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.
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