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?
Using a record with named fields is good style, as you pointed out later.
Stick to it!
Why do you need a type class? What about simply
empty :: Book
empty = Book [] "" Nothing Nothing Nothing "" 0 0
For updates
empty { title = "new title" }
is perfectly ok.
You can also let undefined fields undefined. In
Book {}
all fields are undefined.
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