[Haskell-cafe] Screen scraping with an interactive process: Buffering problems?

Denis Bueno dbueno at gmail.com
Wed Nov 14 09:41:31 EST 2007

On Nov 7, 2007 4:44 PM, David Benbennick <dbenbenn at gmail.com> wrote:
> And
> once you do hGetContents, you have read all the data that will ever
> exist on that handle, so there's nothing to read from it "later on".

I completely misunderstood how hGetContents works.  This now makes
sense.  I first thought hGetContents would return "everything you
could read from the stream right now" as a string.  From reading the
the haddock documentation for System.IO.hGetContents, I failed to
understand that the string it returns will contain characters that
haven't even been written to the (interactive) stream by the time of
the call to hGetContents.  I'm not sure if the hGetContents
documentation could easily be made to make that fact more salient, but
I'd be willing to submit a patch, if other people have been confused.

> Assuming that each call to ACL2 produces exactly one of either "Proof
> succeeded" or "attempt has failed", you can get a list of results like
> this (where aclOutput :: String is the result of hGetContents):
> let results = map (\l -> if l == "Proof succeeded" then True else
> False) $ filter (\l -> elem l ["Proof succeeded", "attempt has
> failed"]) $ lines aclOutput
> Then results :: [Bool], and results !! n is True if the nth call
> succeeded.  Just make sure not to inspect results !! n until after
> making the nth call to ACL2, or it will block.

I used a minor modification of this approach ("Proof succeeded" is not
on a line by itself), and it worked famously.  Thanks for the help,


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