I seem to be forever writing code that looks like this: decode :: String -> (SKI,String) decode (c:cs) = case c of 'S' -> (S,cs) 'K' -> (K,cs) 'I' -> (I,cs) '*' -> let (e0,cs0) = decode cs; (e1,cs1) = decode cs1 in (e0 :@: e1, cs1) In other words, lots and lots of functions with signatures like foo :: [Foo] -> (Bar,[Foo]) that likes to call itself recursively, or that gets called from somewhere else. Is there a Better Way(tm) to do this? It looks like such a common idiom that you'd expect to see "something" in the libraries somewhere. (I remember being puzzled that there was no library function for creating a Cartesian product of two lists. Puzzled until I realised that the monadic nature of lists make it utterly trivial to do this by hand anyway! So it's not always obvious to know what you're looking for...)