Hello, Gabriel!
Real module (which is compilable) is big, so I'll show on email's top
extraction from it (little snippets, with imports and functions) and will
add compilable module (I used it for experiments) at the end of the email -
because it's not very small :) So, compilable code is on the bottom of the
mail.
Snippets:
...
import Streaming
import qualified Streaming.Prelude as S
...
(|>) = flip ($)
...
flow = items
|> S.mapM (liftIO . doRequest connection)
|> S.map (liftM fun1)
|> S.zip fun2
|> S.filter filter1
|> S.mapM fun3
...
So, I supposed that serial "map"s/"zip"s/"filter"s will be compiled into
one loop with serial application of functions-arguments of that
"map"s/"zip"s/"filter"s. My Python background "says" me that
maps/filters/etc are types, not functions, so they are "combinatorable": I
mean no problem to combine serial flat iterations into one iteration. I am
newbie in Haskell and I don't know is it true for Haskell... may be it
should happens on optimization phase of compilation, I don't know.
Does it happen automatically, on GHC side, or developer should take special
actions when works with Streaming/Pipes/Conduits? Is it the same for usual
maps/filters from Prelude/Data.List? I supposed 99% that GHC reduces ASTs
then optimizes the result and we get at the end flat simple iterations and
so on, and developer should not think about such things totally. But I had
a doubt crept in, so I decided to ask...
Another example is:
where flow = getItems connection
|> S.filter getInteresting
|> S.map fun1
|> fixItems fun2
|> S.mapM fun3
|> S.mapM fun4
...
where *fixItems* iterates over stream's items and "yields" them with
*S.yield*, like this:
...
import qualified Control.Monad.Trans.State as ST
import qualified Control.Monad.Trans.Writer as W
...
fixItems :: Monad m => FixItem e ps st -> Stream (Of e) (ST.StateT st m)
Result -> Stream (Of e) (ST.StateT st m) Result
fixItems fi = loop where
loop str = do
e <- lift $ S.next str
e' <- case e of
Left err -> return err
Right (e', str') ->
let (fixes, problems) = W.runWriter $ fixItem fi e'
fix = mconcat fixes
in (lift $ ST.modify $ reportProblems fi problems e')
>> case fix of
ItemFix ff -> (S.yield $ ff e')
ItemSkip -> pure ()
>> loop str'
return NoResult -- FIXME how to return stream result?
...
*Next is compilable module*. I experimented with "fixing" of stream's
items; code is terrible, I suppose, but it was experiment ;):
{-# LANGUAGE FlexibleContexts #-}
module Main where
import Control.Monad
import Control.Monad.Trans.Reader
import Control.Monad.Trans.State
import Control.Monad.Trans.Writer
import Data.Functor.Identity
import Data.Traversable
import Streaming
import qualified Streaming.Prelude as S
type M = StateT [String] IO
subgen :: Int -> S.Stream (S.Of Int) M Int
subgen n =
if odd n then do { return 0 }
else do
S.yield (n*100)
S.yield (n*1000)
lift $ modify (++["!!!"])
return 0
gen :: S.Stream (S.Of Int) M Int
gen = do
S.yield 0; S.yield 100; S.yield 101; S.yield 102; S.yield 103; S.yield
104; S.yield 105; S.yield 106
liftIO $ putStrLn "enter x: "
x <- liftIO getLine
let n = read x::Int
S.yield n
lift $ modify (++["gen1"])
lift $ modify (++["gen2"])
return 0 -- $ do
proc1 :: S.Stream (S.Of Int) M Int -> S.Stream (S.Of Int) M Int
proc1 str = do
st <- lift get
loop str st
where
loop str st = do
e <- lift $ S.next str
e' <- case e of
Left err -> return err
Right (e', str') ->
(if e' == 100 then (lift $ put $ st ++ ["proc1"]) else pure ()) >>
subgen e' >> (S.yield $ e' + 123) >> loop str' st
return 1
data Cr = Cr {
crName :: String
, crAge :: Int
} deriving Show
data FixItem a = FixItem (a -> a) | SkipItem
instance Monoid (FixItem a) where
mempty = FixItem id
mappend SkipItem _ = SkipItem
mappend _ SkipItem = SkipItem
mappend (FixItem f) (FixItem g) = FixItem (f . g)
fixWhen :: Monad m => Bool -> m (FixItem a) -> m (FixItem a)
fixWhen cond act = if cond then act else return $ FixItem id
fixcr1 :: Int -> Writer [String] (FixItem Int)
fixcr1 n =
let (fixes, errs) = runWriter $ sequence [
fixWhen (n == 100) (tell ["panic:"++show n++"==100"] >> return
SkipItem)
, fixWhen (n > 100) (tell ["err1:"++show n ++">100"] >> return
(FixItem (10+)))
, fixWhen (n > 1) (tell ["err2:"++show n++">1"] >> return (FixItem
(100+)))
]
in
writer (mconcat fixes, errs)
fixItems :: (Monad m, Num a) => Stream (Of Int) (StateT [String] m) a ->
Stream (Of Int) (StateT [String] m) a
fixItems = loop where
loop str = do
e <- lift $ S.next str
e' <- case e of
Left err -> return err
Right (e', str') ->
let (fix, errs) = runWriter $ fixcr1 e'
in (lift $ modify (++errs))
>> case fix of
FixItem ff -> (S.yield $ ff e')
SkipItem -> pure ()
>> loop str'
return 1
(|>) = flip ($)
main :: IO ()
main = do
p <- runStateT flow []
print p
print "end."
where flow = gen
|> fixItems
|> S.map (+100)
|> S.mapM_ (liftIO . print)
and to build it I included in cabal 2 dependencies:
...
, transformers >= 0.5
, streaming
...
/Best regards, Paul
Could you provide an example that compiles, including imports? The reason
> I ask is that I'm not sure which mapM that you are using
>
> On Jun 6, 2017, at 1:53 AM, [email protected] <javascript:> wrote:
>
> Hello, everyone. Because I'm newbie, my question may seem naive. but:
> I'm doing something like:
> do
> str' <- S.mapM fun1 str
> str'' <- S.mapM fun2 str'
> -- so on
> and I suppose that real iteration (if I use `mapM` or `iterM`, map other
> functions of streaming) *happens only one* and resulting code after
> compilation will looks like
> for item in str:
> item' = fun1 item
> item'' = fun2 item'
> -- so on
> Am I right? Or are there some pitfalls here which we should remember to
> accomplish such result?
>
>
> /Best regards, Paul
>
>
>
>
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