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
