Re: [Haskell-beginners] First steps with Streaming library

28 May 2017

      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  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