I've got a stylistic question about Haskell.
What's the best way to add optional arguments to a
embedded DSL?
For example, say I've got an library of HTML combinators.
To represent
<H1>This is a Header</H1>
you might write
h1 (stringToHtml "This is a Header")
The types of the functions are
h1 :: Html -> Html
stringToHtml :: String -> Html
Looks OK, but what about if you want this header
right aligned? I can see three ways.
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(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.
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(2) Combinators that take arguments have different names.
So you might have
h1 :: Html -> Html
h1' :: [HtmlAttr] -> Html -> Html
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(3) Have an "inherit attributes combinator".
addAttrs :: [HtmlAttr] -> Html -> Html
So we might write
(h1 (stringToHtml "This is a Header"))
`addAttrs` [align "right"]
This can be cleaned up a bit. define
(<<) :: (Html -> Html) -> [HtmlAttr] -> (Html -> Html)
fn << attr = \ html -> fn html `addAttrs` [attr]
We can now write
(h1 << align "right") (stringToHtml "This is a Header")
or
h1 << align "right" $ stringToHtml "This is a Header"
[You could also write
h1 <| align "right" |> stringToHtml "This is a Header"
if you had <| = << and |> = $, with the right prec.]
However, We're now playing games with syntax, and have
perhaps lost the thread. Working out when to put
<| and |>; only where the first argument is a function,
is a lot easier to get lost that always putting a [].
And the error messages are now obfuscated.
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Has anyone put any thought into this problem? It seems to be general,
not just specific to HTML combinators. How do you allow optional
arguments? Does anyone have a preference to which scheme they like?
Andy Gill
