you would want to use foldr, not foldl
foldl vs foldr baffles me a bit. in Erlang http://erlang.org/doc/man/lists.html#foldl-3 use of *foldl* is suggested. I got used to this and usually use foldl. another reason is: it appears that in Haskell - same as Erlang - *foldl* enumerates items in "natural" order: in [1,2] 1 is passed to the fn, then 2 *foldr* on the other hand begins with 2 and ends with 1. however: *foldr* arg order: a -> acc is more natural. I very rarely deal with large lists and never (so far) with inifinites. for this case for which I consider and use Alternatives - the list would be 2 - 5 items long. The order though is important.
asum [Just 1, Just 2] Just 1 asum [Nothing, Just 1, Nothing, Just 2, Nothing] Just 1
looks "naturally" ordered: tested left to right. I may not understand the workings (memory and such) of foldl vs foldr however I hope that for small lists it is sufficient to focus on the order of element processing. order matters. This example hopefully confirms that foldr begins @ end, foldl begins @ start. Same as in Erlang ;) Prelude Data.Foldable> *foldr* (\i1 acc1 -> i1 + acc1 * 2) 0 [1,2] 5 Prelude Data.Foldable> *foldl* (\acc1 i1 -> i1 + acc1 * 2) 0 [1,2] 4