f for a functor is also very common, as is k for "continuations", very generally speaking (I think this is a borrowing from Lisps?): one often sees monadic bind defined as

    m >>= k = ...

In a somewhat more esoteric vein, I love the use of w for comonads: the rationale is apparently that it looks like an upside-down m.

> Of course, Edward Kmett had a story about winding up with his code telling him `i m a s t a b u`...

Is that an indexed lens ... parameterized over a monadic type? I'd like a link to the video/article.