[Haskell-cafe] Default (or empty) values

17 Jan 2007


      Hello all!

Let
...
data Book = Book {
     authors   :: [String],
     title     :: String,
     editor    :: Maybe String,
     edition   :: Maybe String,
     volume    :: Maybe (Int, Int), -- e.g. volume 1 of 3
     publisher :: String,
     year      :: Int,
     pages     :: Int
}
and
...
convertBook :: Map String String -- a map from field names to values (string representation)
            -> Maybe Book
convertBook takes info about book from some external source (e.g. a BibTeX database) and returns
Just book value or Nothing (if convertion failed). Fields of the Book datatype which are not (Maybe a)
are required to be present. 

convertBook looks like
...
convertBook = (rq "title" (\b v -> b { title = v })         <.>
               rq "publisher" (\b v -> b { publisher = v }) <.>
               ... ) (Just $ Book [] "" Nothing Nothing Nothing "" 0 0)
I don't like the `(Just $ Book [] "" Nothing Nothing Nothing "" 0 0)' part, I would prefer
instead someting like `empty :: Book'. So I define
...
class Empty e where
      empty :: e
But still I have to emplement instances by hand. There are a number of approaches to automatically
derive instances (TH, generic classes in GHC, drift). What would you recommend using in this case?
Or may be it would be better to drop out Empty and use something else?

TIA

-- 
WBR,
Max Vasin.