Hello, I'm the author of elm-transducers, and I will note that I haven't
used in an actual project, nor have I heard of many people using it. So
it's certainly possible that there could be bug in the elm-transducers
package.
The code you mentioned:
transduceList = transduce List.foldr (::) []
does look unexpected; on reading that, I would expect foldl to be used
there instead. Unfortunately I don't remember if there was a reason I used
foldr. But I do remember that the test cases I used when developing it
were pretty thin since my focus at the time was getting the types to work.
On Sun, Dec 18, 2016 at 7:06 PM, Marshall handheld Flax <
[email protected]> wrote:
> Here's code that has a working "transduceList" variant that uses
> "List.foldl", but all of the List.reverse calls doesn't smell nice. (Full
> context at https://github.com/marshallflax/lexical-elm/blob/
> master/src/BowlingScore.elm). Any suggestions welcome!
>
> transduceListL : Transducer a b (List b) s -> List a -> List b
> transduceListL xform list =
> (transduce List.foldl (::) []) xform list |> List.reverse
>
> partitionBy : (List a -> Bool) -> Transducer a (List a) r (List a)
> partitionBy pred =
> { init =
> \reduce r -> ( [], r )
> , step =
> \reduce input ( hold, r ) ->
> let
> merged =
> input :: hold
> in
> if (pred merged) then
> ( [], reduce (List.reverse merged) r )
> else
> ( merged, r )
> , complete =
> \reduce ( hold, r ) ->
> if (List.isEmpty hold) then
> r
> else
> reduce (List.reverse hold) r
> }
>
> Thanks again!
>
> Marshall
>
>
>
> On Sunday, December 18, 2016 at 9:06:37 PM UTC-5, Marshall handheld Flax
> wrote:
>>
>> Suppose I have a *stateful* transducer
>>
>> partitionBy : (List a -> Bool) -> Transducer a (List a) r (List a)
>> partitionBy pred =
>> { init =
>> \reduce r -> ( [], r )
>> , step =
>> \reduce input ( hold, r ) ->
>> let
>> merged =
>> input :: hold
>> in
>> if (pred merged) then
>> ( [], reduce merged r )
>> else
>> ( merged, r )
>> , complete =
>> \reduce ( hold, r ) ->
>> if (List.isEmpty hold) then
>> r
>> else
>> reduce hold r
>> }
>>
>>
>> that "chunkfies" an input stream input into lists, where the supplid
>> predicate (List a -> Bool) determines when to start the next sublist. (My
>> particular toy application is the standard "Bowling Kata" in which I need
>> to partition a stream of bowling throws into frames, but I think it could
>> be generally useful.)
>>
>> However, the "foldr" in the standard
>>
>> transduceList : Transducer a b (List b) s -> List a -> List b
>>
>> transduceList =
>>
>> transduce List.foldr (::) []
>>
>> means that the stateful processing occurs from the end, which is exactly
>> the opposite from what I need. (Since I need my state to be dependent upon
>> *earlier* messages, rather than *later* messages).
>>
>>
>> Am I making too much of this (i.e. should just use transduceArray and be
>> done with it)? Or is there something problematic about stateful
>> transducers, in that different "folder" can result in different semantics?
>>
>>
>> Thanks!
>>
>> Marshall
>>
>>
>> --
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