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.
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.
> 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
>>
>>
>>
>>
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