I would disagree with "not expected in general." In general -- when a and b are different types -- Elm's API design guidelines should set you up to always expect a -> b -> b and never b -> a -> b. If the definition of foldl were changed to take the latter, it would be the only exception to this expectation.
On Fri, Dec 9, 2016 at 7:03 AM, Kasey Speakman <kjspeak...@gmail.com> wrote: > Ok, correction > > List.foldl (-) 0 [1, 2, 3] > -- returns 2 > -- expands to 3 - (2 - (1 - 0)) = 2 > > During my testing last night, I had a typo (foldr instead of foldl) when I > was testing the expansions. That was the center-building behavior. > > Using the form a -> b -> b is right-building regardless of the order the > list is traversed. Traversing from head to tail is equivalent to reversing > the list and building right. This is obviously broken for left-associative > only operations and not expected in general. > > On Friday, December 9, 2016 at 8:44:25 AM UTC-6, Kasey Speakman wrote: >> >> Sorry, that last bit was an example of what happens in Elm when folding >> with string concat (++). That's unexpected behavior from a left fold. >> >> List.foldl (++) "" ["The ", "quick ", "brown "] -- returns "brown quick >> The " >> >> On Friday, December 9, 2016 at 8:26:17 AM UTC-6, Kasey Speakman wrote: >>> >>> You're confusing pipe's syntax and infix. Pipe is defined like this: >>> >>> (|>) x f = f x >>> >>> And used like this >>> >>> x |> f == f x >>> >>> So pipe has an inherent flip because it is used to chain otherwise >>> right-building statements. >>> >>> e.g. >>> >>> List.sum (List.filter isOdd [1, 2, 3]) >>> >>> vs >>> >>> [1, 2, 3] >>> |> List.filter isOdd >>> |> List.sum >>> >>> Pipe is inherently right-building, so operations like subtract or string >>> concatenation are not suitable for it since they are only left associative. >>> >>> List.foldl (++) "" ["The ", "quick ", "brown "] -- returns "brown quick >>> The " >>> >>> On Friday, December 9, 2016 at 1:05:56 AM UTC-6, Aaron VonderHaar wrote: >>>> >>>> What's confusing here is how currying works with infix operators. It's >>>> idiomatic in Elm to have your accumulator be the last argument, and, for >>>> instance, if you were writing your own data type, you would want to write >>>> your functions so that they can be chained together easily: >>>> >>>> myMatrix >>>> |> scale 2 >>>> |> subtract 5 >>>> |> subtractMatrix myOtherMatrix >>>> |> normalize >>>> >>>> >>>> But as an infix operator (-) is not able to follow that convention; >>>> >>>> 5 >>>> |> (-) 3 >>>> |> (-) 1 >>>> >>>> is confusingly equivalent to `(1 - (3 - 5))` rather than to `5 - 3 - 1` >>>> >>>> >>>> If you had a function `subtract` such that >>>> >>>> 5 |> subtract 3 |> subtract 1 == (5 - 3 - 1) >>>> >>>> then you could use that function with fold as you intend >>>> >>>> List.foldl subtract 0 [1, 2, 3, 4] == -10 >>>> >>>> You can achieve the same result with >>>> >>>> List.foldl (flip (-)) 0 [1, 2, 3, 4] == -10 >>>> >>>> >>>> Another way to put it is, in Elm, folds expand in the following way: >>>> >>>> List.foldl f x [b, c, d] == x |> f b |> f c |> f d >>>> List.foldr f x [b, c, d] == f b <| f c <| f d <| x >>>> >>>> >>>> On Thu, Dec 8, 2016 at 7:50 PM, Kasey Speakman <kjspe...@gmail.com> >>>> wrote: >>>> >>>>> (deleted and corrected original post with proper expansion of Elm's >>>>> foldl) >>>>> >>>>> I know this is a really old thread, but I ran into this precise >>>>> question and thought I would add a perspective. >>>>> >>>>> The form a -> b -> b is not left-building, regardless of the direction >>>>> you are traversing the list. >>>>> >>>>> An example: Starting from zero, subtract the numbers 1, 2, and 3. The >>>>> expected answer is -6. >>>>> >>>>> List.foldl (-) 0 [1, 2, 3] >>>>> -> returns -6 in Haskell (well, actually tested in F# which uses same >>>>> order as Haskell) >>>>> expands to: ((0 - 1) - 2) - 3 = -6 >>>>> -> returns 2 in Elm >>>>> expands to: 3 - ((1 - 0) - 2) >>>>> >>>>> Elm's expansion is wonky for this. It appears to be center-building: >>>>> List.foldl (-) 0 [1] -- returns 1, expands 1 - 0 >>>>> List.foldl (-) 0 [1, 2] -- returns -1, expands (1 - 0) - 2 >>>>> List.foldl (-) 0 [1, 2, 3] -- returns 2, expands 3 - ((1 - 0) - 2) >>>>> List.foldl (-) 0 [1, 2, 3, 4] -- returns -2, expands (3 - ((1 - 0) >>>>> - 2)) - 4 >>>>> >>>>> When a and b are the same type it will only return the correct answer >>>>> if the fold operation is also commutative or if flip is used to >>>>> correct the ordering. When a and b are not the same type, the compiler >>>>> will >>>>> provide an error for wrong ordering of course. >>>>> >>>>> I started out on the side that a -> b -> b was correct as that feels >>>>> like proper "reduction" or chainable syntax. But after exploring it, it is >>>>> clearly not left-building. Makes sense when you consider this form is used >>>>> with pipe to convert right-building operations into left-reading code. >>>>> e.g. a >>>>> |> f |> g |> h instead of h (g (f a)) >>>>> >>>>> On Tuesday, July 16, 2013 at 6:13:01 AM UTC-5, Evan wrote: >>>>>> >>>>>> Gotcha, I definitely see the reasoning :) >>>>>> >>>>>> >>>>>> On Tue, Jul 16, 2013 at 12:54 PM, Balazs Komuves <bkom...@gmail.com> >>>>>> wrote: >>>>>> >>>>>>> >>>>>>> I was not engaging in debate, religious or not (though I tend to >>>>>>> have very strong opinions about these questions). I was explaining why I >>>>>>> think Haskell uses the order it uses (because it is distinguished from a >>>>>>> mathematical viewpoint). Of course you are not required to follow that >>>>>>> convention, I was just pointing out that it is not simply an ad-hoc >>>>>>> choice. >>>>>>> >>>>>>> Balazs >>>>>>> >>>>>>> >>>>>>> >>>>>>> On Tue, Jul 16, 2013 at 12:21 PM, Evan Czaplicki <eva...@gmail.com> >>>>>>> wrote: >>>>>>> >>>>>>>> I think this might be a religious debate on some level. My first >>>>>>>> functional languages were Scheme >>>>>>>> <http://docs.racket-lang.org/reference/pairs.html#(def._((lib._racket/private/list..rkt)._foldl))> >>>>>>>> and Standard ML <http://www.standardml.org/Basis/list.html>. The >>>>>>>> libraries I just linked both use the same argument order for foldl and >>>>>>>> foldr as in Elm. I was raised a certain way and it just stuck in my >>>>>>>> mind. I >>>>>>>> suspect that everyone prefers the order they learned first because it >>>>>>>> matches their mental model. >>>>>>>> >>>>>>>> I wrote up a bunch of "reasoning", but really, I am just engaging >>>>>>>> in the religious debate. I'd feel bad deleting it all though, so here >>>>>>>> is >>>>>>>> some of it: >>>>>>>> >>>>>>>> OCaml's list library >>>>>>>> <http://caml.inria.fr/pub/docs/manual-ocaml/libref/List.html> does >>>>>>>> it the way you suggest. I find this order offensive on some level. >>>>>>>> >>>>>>>> The big questions for "physical" argument order are as follows: >>>>>>>> >>>>>>>> - What is the type of `fold` or `reduce`? When you fold an >>>>>>>> unordered thing, is it from the right or the left? >>>>>>>> - What is the type of `foldp`? Which way does time go? Is this >>>>>>>> cultural? >>>>>>>> >>>>>>>> I don't find these questions particularly useful, and I don't think >>>>>>>> programmers should have to wonder about them to use fold and foldp. >>>>>>>> >>>>>>>> At the end of the day, I chose the types on purpose. I find them >>>>>>>> easier to use, easier to teach, easier to understand. I want to keep >>>>>>>> them >>>>>>>> this way. >>>>>>>> >>>>>>>> >>>>>>>> On Tue, Jul 16, 2013 at 10:40 AM, Balazs Komuves <bkom...@gmail.com >>>>>>>> > wrote: >>>>>>>> >>>>>>>>> >>>>>>>>> The Haskell version of the foldl is the "right one" in the >>>>>>>>> following sense: >>>>>>>>> >>>>>>>>> foldl makes sense in general for left-associative operators, and >>>>>>>>> foldr makes sense for right-associative operators. >>>>>>>>> Left-associative operators must have the type (a -> b -> a), while >>>>>>>>> right-associative operators must have type (a -> b -> b). >>>>>>>>> >>>>>>>>> I think the fact that you cannot change a foldr to foldl without >>>>>>>>> changing the types is actually an advantage: it forces you to think >>>>>>>>> about >>>>>>>>> which version is the "proper" one, and you cannot accidentally do the >>>>>>>>> wrong >>>>>>>>> one. Of course sometimes it can be inconvenient. >>>>>>>>> >>>>>>>>> What I somewhat dislike in the Haskell version of foldr (not >>>>>>>>> foldl), is that while >>>>>>>>> >>>>>>>>> (foldl . foldl . foldl) etc makes sense, (foldr . foldr) does not; >>>>>>>>> for that to work you would have to flip the last two arguments: >>>>>>>>> >>>>>>>>> myfoldr :: (a -> b -> b) -> ([a] -> b -> b) >>>>>>>>> myfoldr f xs y = foldr f y xs >>>>>>>>> >>>>>>>>> But the practicality of this change is debatable, I guess. >>>>>>>>> >>>>>>>>> Balazs >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> On Wed, Jul 10, 2013 at 4:38 PM, Evan Czaplicki <eva...@gmail.com> >>>>>>>>> wrote: >>>>>>>>> >>>>>>>>>> It's partly about composability (i.e. the data structure should >>>>>>>>>> be last). >>>>>>>>>> >>>>>>>>>> It is also about reuse. In Elm it is valid to say: >>>>>>>>>> >>>>>>>>>> foldl (::) [] >>>>>>>>>> foldr (::) [] >>>>>>>>>> >>>>>>>>>> If I want to change the order of my traversal, I should not >>>>>>>>>> *also* need to change the definition of mildly related functions >>>>>>>>>> or start using flip on things. >>>>>>>>>> >>>>>>>>>> Finally, once you know that the accumulator is always the second >>>>>>>>>> argument, you do not have to look at docs anymore. Even now I forget >>>>>>>>>> the >>>>>>>>>> order of arguments in Haskell's folds and need to look it up. >>>>>>>>>> >>>>>>>>>> I first learned this way from Standard ML >>>>>>>>>> <http://www.standardml.org/Basis/list.html>, and it is my >>>>>>>>>> favorite by far. >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> On Wed, Jul 10, 2013 at 4:12 PM, Tim hobbs <tim.t...@gmail.com> >>>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>>> Well, elm's ordering is more useful. For example, I recently >>>>>>>>>>> had a case where I wrote: >>>>>>>>>>> >>>>>>>>>>> let >>>>>>>>>>> irrelivantFuncitonName fold = fold blabla default list >>>>>>>>>>> in >>>>>>>>>>> irrelivantFunctionName foldl + irrelivantFuncitonName foldr >>>>>>>>>>> >>>>>>>>>>> In Haskell, the same example ends up being >>>>>>>>>>> >>>>>>>>>>> let >>>>>>>>>>> irrelivantFuncitonName fold = fold blabla default list >>>>>>>>>>> in >>>>>>>>>>> irrelivantFunctionName foldl + irrelivantFuncitonName (\f d l-> >>>>>>>>>>> foldr (\a b->f b a) d l) >>>>>>>>>>> >>>>>>>>>>> Tim >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> On Wednesday, July 10, 2013 4:03:23 PM UTC+2, Zsombor Nagy wrote: >>>>>>>>>>>> >>>>>>>>>>>> Hi! >>>>>>>>>>>> >>>>>>>>>>>> I wonder why is the foldl in Elm and in Haskell calling the >>>>>>>>>>>> binary operator with arguments in a different order? >>>>>>>>>>>> >>>>>>>>>>>> foldl (\t acc -> acc + 1) 0 [1, 1, 1, 1, 1, 1] >>>>>>>>>>>> haskell: 2 >>>>>>>>>>>> Elm: 6 >>>>>>>>>>>> >>>>>>>>>>>> For me the haskell way seems more straightforward, but maybe >>>>>>>>>>>> that "optimal composibility guideline" makes this turn around? >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> zs >>>>>>>>>>> >>>>>>>>>>> -- >>>>>>>>>>> You received this message because you are subscribed to the >>>>>>>>>>> Google Groups "Elm Discuss" group. >>>>>>>>>>> To unsubscribe from this group and stop receiving emails from >>>>>>>>>>> it, send an email to elm-discuss...@googlegroups.com. >>>>>>>>>>> For more options, visit https://groups.google.com/groups/opt_out >>>>>>>>>>> . >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>> >>>>>>>>>> -- >>>>>>>>>> You received this message because you are subscribed to the >>>>>>>>>> Google Groups "Elm Discuss" group. >>>>>>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>>>>>> send an email to elm-discuss...@googlegroups.com. >>>>>>>>>> For more options, visit https://groups.google.com/groups/opt_out. >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> >>>>>>>>> >>>>>>>>> -- >>>>>>>>> You received this message because you are subscribed to the Google >>>>>>>>> Groups "Elm Discuss" group. >>>>>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>>>>> send an email to elm-discuss...@googlegroups.com. >>>>>>>>> For more options, visit https://groups.google.com/groups/opt_out. >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>> >>>>>>>> -- >>>>>>>> You received this message because you are subscribed to the Google >>>>>>>> Groups "Elm Discuss" group. >>>>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>>>> send an email to elm-discuss...@googlegroups.com. >>>>>>>> For more options, visit https://groups.google.com/groups/opt_out. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> -- >>>>>>> You received this message because you are subscribed to the Google >>>>>>> Groups "Elm Discuss" group. >>>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>>> send an email to elm-discuss...@googlegroups.com. >>>>>>> For more options, visit https://groups.google.com/groups/opt_out. >>>>>>> >>>>>>> >>>>>>> >>>>>> >>>>>> -- >>>>> You received this message because you are subscribed to the Google >>>>> Groups "Elm Discuss" group. >>>>> To unsubscribe from this group and stop receiving emails from it, send >>>>> an email to elm-discuss...@googlegroups.com. >>>>> For more options, visit https://groups.google.com/d/optout. >>>>> >>>> >>>> -- > You received this message because you are subscribed to the Google Groups > "Elm Discuss" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to elm-discuss+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Elm Discuss" group. 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