[Haskell-beginners] Alternative

[Tony Morris]

foldr doesn't begin anywhere.

https://vimeo.com/64673035


On 29/12/16 06:43, Imants Cekusins wrote:
> > 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
>
>
>
>
>
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