So, my suspicions are confirmed. Thanks, all.
Michael --- On Fri, 1/28/11, Daniel Peebles <pumpkin...@gmail.com> wrote: From: Daniel Peebles <pumpkin...@gmail.com> Subject: Re: [Haskell-cafe] Merry monad mixup? To: "Chris Smith" <cdsm...@gmail.com> Cc: "michael rice" <nowg...@yahoo.com>, "haskell cafe" <haskell-cafe@haskell.org> Date: Friday, January 28, 2011, 2:47 PM And by "works", I mean, ListT is is a monad only if the internal monad is commutative. On Fri, Jan 28, 2011 at 2:46 PM, Daniel Peebles <pumpkin...@gmail.com> wrote: Beware of ListT. It only works if your internal monad is commutative, which IO is not. (Maybe would work, for example) On Fri, Jan 28, 2011 at 2:41 PM, Chris Smith <cdsm...@gmail.com> wrote: On Fri, 2011-01-28 at 11:20 -0800, michael rice wrote: > The first and third work, but not the second. Why? When you use a do block, it can be the syntactic sugar for whatever monad you like; but you do have to make a choice. Your first example had a do block for the IO monad. Your third example used the [] monad. Both are fine. The second, though, wasn't clear on what monad it was using. When you used the (<-) syntax to nondeterministically choose from a list, the compiler settled upon the [] monad. But then the next line was a statement in the IO monad. That's inconsistent, hence the error. Perhaps you wanted to build a monad out of both behaviors? In this case, you should likely look into monad transformers, and in particular, the ListT monad transformer in the List package. This would allow you to write the code you did in a monad called ListT IO, except that the IO actions would need to be lifted through the use of either `lift` or `liftIO`. -- Chris Smith _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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