-cli .:
Start2
End
Evaluating {
scala.Predef.println("Start2")
java.lang.Thread.sleep(1000L)
scala.Predef.println("End")
} took: 1076ms
Evaluating App$package.f(1) took: 503ms
Evaluating App$package.f(2) took: 1002ms
Customizing the macro output#
Macro Definition - Macro.scala#
import scala.quoted.*
object Macro:
inline def timed[T](inline expr: T): T = ${ timedImpl('expr) }
private def timedImpl[T: Type](expr: Expr[T])(using Quotes): Expr[T] =
'{
val start = System.currentTimeMillis()
try $expr
finally {
val end = System.currentTimeMillis()
val exprAsString = ${ Expr(exprAsCompactString(expr)) }.replaceAll("\\s+", " ").trim()
val exprAsStringShort = if (exprAsString.length > 50) exprAsString.take(50) + "..." else exprAsString
println(s"Evaluating $exprAsStringShort took: ${end - start}ms")
}
}
private def exprAsCompactString[T: Type](expr: Expr[T])(using Quotes): String =
import quotes.reflect.*
expr.asTerm match {
case Inlined(_, _, Apply(method, params)) => s"${method.symbol.name}(${params.map(_.show).mkString(",")})"
case _ => expr.show
}
Macro Usage - App.scala#
//> using scala "3.2.2"
import Macro.*
def f(n: Int) = {
Thread.sleep(500L * n)
20
}
@main
def main =
timed {
println("Start2")
Thread.sleep(1000L)
println("End")
}
timed(f(1))
timed(f(2))
Run scala-cli .:
Start2
End
Evaluating { scala.Predef.println("Start2") java.lang.Thread.... took: 1069ms
Evaluating f(1) took: 504ms
Evaluating f(2) took: 1004ms