[Haskell-cafe] streaming with logs / metadata

[Olaf Klinke]

>> Anything beyond the Functor instance for (Either e), though, models exceptions that abort the computation as soon as the first Left arises. You seem to use the type in a different way which may be one reason why you do not find your functions implemented elsewhere. For logging or warnings, most Haskellers use something like a writer monad transformer. The underlying monad,
>> 
>> Writer w a = (w,a)
> 
> That's the thing I was saying destroys streaming.  The problem is that
> it returns ([log], [a]), instead of [Either log a], so the
> interleaving has been lost.
> 
Evan, 

When you say interleaving, do you think of lists like the following? 
[Left warning1,Right value 1,Left warning 2,Right value2,...] 
The above is isomorphic to 
[(warning1,value1),(warning2,value2),...]

I don't think that the writer monad destroys streaming. Below is a somewhat contrived, but ordinary use case. 

{-- begin example --}
import Control.Monad.Writer -- from mtl

type Log = [String] -- some Monoid

half :: Int -> Writer Log Int
half x = case x `quotRem` 2 of
    (y,0) -> return y
    (y,1) -> writer (y,["Warning: "++(show x)++" was uneven."])

stream :: [Int]
stream = 4:8:5:3:7:2: error "broken stream"

(ys,warnings) = runWriter (mapM half stream)
{-- end example --}

ghci> -- load the example above
ghci> ys
[2,4,2,1,3,1*** Exception: broken stream

ghci> mapM_ putStrLn warnings
Warning: 5 was uneven.
Warning: 3 was uneven.
Warning: 7 was uneven.
*** Exception: broken stream

You see that there is some output before the error is reached, proving that both ys and warnings could be consumed in a streaming manner. Here it is important that the Log monoid's `mappend` function is not strict in the second argument. It would not, for example, work with 
Log = Data.Sequence.Seq String
With mtl you could easily throw in a State monad component that keeps track of the position in the stream, so that warning messages can hint at the place where the warning originated. 

Olaf