[Haskell-cafe] 'Progress bar' enumeratee

[David Hotham]

Very interesting - thanks!


"Gregory Collins" <greg at gregorycollins.net> wrote in message 
news:BANLkTikEh7aQfe2-JDq6DoCZ+SYw5MJ5Lg at mail.gmail.com...
> My enumerator style may not be the best (I'm long-winded), but
> personally when the stream types are the same on input and output I
> often skip the Enumeratee stuff and just write an Enumerator wrapper.
> To address your complaint here:
>
>> PS Implementations which involve "EB.take count" seem to me 
>> unsatisfactory;
>> one surely oughtn't need to have a large buffer to solve this problem
>
> I'd write a helping combinator:
>
>> module Main where
>>
>> import           Control.Monad              (when)
>> import           Control.Monad.Trans
>> import           Data.ByteString.Char8      (ByteString)
>> import qualified Data.ByteString.Lazy.Char8 as L
>> import           Data.Enumerator
>> import qualified Data.Enumerator.List       as EL
>> import           System.IO
>>
>> takeUpTo :: Monad m =>
>>             Int
>>          -> Iteratee ByteString m (Stream ByteString, Int)
>> takeUpTo n' = continue k
>>   where
>>     n = toEnum n'
>>
>>     k EOF         = yield (EOF,0) EOF
>>     k (Chunks xs) = if taken == 0
>>                       then takeUpTo n'
>>                       else yield (stream, taken) rest
>>       where
>>         s      = L.fromChunks xs
>>         (a,b)  = L.splitAt n s
>>         taken  = fromEnum $ L.length a
>>         stream = Chunks $ L.toChunks a
>>         rest   = Chunks $ L.toChunks b
>>
>
> The code to run a side effect every N bytes is then pretty short (and
> should be efficient):
>
>> sideEffectEveryNBytes :: Monad m =>
>>                          Int      -- ^ run the side effect every N bytes
>>                       -> m ()     -- ^ side effect
>>                       -> Step ByteString m a
>>                       -> Iteratee ByteString m a
>> sideEffectEveryNBytes n act = flip checkContinue1 n $ \loop i k -> do
>>         (str, taken) <- takeUpTo i
>>         when (taken == i) $ lift act
>>         (lift $ runIteratee $ k str) >>= loop (nextI $ i - taken)
>>   where
>>     nextI 0 = n
>>     nextI i = i
>
> Here's your particular example:
>
>> example :: IO [ByteString]
>> example = run_ $ enumList 1 [ "the quick brown "
>>                             , "fox "
>>                             , "jumped "
>>                             , "over "
>>                             , "the lazy dog" ] $$ it
>>   where
>>     it = do
>>         xs <- sideEffectEveryNBytes 10 (putStr "." >> hFlush stdout) $$
>>               EL.consume
>>         lift $ putStrLn ""
>>         return xs
>
> Running it:
>
>> *Main> example
>> ....
>> ["the quick ","brown ","fox ","jumped ","ove","r ","the lazy"," dog"]
>
> G
> -- 
> Gregory Collins <greg at gregorycollins.net>