drtomc:
Well, maybe I shoud be asking a higher level question then.
I have a function
tidy = reverse . dropWhile punk . reverse . dropWhile punk where punk = isPunctuation . chr . fromIntegral
which is leading to a significant amount of allocation, and you can see why.
The way I'd like to write it is
tidy = dropWhile punk . dropWhileEnd punk where ....
which has the obvious advantage of avoiding quite a bit of intermediate allocation.
Is there a another way?
I note that since I'm using a nice declarative language, the compiler CLEARLY should be transforming the first form into the second. :-)
I'd just manually write a 'tidy' loop (in the Data.ByteString style) (which would avoid all allocations), since it seems pretty useful. Something in this style: findIndexOrEnd :: (Word8 -> Bool) -> ByteString -> Int findIndexOrEnd k (PS x s l) = inlinePerformIO $ withForeignPtr x $ \f -> go (f `plusPtr` s) 0 where go !ptr !n | n >= l = return l | otherwise = do w <- peek ptr if k w then return n else go (ptr `plusPtr` 1) (n+1) If its costly, since that'll make it non-costly. -- Don