thank you Tony and Yitzchak. I may understand it one day ;) it is important to understand it alright. I remember spending half a day over unexpected (for me) results of recursive functions that contained folds. In the end I settled for TChan - a queue with predictable behaviour (my view of it - not necessarily correct). Yitzchak, how about this: Prelude Control.Applicative> *foldl* (<|>) Nothing [Just 1, Nothing, Just 2, Nothing] Just 1 Prelude Control.Applicative> *foldr* (<|>) Nothing [Just 1, Nothing, Just 2, Nothing] Just 1 ? I guess this is due to the nature of (<|>), no? Anyway, I'd use asum instead of fold_ for Alternatives. for non-Alternative cases, Erlang analogy seems to be a useful rule of thumb for foldl & foldr over shorter lists . After all, what matters most is what results to expect. *foldl* and *foldr* may yield different results for the same list & similar processing fn (save for different arg order).