b.3.8.scala

implicit object ExprApplicative extends Applicative[Expr] {
  def pure[A](a: => A): Expr[A] = Lit(a)
  
  override def fmap[A, B](e: Expr[A], f: A => B): Expr[B] = new Expr[B] {
    def eval(st: SymbolTable): B = f apply e.eval(st)
  }

  override def apply[A, B](f: Expr[A => B], e: Expr[A]): Expr[B] = new Expr[B] {
    def eval(st: SymbolTable): B = f.eval(st) apply e.eval(st)
  }
}