[Haskell-beginners] First steps with Streaming library

[aquagnu]

I'll try to clarify the question. Sure I can traverse the stream items
with function like `a -> m b` applying it with `mapM`. But in this case
I will not have access to return of previous stream node, which I'm
planning to use as global state for whole pipe workflow:

     .--state-------+--state'--------+--state''-->
     |              |                |
  [e0..eN] ==> [e0'...eN'] ==> [e0''..eN''] =====>

This "state" will be used for statistics, errors, whatever - through
the whold workflow. This "pipe" will iterate over `eN` items (which
will be streams too), concatenates results... How this can be achieved
with Streaming library? I mean each "node" should have access to stream
items but to "global" state (result of prev. node return?) too.

Is it possible?


В Sat, 27 May 2017 18:30:28 +0300
aquagnu <aquagnu at gmail.com> пишет:

> I'm trying to start working with Streaming package. First attempt
> is to simulate source of stream items (I call the function "gen")
> and some processor ("proc"). Both should be stateful: to be able
> to sabe some info about processing steps, etc.
> 
>   ...
>   import Streaming
>   import qualified Streaming.Prelude as S
>   ...
> 
>   gen :: S.Stream (S.Of Int) IO [String]
>   gen = do
>     S.yield 1000
>     S.yield 2000
>     x <- lift getLine
>     return ["a", "b", "c", x] -- results
> 
>   proc :: S.Stream (S.Of Int) IO [String] -> S.Stream (S.Of Int) IO
> [String] proc str = do
>     e <- str
>     lift $ print "Enter x:"
>     x <- lift getLine
>     return $ e ++ [" -- " ++ x] -- put stream items in result
>   
>   main :: IO
>   main = do
>     s <- S.mapM_ print $ S.map show gen
>     p <- S.mapM_ print $ proc gen
>     putStr "s: " >> print s
>     putStr "p: " >> print p
> 
> And I try to simulate "piping" between "gen" and "proc", seems that
> function application is enought to compose producers and consumers.
> But this snippet is not correct: "e <- str" extracts element not from
> stream, but from results (of "gen"). Even more, I don't know how to
> "yield" new items, based on stream items. Something like "await" of
> Conduit, or like in Python "for e in str: ... yield modified(e)...".
> Is it possible to do it with "do" notation?
> 
> ---
> Best,
>   Paul


-- 
Best regards,
  Paul a.k.a. 6apcyk