Re: [Haskell-beginners] help with IO guards

Not really, for the same reason: a guard needs a Bool, and you can't get a Bool from IO Bool. On Thursday, January 15, 2015, Miro Karpis wrote:

many thanks,....but then I unfortunately don't understand how can I fix my initial problem:

to use IO check in guards - is that possible?

Regards, Miro

On Thu, Jan 15, 2015 at 10:12 PM, Julian Birch javascript:_e(%7B%7D,'cvml','julian.birch@gmail.com');> wrote:

...

Going back to an earlier question: a monad is a bit like a roach motel. You can check in but you can't leave. (This isn't true of every Monad, but the point is there's no guarantees.) In particular, you can't go from IO String to String _at all_. But you can, through Functor, pass it to a function that takes a plain String. And through Monad, you can turn IO (IO String) back to IO String.

Hope this helps.

On Thursday, January 15, 2015, Marcin Mrotek javascript:_e(%7B%7D,'cvml','marcin.jan.mrotek@gmail.com');> wrote:

...

Hello,

A list ([]) is also a monad, and a String is defined as a list of characters ([Char]). So in your example, it's as if you were trying to use (>>=) operator on two different monads ([] and IO), which is impossible. To make a pure value a monadic value, you need to use return:

g = readLn >>= (\a -> return (f a))

which is equivalent to composing f with return:

g = readLn >>= return.f

Regards, Marcin _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- Sent from an iPhone, please excuse brevity and typos.

-- Sent from an iPhone, please excuse brevity and typos.