> They will compile successive transformations (i.e. multiple
> maps/mapMs/filters, for example) into a single pass over the data.
> This is true for the
> `streaming`/`pipes`/`list-transformer`/`logict`/`conduit`/`turtle`
> libraries or any "ListT-done-right" implementation. All of these
> libraries do not rely on any special GHC optimizations or rewrite
> rules. They will still go over the data in one pass even if you
> disable all optimizations.
So, this is so, because of implementation of their Functor and
Applicative instances? Am I right, that there occurs this combination
("single pass")?
But what about Data.List and Prelude's `map` (in GHC.Base)? If I'll
do
map fn1 (map fn2 lst)
?
>
> This is *not* true for `Control.Monad.mapM` or the `ListT` type from
> `transformers` (a.k.a. "ListT done wrong"), which is why I wanted to
> make sure which `mapM` you were talking about.
OK, I got it.
>
> > On Jun 7, 2017, at 12:32 AM, aqua...@gmail.com wrote:
> >
> > 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, aqu...@ <>gmail.com
> >> <http://gmail.com/> 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
> >>
> >>
> >>
> >>
> >> --
> >> You received this message because you are subscribed to the Google
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> >> group, send email to haskel...@ <>googlegroups.com
> >> <http://googlegroups.com/>.
> >
> >
> > --
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>
--
Best regards,
Paul a.k.a. 6apcyk
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