[Haskell-cafe] Re: Default (or empty) values

...

...

...

...

"Henning" == Henning Thielemann writes:

Henning> On Wed, 17 Jan 2007, Max Vasin wrote:

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?

Henning> Using a record with named fields is good style, as you Henning> pointed out later. Stick to it! Why do you need a type Henning> class? What about simply Henning> empty :: Book empty = Book [] "" Nothing Nothing Nothing Henning> "" 0 0 Well the type class is not necessary, a TH function generating an empty value for given datatype. OTOH convertBook can be rewritten (using type class) as:

convertBook = (rq "title" (\b v -> b { title = v }) <.> rq "publisher" (\b v -> b { publisher = v }) <.> ... )

and it will be used like 'convertBook empty'. I like this way more since convertBook looks more declarative. Henning> For updates empty { title = "new title" } is perfectly Henning> ok. Henning> You can also let undefined fields undefined. In Henning> Book {} This will be enough if there were no optional fields. -- WBR, Max Vasin.