> I've got a stylistic question about Haskell.
> What's the best way to add optional arguments to a
> embedded DSL?
>
[..]
> --------------------------------------------------
> (1) Combinators takes a list of attributes.
> Now we have
>
> h1 :: [HtmlAttr] -> Html -> Html
>
> and might write
>
> h1 [align "right"] (stringToHtml "This is a Header")
>
> but now there are lots of [] scattered needlessly all over
> the code.
>
> --------------------------------------------------
This is (as Tommy Thorn has already noted) what TkGofer and TclHaskell
use at present, and it seems quite convenient. One clever thing is
done as well, to prevent illegal attributes being passed:
class Has_alt w where
alt :: String -> Conf w
alt s = ...
class Has_align w where
align :: Alignment -> Conf w
align a = ...
instance Has_alt IMG
instance Has_align IMG
instance Has_align H1
img :: URL -> [Conf IMG] -> HTML
h1 :: String -> [Conf H1] -> HTML
img "purple.gif" [alt "purple picture", align Right] -- OK
h1 "My heading" [align Left] -- OK
h1 "My illegal heading" [align Left, alt "illegal"] -- type error
This is quite a cunning trick.
An override can be provided, too, as follows:
(%!) :: String -> String -> Conf w
tag %! val = ...
h1 "My illegal heading" [align Left, "alt" %! "illegal"] -- OK
HTH.
--KW 8-)
