The closest thing is the mealy machine type from this library, which is equivalent, although possibly less efficient (and missing the `focus` utility):
https://hackage.haskell.org/package/machines-0.6.1/docs/Data-Machine-Mealy.html <https://hackage.haskell.org/package/machines-0.6.1/docs/Data-Machine-Mealy.html> This is something I've been meaning to write up in a small library at some point, but I keep forgetting. > On Aug 29, 2016, at 4:49 PM, Louis Pan <lo...@pan.me> wrote: > > 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 > <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 <http://pan.me/>> >> 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 >> <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 >>> >>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "Haskell Pipes" group. >>> To unsubscribe from this group and stop receiving emails from it, send an >>> email to haskell-pipe...@ <>googlegroups. <http://googlegroups.com/>com >>> <http://googlegroups.com/>. >>> To post to this group, send email to haskel...@ <>googlegroups.com >>> <http://googlegroups.com/>. >> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Haskell Pipes" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to haskell-pipe...@googlegroups.com <>. >> To post to this group, send email to haskel...@googlegroups.com <>. > > > -- > You received this message because you are subscribed to the Google Groups > "Haskell Pipes" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to haskell-pipes+unsubscr...@googlegroups.com > <mailto:haskell-pipes+unsubscr...@googlegroups.com>. > To post to this group, send email to haskell-pipes@googlegroups.com > <mailto:haskell-pipes@googlegroups.com>. -- You received this message because you are subscribed to the Google Groups "Haskell Pipes" group. 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