Haskell: Picker Combinators

One more illustration of combinatorial approach to a problem - Pickler Combinators.
This time the matter is serialization/deserialization of data.

Most of all I was surprised at the unusual using of parameter data types.

 data PU a = PU {appP :: (a,String) -> String,
                 appU :: String -> (a, String)}

Where I'd declare a class without a hesitation, a type consisting of two functional type fields is used instead.