The install script in the package complains of:
./gen_contents_index: line 50: ../../../../../bin/haddock: No such file or directory
That's an unfortunate consequence of adopting ghc straight from [extra] without massaging. As I mentioned in an email to the list I've had problems compiling the package myself.
Once you are done upgrading to ghc 7.4.2 these error messages will cease.
Ah, I didn't know it was the [extra] one, it came from [haskell] I thought. Before the upgrade to 7.4.2, pacman complained /usr/bin/haddock already existed on the filesystem, so I rm'd it. Now apparently I have no /usr/bin/haddock, so all these install scripts are complaining. So I installed haskell-haddock from [haskell], but that just provided a /usr/bin/haddock-haddock (??), and the install scripts still complain. I also have a /usr/bin/haddock-ghc-7.4.2 in there from GHC. I can just symlink one of those to /usr/bin/haddock - would you know which one we should be using?
Yeah, it is the [extra] one, but I copied it over to [haskell] to insulate from any changes made to [extra].
The whole situation with haddock is a bit of a mess. On a cleanly installed system it will look like this:
$ ll /usr/bin/haddock*
lrwxrwxrwx 1 root root 17 Jun 11 01:47 /usr/bin/haddock -> haddock-ghc-7.4.2
-rwxr-xr-x 1 root root 225 Jun 11 01:47 /usr/bin/haddock-ghc-7.4.2
-rwxr-xr-x 1 root root 34M Jun 17 14:38 /usr/bin/haddock-haddock
Recreate that symlink and you should be good.
Even with 7.4.2 and everything for haddock appearing to be correct, xmonad-contrib-darcs still wouldn't install.
So, for anyone finding this thread and just wanting to just get past it,
Assuming you can run haddock from the command line without issue,
HADDOCK=which haddock # Those are backticks
Hope that helps.