"Neil Mitchell" <[EMAIL PROTECTED]> writes:
> > To make things concrete, the example I'm really thinking of is a "send
> > an email" function, which would take a subject, a body, a list of
> > recipients, optional lists of cc and bcc recipients, an optional
> > mailserver (default localhost), an optional port (default 25), and
> > possibly optional authentication details.
>
> Records are your friend.
>
> data Email = Email {subject :: String, body :: String, to ::
> [Address], cc = [Address], bcc = [Address], mailserver :: String, port
> :: Int}
>
> defaultEmail = Email{subject = "No subject", body = "", to = [], cc =
> [], bcc = [], mailserver = "localhost", port = 25}
>
> The user can then go:
>
> sendEmail defaultEmail{subject="Subject here", body = "body here", to
> = ["haskell-cafe"], mailserver = "server.haskell.org"}
>
> Now things which are't specified (port) keep their default value.
If you do this for more than one function (and consequently
more than one datatype) there's a case for a class --
something like:
class Defaultable t where
defaults:: t
instance Defaultable Email where
defaults = Email{subject = "No subject", body = "", to = [],
cc = [], bcc = [], mailserver = "localhost", port = 25}
which would save having a defaultFoo for every Foo (at the
possible expense of occasional explicit types).
--
Jón Fairbairn [EMAIL PROTECTED]
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