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

John Lato jwlato at gmail.com
Sat Feb 28 17:51:46 EST 2009

Hello Günther,

I think the largest reason Haskellers don't use left-fold enumerators
is that there isn't a ready-to-use package on Hackage.  Oleg's code is
extremely well commented and easy to follow, but it's not cabalized.

In addition to Takusen, Johan Tibbe's hyena application server uses
enumerators for IO:

There is a darcs repo of a cabalized iteratee package available at
This is essentially Oleg's code, slightly modified and reorganized.
If anyone is interested in using left-fold enumerators for IO, please
give it a look and let me know what you think.  I'd like to put this
on hackage in about a week or so, if possible.  I would especially
appreciate build reports.

There are a few iteratee/enumerator design questions that remain,
which Oleg and others would like to explore more fully.  The results
of that research will likely find there way into this library.

John Lato

> Hi all,
> in the last few months I was looking for haskell database library,
> eventually settling for HDBC (thanks John btw).
> Takusen also caught my eye although I even failed installing it.
> Nevertheless a particular property of takusen, left-fold-enumerator,
> also sparked  my interest and I tried to follow it up.
> I had the impression this is something of relevance but something that
> is not mentioned or made use of often. There are actually only a few
> references on the net and most of those by one person only, Oleg.
> There was one post from John Goerzen about Haskell HTTP libraries in
> which he hinted using left-fold enumerators.
> Anyway what I'm saying is that the whole topic is somewhat off the radar
> of the larger haskell community, how come, or am I merely overestimating
> its relevance and usefulness?
> Günther

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