IntCodeComputer.scala

  case class ComputerState(memory: Vector[Int], ptr: Int)

  type IntCodeError[A] = Either[String,A]
  type IntCodeProgram[A] = StateT[IntCodeError,ComputerState,A]

  def pure[A](a: A): IntCodeProgram[A] = StateT.pure(a)
  def inspect[A](f: ComputerState => A): IntCodeProgram[A] = StateT.inspect(f)
  def mod(f: ComputerState => ComputerState): IntCodeProgram[Unit] = StateT.modify(f)
  def error[A](msg: String): IntCodeProgram[A] = StateT.liftF(Left(msg))

  def read: IntCodeProgram[Int] = for {
    ptr  <- inspect(_.ptr)
    next <- readAt(ptr)
    _    <- mod(s => s.copy(ptr = ptr + 1))
  } yield {
    next
  }

  def readAt(ptr: Int): IntCodeProgram[Int] = inspect(s => s.memory(ptr))
  def write(value: Int)(ptr: Int): IntCodeProgram[Unit] = mod(s => s.copy(memory = s.memory.updated(ptr, value)))

  def run_(opCode: Int): IntCodeProgram[Unit] = FlatMap[IntCodeProgram].tailRecM(opCode) {
    case 1 => for {
      op1  <- read >>= readAt
      op2  <- read >>= readAt
      _    <- read >>= write(op1 + op2)
      next <- read
    } yield { Left(next) }
    case 2 => for {
      op1  <- read >>= readAt
      op2  <- read >>= readAt
      _    <- read >>= write(op1 * op2)
      next <- read
    } yield { Left(next) }
    case 99 => pure(Right())
    case _  => error("Unrecognized op code")
  }

  def run: IntCodeProgram[Unit] = read >>= run_