You can directly edit Haskell in Xcode (with very basic syntax highlighting)
Here's how I got (basic) syntax highlighting for Haskell in Xcode:
I should start by saying that unlike my other post, this is totally unsupported and that this involves modifying Xcode's bundle. While it mainly just involves adding a plist file to the bundle and modifying an existing plist, it is still potentially dangerous and you will likely have to re-do it every time you update Xcode. You have been warned.
Also, this only adds syntax coloring for keywords, strings, characters, and numbers. Unfortunately, it doesn't do coloring of variables, types, and other identifiers. Xcode doesn't generally color operators, though you could add the built-in ones to the list of keywords if you want it to also color them.
Given all that, if you still want to proceed, here's what you need to do:
1) Copy Haskell.xclangspec (included at the bottom of this email) into /Applications/Xcode/Contents/SharedFrameworks/DVTFoundation.framework/Versions/A/Resources/. (Assuming you've installed Xcode into /Applications/.) This will require admin privileges on the machine you're running Xcode on.
2) Save the following as "~/Desktop/Haskell.plist" (or wherever is convenient for you) :
and put it somewhere safe in case you mess it up in the next step.
4) Using PlistBuddy, add the above into DVTFoundation.xcplugindata (in the same directory as the stuff in step 1), by doing the following on the command-line (this should all be one line) :
You may need to use "sudo" to run the above command. (In other words type "sudo /usr/libexec/PlistBuddy …<rest of above command>…") Note that if you saved the above file somewhere other than your desktop, you'll need to change the path in the "Merge" command to match wherever you put it. If you somehow messed it up and need to undo it, you can use the following command to remove what you just added:
Again, you'll probably need to use 'sudo' to run that command, too.
5) Once you've done the above, you'll notice Xcode's "Editor" > "Syntax Coloring" menu now includes "Haskell" as an option. Simply open your Haskell source file and choose "Haskell" from the "Syntax Coloring" submenu!
Here is the Haskell.xclangspec. It's probably not perfect. I created it by looking at the C and Fortran .xclangspec files and copying their format. I used the information from the Haskell Language Spec <http://www.haskell.org/haskellwiki/Language_and_library_specification>. If you have suggestions for changes, let me know, or just post them: