Re: [Haskell-cafe] Re: Left fold enumerator - a real pearl overlooked?

jwlato:

On Tue, Mar 3, 2009 at 1:03 AM, Henning Thielemann wrote:

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On Mon, 2 Mar 2009, John Lato wrote:

While I think that the Iteratee pattern has benefits, I suspect that it can't be combined with regular lazy functions, e.g. of type [a] -> [a]. Say I have a chain of functions: read a file, parse it into a tag soup, parse that into an XML tree, transform that tree, format that into a string, write that to a file, and all of these functions are written in a lazy way, which is currently considered good style, I can't use them in conjunction with iteratees. This means, almost all Haskell libraries have to be rewritten or extended from lazy style to iteratee style. The question for me is then: Why having laziness in Haskell at all? Or at least, why having laziness by default, why not having laziness annotation instead of strictness annotation.

I'm not sure that this is a problem, at least not for all cases. When reading seekable streams, it is possible to have IO on demand provided that all processing take place within the context of the Iteratee (see Oleg's Tiff reader, http://okmij.org/ftp/Haskell/Iteratee/Tiff.hs, and my wave reader, http://inmachina.net/~jwlato/haskell/iteratee/src/Data/Iteratee/Codecs/Wave....).

BTW, I've started (with his blessing) packaging up Oleg's Haskell code: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/liboleg-0.1.0.2 So you can use, e.g., the left-fold based TIFF parser: http://hackage.haskell.org/packages/archive/liboleg/0.1.0.2/doc/html/Codec-I... I'm walking backwards over his released modules, adding a few each day. Enjoy. -- Don