Thanks for an excellent clarification. I have been known to be rather daft at times so I just wanted to make sure I understood everything correctly. Thanks!
/M On Sun, Jan 28, 2007 at 12:56:56 +0200, Yitzchak Gale wrote: >Hi Magnus, > >You wrote: >>This piece has type problems. I couldn't get ghci to >>accept it without making some changes... > >You are absolutely correct, and I apologize for the errors. >I will try one more time to give a corrected version below. > >Let me point out, though, that this does not exactly solve the >problem you originally stated. Here is a summary and >clarification of what we have come up with together so far: > >First, I said that it sounds like what you really want is ListT. > >But you pointed out that my ListT solution gave you the wrong >order of interaction. > >The reason for this problem is a bug in the implementation >of ListT in the standard libraries, which is explained on the >wiki page: > >http://www.haskell.org/haskellwiki/ListT_done_right > >Dan Piponi posted a solution to your problem that works >the way you want it, and without unsafeInterleaveIO. >Dan's solution is just ListT in disguise, except that Dan >used the corrected form of ListT like on the wiki page. > >My code in which you found the errors is a translation >of Dan's solution back into ListT notation. So if you >use that with the broken ListT currently in the standard >libraries, you'll go back to the problem we had at the >beginning. > >Here is the bottom line: > >You can solve your problem - without unsafeInterleaveIO - >either by using one of the corrected versions of ListT >listed on the wiki page, or by writing out an implementation >of ListT that is hard-wired for your application like Dan >did. > >OK, so here is my translation of Dan's code back >into ListT notation again. I hope this version is now correct: > >test = do >a <- liftIO getChar >guard $ a /= 'q' >return a `mplus` test > >test2 = (>>= liftIO . print) > >Run it with: > >runListT $ test2 test > >Note that you probably want putChar instead of print. >Also, Dan's version drops the 'q' at the end, while >your code prints the 'q'. > >Hope this helps. > >Regards, >Yitz >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe@haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe -- Magnus Therning (OpenPGP: 0xAB4DFBA4) [EMAIL PROTECTED] Jabber: [EMAIL PROTECTED] http://therning.org/magnus
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