Implement the transactional pair abstraction, represented with the TPair class:

class TPair[P, Q](pinit: P, qinit: Q) {
  def first(implicit txn: InTxn): P = ???
  def first_=(x: P)(implicit txn: InTxn): P = ???
  def second(implicit txn: InTxn): Q = ???
  def second_=(x: Q)(implicit txn: InTxn): Q = ???
  def swap()(implicit e: P =:= Q, txn: InTxn): Unit = ???
}

In addition to getters and setters for the two fields, the transactional pair defines the swap method that swaps the fields, and can only be called if the types P and Q are the same.

Use ScalaSTM to implement the mutable location abstraction from Haskell, represented with the MVar class:

class...