How to feed one of these Streams to a Fold from foldl? On Wednesday, August 26, 2015 at 6:15:25 PM UTC+2, Michael Thompson wrote: > > I uploaded two packages to hackage that may interest readers of this list. > > - https://hackage.haskell.org/package/streaming > - https://hackage.haskell.org/package/streaming-bytestring > > > It's probably a terrible idea! > > `streaming` is an attempt to implement `FreeT` in the style of > `Pipes.Internal`, with a zillion more > associated functions. There is a Prelude especially for the fundamental > 'Producer' case - `Stream ((,) a) m r` and > its iterations, `Stream (Stream ((,)a) m) m r` . Functor-general > operations are in `Streaming` and use a pipes-like > nomenclature of using an `s` to express functor-generality, e.g. maps, > splitsAt, folds etc etc. > The `Streaming.Prelude` uses regular prelude names and replicates > `Pipes.Prelude` and `Pipes.Group` as far as is > possible -- but turning the pipes into functions as you would expect. > > `streaming-bytestring` is just the obviously correct implementation > `Data.ByteString.Lazy` > (but with the same `Pipes.Internal` maneouver.) It tries to follow the > api of the bytestring library > as far as possible, with some us of typical pipes language. Here > > Producer ByteString m r > > as it is used in `Pipes.ByteString`, passes over into the monadic > > ByteString m r > > I'm not sure I've succeeded yet in hiding the implementation in either > case; it is only > in the much more general `streaming` case that there may be some genuine > trouble I am overlooking. > > Strangely I had hit on the idea of naming the strict pair `Of a b` before > seeing the similar attempt > of ertes' `fuse`; it is almost inevitable where you re-express > > Producer a m r > > as > > Stream (Of a) m r > > but I adopted his contructor, `a :> b`. > > I conceived this scheme ages ago, but was bent on using fancy optimization > schemes. When it occurred to > me just to follow Gabriel's method in Pipe.Internal - and that > `Data.ByteString.Lazy` already incorporated > highly optimized versions of the natural Prelude of functions - it was > mostly mechanical. I was amazed by > the speed of the `ByteString m r` operations. (In some places I don't have > the well-thought-out > material from Data.ByteString.Lazy to work with, so there are no doubt > some really bad operations in there!) > > Anyway, part of interest is that it de-pipes (and de-lensifies) some of > the material in > Pipes.Prelude, Pipes.Group and Pipes.ByteString so that you can see what > Gabriel is thinking more clearly. Pipes is incapable of expressing the > distinction between > > ByteString m r > Stream (Of B.ByteString) m r > > and uses the latter to implement the former, which is the basis of much of > the difficulty people > have with the library, for example, the chronic difficulty with the type > of lines, which here appears as > > ByteString m r -> Stream (ByteString m) m r > > exactly corresponding to the type in Data.ByteString.Lazy > > LB.ByteString -> [LB.ByteString] > > The pipes user naturally expects the equivalence > > Producer ByteString m r ~ Stream (Of B.ByteString) m r > > -- since after all that's what it is! -- but Gabriel is systematically > forcing the equivalence > > Producer ByteString m r ~ ByteString m r > > The pipes-group/pipes-bytestring correspondence > > ([a],[b]) ~ Stream (Of a) m (Stream (Of a) m r) ~ > Producer a m (Producer a m r) > (ByteString, ByteString) ~ ByteString m (ByteString m r) ~ > Producer ByteString m (Producer ByteString m r) > [[a]] ~ Stream (Stream (Of a) m) m r ~ > FreeT (Producer a m) m r > [ByteString] ~ Stream (ByteString m) m r ~ > FreeT (Producer ByteString m) m r > > emerges very naturally from the material. (In ertes' library FreeT is > called List, which is perhaps better). > > I implemented some of the shell-like examples from the io-streams tutorial > here https://gist.github.com/michaelt/6c6843e6dd8030e95d58 > The Streaming.Prelude module could use a tutorial, but the little ghci > examples in the haddocks might be of use. > > Again, properly arranged, they might operate as a sort of preliminary > tutorial for pipes-group and pipes-bytestring, I don't know. > > yours Michael > > > >
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