Wouldn't the implementation hiding feature of the newtype idiom be broken, if field selectors were not first class functions? For instance, the following code (taken shamelessly from Ch. 10 of Real World Haskell):

module Parse (
runParser

) where

data ParseState = ParseState {

string :: String
} deriving (Show)

newtype Parser a = Parser {
runParser :: ParseState -> Either String (a, ParseState)

}

has the attractive feature of hiding the internal implementation of the ParseState and Parser types from the user, preventing him from, for instance, pattern matching on either and thus writing code, which may break when we change the implementation. I believe this is only possible, because the runParser accessor is exportable as a first class function.