The reason formlet placements (and page placements {|...|}) are allowed
in page literals but not in plain XML literals is because of the way
formlet validation works. Suppose there are multiple formlets, each with
a separate handler, on a page. If submitting one of the formlets leads
to an error then we need to re-render the entire page as XML with the
error reported in the offending formlet, but all the other formlets left
in tact. In order to implement this behaviour, the Page type has more
structure than plain XML. Rendering a page as XML loses that structure.
The renderPage function is really an internal implementation detail that
you shouldn't use directly. It is called at the last minute to render an
entire page as XML. It will not necessarily do the right thing if you
use it inside your own XML.
We should certainly improve the error messages for this stuff!
Sam
James Cheney wrote:
Great, that works, though it makes my head hurt a little.
I'm curious about the restriction that formlet placements have to be
directly within a page (by which I guess you mean no intervening {}
brackets). Could this restriction in principle be lifted (at least for
simple cases like my situation where I want to make a bunch of formlets
using a for loop)?
Another, related question: Assuming {| - |} brackets are meant to allow
a Page to be embedded into an Xml value, why can't we do this instead:
page
<html>
<body>
{
for (x <- tbl)
<#>{|
page <#>{f => main}</#>
|}</#>
}
</body>
</html>
This seems to have the same effect as the concatPages approach, but
avoids explicitly building a list and then traversing it. But this also
gives me an Undefined_recursive_module error. In fact, this happens
even if I get rid of the formlet stuff and just stick some random XML
inside the inner {} block. So I guess formlets were a red herring here.
Here's a smaller example demonstrating the Undefined_recursive_module
error:
<foo> {| page <bar/>|}</foo>
Indeed, even doing this:
sig pageToXml : (Page) ~> Xml
fun pageToXml(p) {
<#>{|p|}</#>
}
yields an error, this time:
*** Error: File "sugartoir.ml", line 771, characters 14-20: Assertion
failed
(This is in links 0.5.0.)
But looking at the prelude, the function renderPage : (Page) ~> Xml also
seems to do what I need. So maybe it would be enough to desugar
<#>{|p|}</#> to renderPage(p)?
--James
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