Scala3-InvariantMinInline.scala

// Adapted from "Programming Scala, Third Edition" (http://programming-scala.com)
// https://github.com/deanwampler/programming-scala-book-code-examples/blob/master/src/main/scala/progscala3/meta/Invariant.scala
import scala.quoted.*

object invariantMinInline:
  val ignore = false

  inline def apply[T](         // Use of "minimal" inlining. Does it still work?
      predicate: => Boolean,   // Now we need by-name parameters!
      message: => String = "")(
      block: => T): T =
    if !ignore then
      if !predicate then fail(predicate, message, block, "before")
      val result = block
      if !predicate then fail(predicate, message, block, "after")
      result
    else
      block

  inline private def fail[T](
      predicate: => Boolean,
      message: => String,
      block: => T,
      beforeAfter: String): Unit =
    ${ failImpl('predicate, 'message, 'block, 'beforeAfter) }

  case class InvariantFailure(msg: String) extends RuntimeException(msg)

  private def failImpl[T](
      predicate: Expr[Boolean], message: Expr[String],
      block: Expr[T], beforeAfter: Expr[String])(
      using Quotes): Expr[String] =
    '{ throw InvariantFailure(
      s"""FAILURE! predicate "${${showExpr(predicate)}}" """
      + s"""failed ${$beforeAfter} evaluation of block:"""
      + s""" "${${showExpr(block)}}". Message = "${$message}". """)
    }

  private def showExpr[T](expr: Expr[T])(using Quotes): Expr[String] =
    val code: String = expr.show
    Expr(code)