[web-devel] BlazeHtml: a question about API design

Jasper Van der Jeugt jaspervdj at gmail.com
Sun Apr 25 04:54:49 EDT 2010

Hey web-devel mailing list,

I'm currently working on the BlazeHtml HTML combinator library, and
currently a bit stuck on a piece of API design, for which I need some
community feedback. I'll try to explain the problem here as clear as

The base idea is, for an HTML combinator library, that you provide
combinators that the end user can use to "combine" his document. More
specifically, we would have combinators for every HTML element.

So, say you have the regular div tag. This would be, in our library,
Text.Blaze.Html.Strict.div. That makes sense, but it seems possible
that a user wants to use a div as a leaf node (e.g. `<div />`). So
there are two possible signatures for `div`:

div :: Html -- An empty div element.
div :: Html -> Html -- The argument is the content of the div element.

I'm not sure what we want to do in that case. I so far see two major options:

Option 1: We can provide leaf and non-leaf combinators for every tag.
I'm not sure if this is overkill or not, but it is, for example not
forbidden to have content in an `<img>` tag. We could have these
functions in Text.Blaze.Html.Strict and Text.Blaze.Html.Strict.Leaf,
for instance. But it must be kind of annoying for the end user to have
to write `L.div` instead of just `div`. On the other hand, if we put
the *common* uses for the tags (e.g. `img` as leaf, `div` as non-leaf)
in the main module, we would get a very inconsistent mess, I assume.

So this solution would include:
- a module of html combinators as non-leaf elements
- a module of html combinators as leaf-elements
- the possibility for the end-user to import and qualify them as
he/she sees fit; or
- add a module with the most common uses and re-export the combinators

Option 2: We could only support parent nodes (of the type `Html ->
Html`), and introduce another combinator:

    (/>) :: Html
    (/>) = mempty

I have chosen `/>` here because it resembles the end of a leaf HTML
tag (e.g. `<img />`). Then, we would introduce a custom rule.

    {-# RULES
        "tag/empty" forall x y. tag x y (/>) = leaf x

The `y` here is the closing tag, we pass it as an argument for
performance reasons, and you can safely ignore it. This code results
in the fact that if we write

    img (/>)

somewhere in our template, it would be rendered to `<img></img>` when
we don't pass `-fenable-rewrite-rules` to the compiler, and `<img />`
otherwise. Note that `-O` implies `-fenable-rewrite-rules`. I'm not
sure about this solution either, because it sort of feels like a
(slightly elegant) hack.

I have my doubts with both options, but I would tend to go for (1),
because (2) feels more unstable. Anyway, feedback and more ideas would
be appreciated :-)

Kind regards,
Jasper Van der Jeugt

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