ronguida:
Monads are undoubtedly more pervasive, and that could be because there aren't as many arrow and comonad tutorials, atomic ones or otherwise.
Moreover, Comonad isn't even in the standard libraries (Hoogle returns no results for it).
When I searched for tutorials on monads, I found lots of them. In fact, I have heard that writing (yet another) monad tutorial is part of standard Haskell initiation.
Regarding the available tutorials, I've collected them under each flavour here: haskell.org/haskellwiki/Blog_articles/Monads Including 42 monad tutorials, 6 arrow tutorials, and 3 comonad tutorials, and 0 on applicative functors. The source for the `standard' (only) comonad library is available from one of the sigfpe tutorials, but really should be on hackage. -- Don