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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