[Haskell-cafe] Default (or empty) values

[Pedro Baltazar Vasconcelos]

On Wed, 17 Jan 2007 15:58:04 +0300
Max Vasin <max.vasin at gmail.com> 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
> > } 

One suggestion: why not make the Book type more general, e.g. a list of labeled fields:

> data Label = Author | Title | Editor | ....
> type Field = String
> newtype Book = Book [(Label,Field)]

The idea is that e.g. multiple authors would be represented by multiple entries with an "Author" label and optional fields can be omitted.
Then the empty book is simply

> empty = Book []

You might also find that this makes the processing of fields more uniform and easier to extend. 

Best regards,

Pedro Vasconcelos.