Hello John, Thanks for your detailed answer. I finally understood this behaviour and got to the point where I need something like enumPair. About the scala implementation, it was this same blog post that arose my interest in iteratees :-)
Regards, Arnaud On Wed, Oct 27, 2010 at 5:26 PM, John Lato <jwl...@gmail.com> wrote: > Hi Arnaud, > >> >> From: Arnaud Bailly <arnaud.oq...@gmail.com> >> >> Hello, >> I am trying to wrap my head around the concept of Iteratee reading the >> article by John Lato in Monad Reader issue 16 >> (http://themonadreader.files.wordpress.com/2010/05/issue16.pdf). I >> followed the "advice" on page 34: >> >> "I have frequently heard reports from Haskellers (including >> highly-talented and >> experienced users) that the only way they could understand >> enumeration-based >> I/O was by re-implementing it themselves." >> >> and so reimplemented this code in Java which is, for better and worse, >> my current professional language (and there really is no point in >> writing my own 50th iteratee code...). >> >> I have a question about iteratees composition and their behaviour. I >> tried to use the throbber code to display progress while writing a >> file, without using enumeratees which are introduced later on in the >> article, but failed. I can bind the two iteratees together, but the >> writer iteratee consumes the stream of characters to write before >> handing over the control to the throbber iteratee which has nothing to >> count. Did I miss something ? > > This is the expected behavior. When iteratees are composed with bind, the > first will consume as much of the stream as it needs to calculate its > result, and only then will it pass the remaining stream to the next iteratee > in the composition. Since "streamToFile" and "throbber" both consume an > entire stream, they won't complete until EOF, and then will pass just that > EOF to the next iteratee in the composition. > The behavior you want is pretty common, and it can be accomplished by > writing either throbber or streamToFile as an enumeratee. Then the > enumeratee version would do the work it's supposed to but also pass the > stream along to another iteratee. You could write a library function with > type (following the types from the article) > iterToEnum :: Monad m => Iteratee el m () -> EnumerateeM el el m a > which would do this for any data-sink type iteratee. > A very similar approach is used in the "iteratee" package to define the > "enumPair" function, > enumPair :: Monad m => Iteratee s m a -> Iteratee s m b -> Iteratee s m > (a,b) > I consider "enumPair" to be one of the most powerful functions supplied in > iteratee. > Since you use Java, you may be interested in an iteratee implementation in > Scala, http://apocalisp.wordpress.com/2010/10/17/scalaz-tutorial-enumeration-based-io-with-iteratees/ > Best, > John _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
