[Haskell-cafe] 'Progress bar' enumeratee

[Ertugrul Soeylemez]

"David Hotham" <david.hotham at blueyonder.co.uk> wrote:

> I've spent some time over the last couple of days trying to write an
> enumeratee that prints a "." every n bytes (with obvious intended use
> as a progress tracker).  Seems like it oughtn't be hard, but it has
> been a steep learning curve...
>
> I have come up with something that seems to do the job but I don't
> know that I'm completely happy with it (or even that I completely
> understand it, to be honest).
>
> If anyone more expert would be kind enough either to reassure me that
> I'm doing it right or - more likely - to offer improvements /
> suggestions on what obvious simplifications I have overlooked, I'd be
> grateful.

I think that using lazy bytestrings does not have any advantage here,
since the enumerator creates the strict bytestrings at some point, then
your enumeratee converts them into lazy ones just for counting.  Just
use the straightforward approach:  Take the chunks and count the bytes
like here:

    {-# LANGUAGE ScopedTypeVariables #-}

    dotsAt :: forall b m. MonadPeelIO m =>
              Int -> Enumeratee ByteString ByteString m b
    dotsAt n =
        loop 0

        where
        loop :: Int -> Enumeratee ByteString ByteString m b
        loop i' step@(Continue k) =
            continue go

            where
            go :: Stream ByteString ->
                  Iteratee ByteString m (Step ByteString m b)
            go EOF = return step
            go ch@(Chunks strs) = do
                let (numDots, i) = divMod (i' + sum (L.map BC.length strs)) n
                tryIO $ BC.putStr (BC.replicate numDots '.')
                k ch >>== loop i
        loop i' step = return step

I think, this is about the most straightforward and also the fastest
approach.


Greets,
Ertugrul


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