Deriving Backtracking Monad Transformers Functional Pearl

In a paper about pretty printing J. Hughes introdu ed two fundamental te hniques for deriving programs from their spe i ation, where a spe i ation onsists of a signature and properties that the operations of the signature are required to satisfy. Brie y, the rst te hnique, the term implementation, represents the operations by terms and works by de ning a mapping from operations to observations | this mapping an be seen as de ning a simple interpreter. The se ond, the ontext-passing implementation, represents operations as fun tions from their alling ontext to observations. We apply both te hniques to derive a ba ktra king monad transformer that adds ba ktra king to an arbitrary monad. In addition to the usual ba ktra king operations | failure and nondeterministi hoi e | the prolog ut and an operation for delimiting the e e t of a ut are supported.