foldr doesn't begin anywhere.
> you would want to use foldr, not foldl
foldl vs foldr baffles me a bit.
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]5Prelude Data.Foldable> foldl (\acc1 i1 -> i1 + acc1 * 2) 0 [1,2]4
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