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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