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I have the following code to generate a blank html page with a series of divs with id's and classes in Haskell using the Text.XHtml.Strict library:
module Main where
import Text.XHtml.Strict
import Text.Printf
page :: Html
page = pHeader +++ pTop +++ pBody +++ pFooter
pHeader :: Html
pHeader = header << thetitle << "Page title"
pTop :: Html
pTop = (dC "header") << (dI "title")
pFooter :: Html
pFooter = (dC "footer") << (dI "foottext")
pBody :: Html
pBody = body << (dC "main") << (dI "window") << (dI "content")
dC :: String -> Html
dC x = (thediv noHtml)! [theclass x]
dI :: String -> Html
dI x = (thediv noHtml) ! [identifier x]
main :: IO ()
main = do
printf $ prettyHtml $ page
The functions dC and dI should make an empty with a class or id respectively. In the interpreter these functions work fine when concatenating, as in:
printf $ prettyHtmlFragment $ dC "1" +++ dC "2"
<div class="1">
</div>
<div class="2">
</div>
But not when I try to nest them using << instead of +++, I get an error:
<interactive>:1:28:
Couldn't match expected type `Html -> b'
against inferred type `Html'
This is what I think the cause of the problem in the main part of the code is, but I don't know how to fix it. Any ideas?
631
Just take out the nohtml parts, and change the signatures:
dC :: String -> Html -> Html
dC x = thediv ! [theclass x]
dI :: String -> Html -> Html
dI x = thediv ! [identifier x]
The ! operator can add attributes not only to an Html object, but also to a function that returns Html. thediv is a function that takes Html and returns it wrapped in a <div> element. By applying ! to it, you create a function that takes Html and wraps a <div class="…"> (or id="…") around it.
> :type thediv
thediv :: Html -> Html
> let dC c = thediv ! [theclass c]
> let dI i = thediv ! [identifier i]
> :type dC
dC :: String -> Html -> Html
> :type dI
dI :: String -> Html -> Html
> putStr $ prettyHtmlFragment $ body << dC "main" << dI "window" << dI "content" << "Hi"
<body>
<div class="main">
<div id="window">
<div id="content">
Hi
</div>
</div>
</div>
</body>
Note that you don't need the extra parenthesis.
162
dC and cI can't nest because they're defined to be empty. You need to leave a hole that the nested piece will go into. The hole is expressed by parameterizing it:
dC classx child = (thediv child)! [theclass classx]
dI x y = (thediv y) ! [identifier x]
likewise for body:
pBody x = body << (dC "main") << (dI "window") << (dI "content" x)
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