I participate in writing the Wikibook about the Haskell programming language.
GeSHi doesn't support Haskell yet, but it might in the near future (see the feature request - http://sourceforge.net/tracker/index.php?func=detail&aid=1720310&group_id=114997&atid=670234 ).
In the meantime, if i try to write <source lang="haskell">SOME CODE</source>, it is rendered as a Big Red Error that doesn't show any code. I think that it would be better to display the source in the default (black) color with a small warning saying that the language is not supported. This way, when support for that language is added, the warning will go away and the coloring will just start working.
Version: unspecified
Severity: enhancement
URL: http://www.mediawiki.org/wiki/Extension:SyntaxHighlight_GeSHi