Hi Gabriel, thank you for taking the time to answer my questions.
Is your Scan encoding available anywhere? The closest thing I can find is
ekmett's "strict Mealy Machine" at
https://hackage.haskell.org/package/folds-0.7.1/docs/Data-Fold-L1'.html but
it looks quite different.
On Saturday, 27 August 2016 02:15:25 UTC+10, Gabriel Gonzalez wrote:
>
> Yeah, what you wrote looks correct. The general idea is that you compose
> `Fold`s and then convert to a `Pipe` at the last minute.
>
> There's another related type which also has nice properties for this sort
> of thing, which is basically a mealy machine. I usually encode it like
> this:
>
> {-# LANGUAGE ExistentialQuantification #-}
>
> data Scan a b = forall m . Monoid m => Scan { begin :: s, step :: s -> a
> -> (s, b) }
>
> instance Category Scan where ...
> instance Arrow Scan where ...
> instance Functor (Scan a) where ...
> instance Applicative (Scan a) where ...
>
> focus
> :: (forall x. LensLike (State x) s t a b)
> -> Scan a b
> -> Scan s t
> focus k (Scan s f) = Scan s (Control.Lens.mapAccumLOf k f)
>
>
> Then you can use `focus traverse` to lift a `Scan` to work on anything
> that is `Traversable` (like `ZipList`):
>
> focus traverse :: Traversable t => Scan a b -> Scan (t a) (t b)
>
>
> On Aug 26, 2016, at 3:37 AM, Louis Pan <lo...@pan.me <javascript:>> wrote:
>
> I think I understand what you mean by using Control.Fold.Foldl.
>
> It's not possible to "applicatively" lift a Pipe from "Pipe a b m r" to
> "Applicative f => Pipe (f a) (f a) m r", but it is possible to lift a
> Control.Foldl.Fold from "Fold a b" to "Applicative f => Fold a b -> Fold (f
> a) (f b)" (see below).
>
> This means it's better in my case to code the consumer as a Fold (and then
> convert it to a Pipes Parse), as opposed to a Pipe Consumer.
>
> -- requires a function to force the applicative to prevent space leaks.
> liftFold :: Applicative f => (forall x. f x -> f x) -> L.Fold a b ->
> L.Fold (f a) (f b)
> liftFold f (L.Fold step begin done) = L.Fold step' begin' done'
> where
> step' xs as = f (step <$> xs <*> as)
> begin' = pure begin
> done' xs = done <$> xs
>
> forceZipList :: ZipList a -> ZipList a
> forceZipList (ZipList xs) = ZipList (forceFoldable xs)
>
> -- https://ro-che.info/articles/2015-05-28-force-list
> forceFoldable :: Foldable t => t a -> t a
> forceFoldable xs = case foldr seq () xs of
> () -> xs
>
> liftZipList :: L.Fold a b -> L.Fold (ZipList a) (ZipList b)
> liftZipList = liftFold forceZipList
>
> I've confirmed this prevents space leaks by profiling with "+RTS -h -p"
> and using hp2ps.
>
> However, I'm a novice with the significance of using seq and $!.
>
> Am I doing anything dangerously? Is there something I should be careful of?
>
>
> On Wednesday, 24 August 2016 20:16:29 UTC+10, Louis Pan wrote:
>
>> My code doesn't stream properly - it forces consumption of the original
>> producer before outputting anything. :(
>>
>> On Wednesday, 24 August 2016 04:24:03 UTC+10, Louis Pan wrote:
>>>
>>> I managed to convert between `P.Producer (ZipList a) m r` and `ZipList
>>> (P.Producer a m ()` using the StateT trick used in Pipes.Parse
>>>
>>> Does the following code make sense? Am I breaking any laws?
>>>
>>> import Control.Applicative
>>> import Control.Monad.Trans.Class
>>> import Control.Monad.Trans.State.Strict
>>> import qualified Pipes as P
>>> import qualified Pipes.Lift as PL
>>>
>>> -- ZipList Traversable is only in 4.9.0.0
>>> sequenceAZipList :: Applicative f => ZipList (f a) -> f (ZipList a)
>>> sequenceAZipList xs = ZipList <$> sequenceA (getZipList xs)
>>>
>>> -- | Similar to Pipes.Parse.Parser, except it stores a ZipList of
>>> Producers.
>>> type ZipParser a m r = forall x . StateT (ZipList (P.Producer a m x)) m r
>>>
>>> -- | Draw one element from each underlying Producer, returning 'Nothing'
>>> if any of the producers are empty
>>> drawZ :: Monad m => ZipParser a m (Maybe (ZipList a))
>>> drawZ = do
>>> ps <- get
>>> rs <- lift (sequenceAZipList (P.next <$> ps))
>>> case sequenceAZipList rs of
>>> Left _ -> pure Nothing
>>> Right rs' -> do
>>> put $ snd <$> rs'
>>> pure . Just $ fst <$> rs'
>>>
>>> -- | Push back a Ziplist element onto the underlying ZipList of Producers
>>> unDrawZ :: Monad m => ZipList a -> ZipParser a m ()
>>> unDrawZ as = modify (\ps -> appendA <$> ps <*> as)
>>> where
>>> appendA p a = do
>>> r <- p
>>> P.yield a
>>> pure r
>>>
>>> toZipList :: Monad m => P.Producer (ZipList a) m r -> m (ZipList
>>> (P.Producer a m ()))
>>> toZipList p = execStateT (toZipParser p) (pure (pure ()))
>>> where
>>> toZipParser :: Monad m => P.Producer (ZipList a) m r -> ZipParser a m r
>>> toZipParser p' = do
>>> r <- lift $ P.next p'
>>>
>>>
>>> On Tuesday, 23 August 2016 15:01:54 UTC+10, Louis Pan wrote:
>>>>
>>>> Sorry, could you please elaborate? I'm a bit slow.
>>>>
>>>> How do I use Control.Foldl.Fold to convert `Pipes a b m r` into `Pipes
>>>> (ZipList a) (ZipList b) m r` ?
>>>>
>>>>
>>>> On Tuesday, 23 August 2016 13:20:24 UTC+10, Gabriel Gonzalez wrote:
>>>>>
>>>>> I would do what you're already doing with `Control.Foldl.Fold`.
>>>>> That's the approach I usually recommend for this.
>>>>>
>>>>> On Aug 22, 2016, at 6:45 PM, Louis Pan <lo...@pan.me> wrote:
>>>>>
>>>>> One approach I can think of is the use Pipes.Concurrent to manually
>>>>> split the ZipList and then combine then results. Is there a nicer way?
>>>>>
>>>>> On Tuesday, 23 August 2016 11:35:50 UTC+10, Louis Pan wrote:
>>>>>>
>>>>>> Say I have a `Pipes a b m r` called 'p'.
>>>>>>
>>>>>> How do I convert it to a `Pipes (ZipList a) (ZipList b) m r`?
>>>>>> That is, I want to apply the pipe 'p' and apply zip-wise to an input
>>>>>> ziplist, without losing the state of each pipe in the ziplist.
>>>>>>
>>>>>> The reason I want this, is that I want to run multiple attoparsec
>>>>>> parsers in parallel in one pass of a file.
>>>>>>
>>>>>> My approach for this is to use Control.Foldl to combine the
>>>>>> attoparsec parsers into a `Control.Foldl.Fold ByteString
>>>>>> [PartialParseResult]`, then convert it (using Control.Foldl.purely
>>>>>> Pipes.Prelude.scan) to a `Pipe ByteString [PartialParseResult]`
>>>>>>
>>>>>> My other requirement is that I want to look at intermediate parser
>>>>>> output (eg. each line of a file as it's parsed), in order to summarise
>>>>>> each
>>>>>> line, in a stateful way as I'm keeping track of the current line number.
>>>>>>
>>>>>> My approach for this is to use Control.Foldl.Fold for the summary
>>>>>> logic, and then convert it to a pipe. I am able to create a `Pipe
>>>>>> PartialParseResult AnalysisResult m r` for this.
>>>>>>
>>>>>> However, how do I connect a `Pipe ByteString [PartialParseResult] m
>>>>>> r` to a `Pipe PartialParseResult AnalysisResult m r`?
>>>>>>
>>>>>> Regards,
>>>>>>
>>>>>> Louis
>>>>>>
>>>>>>
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>>>>>
>>>>>
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