[Haskell-beginners] Using Iteratees to write to a file

Magnus Therning magnus at therning.org
Sat Mar 19 17:29:54 CET 2011

On Sat, Mar 19, 2011 at 01:01:46AM -0700, Richard Wallace wrote:
> Hello everyone,
> I've been reading "Iteratee: Teaching an Old Fold New Tricks" from
> the Monad Reader Issue 16 and am having a problem figuring out how
> to use the streamToFile function.  Specifically, I can't figure out
> how to feed the Iteratee data to write to the file.  If I run
> ghci> runIter $ streamToFile "/tmp/stream"
> Then the file is created but not populated with any data - which is
> natural since I didn't give it any data.  I have no clue how to give
> it data though.  I'd really appreciate it if someone could show me
> how to write "Hello world!" or something else equally mundane to get
> me going.  I've been banging my head on this for hours now.

I think you need to get your data from somewhere, an Enumerator, e.g.
an Enumerator for a file handle.  I haven't looked at the Iteratee API
for a while, but I believe that an Enumerator basically is of the type
Iteratee -> Iteratee, which means that something along the lines of

  runIter $ enumerateHandle h (streamToFile "/tmp/stream")

ought to copy the content of the handle into the file.

The documentation for Enumerator[1] is an excellent resource for
understanding iteratees.  Even if you don't plan on using the
Enumerator package itself.


[1] http://hackage.haskell.org/package/enumerator
Magnus Therning                      OpenPGP: 0xAB4DFBA4 
email: magnus at therning.org   jabber: magnus at therning.org
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Most software today is very much like an Egyptian pyramid with
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integrity, but just done by brute force and thousands of slaves.
     -- Alan Kay
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