Scala3-InvariantEnabled.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 invariantEnabled:
  inline val ignore = false

  inline def apply[T](
      inline predicate: Boolean, 
      inline message: String = "")(
      inline block: T): T =
    inline 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](
      inline predicate: Boolean,
      inline message: String,
      inline block: T,
      inline 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)