http://www.extremeprogramming.org/rules/simple.html ;-) So IMHO a simple, old-skool '\x -> case x of Left _ -> True; _ -> False' (probably item 2.5 in the list above) is by far the "best" way: It doesn't use any kind of extension, it doesn't pull in a dozen of packages for a trivial task, and is readable by anyone. My personal experience is that things which look very clever and advanced (like using meta/reflection facilities) only increase the WTF factor in the long run. Those facilities have their uses, but they come with a heavy cost which must be justified somehow. Boring code is good code! :-D
+1 for readability That's the problem with Haskell. You can do things in so many beautiful and wondrous ways that you tend to get lost very easily.