I find myself writing things like,
splitListOn :: Eq a => a -> [a] -> [[a]]
splitListOn delimiter = unfoldr splitter . (delimiter :) where splitter [] = Nothing splitter xs = Just (span (/= delimiter) (tail xs))
This is a sort of intersperse-opposite, in that...
myId delimiter = concat . intersperse [delimiter] . splitListBy delimiter
...myId foo turns into a sort of id for lists of the correct type.
With this, and other things, I always have a feeling that it's probably in the standard library somewhere but I didn't notice - maybe because it's abstracted out of recognition, just like "sequence" does Cartesian products for us, but not so that I noticed right away.
Is there a good place to put these things - little things that feel like that maybe they're already in the standard library, and maybe they should be? I'd hate to be unknowingly reinventing the wheel all the time. I don't see an obviously-appropriate page for these on the Haskell Wiki, but maybe I missed it, or maybe I should create one.
I created a page a short while back that I haven't linked in anywhere (so it's currently an orphan page modulo being in CategoryCodeSnippet) called http://www.haskell.org/hawiki/CodeOnTheWiki that links to various parts of the wiki containing good chunks of code. The first three links seem most appropriate. A page about Haskell's "hidden" library functions may be useful. There are many functions that seem to be reimplemented that are, as you well put it, "abstracted out of recognition," to varying degrees and often reimplemented. For example, another is the mplus instance of Maybe which is useful but has no (other) equivalent library function and is often reimplemented (e.g. darcs' or_maybe http://www.haskell.org/pipermail/haskell-cafe/2003-December/005617.html).