Question

I’m just totally confused with lists and monads, so maybe my question isn’t correct or very naive.
I’ve seen the way to do it using mapM_ func here:

mapM_ print [1, 2, 3, 4]

But I don’t know exactly how it works and want to know how can I do this in a way like this:

x <- [1, 2, 3]
print x

or, if I understood it right:

[1, 2, 3] >>= print

I understand that [1, 2, 3] has type [a] and print has type Show a => a -> IO (). Also I understand that for using monad List we need type List a on the left and func with type a -> List b on the right. Am I right?
Can you help me with this?

UPD. Thanks @MathematicalOrchid for explanation how mapM_ works. From my side I want to explain that the real problem is not printing any results in different lines but do some monadic actions(because now I’m hanging around OpenGL stuff) in a way monad List provides it. But I got that the root of misunderstanding was in mixing monads.

UPD2. Thanks everyone for answers. I apologize for this kinda fuzzy question. I dind’t exactly know what answer I need and what is the question. It’s because I didn’t understand some basics. So it’s hard to choose “the correct answer” now because every answers have a small peace of what I was looking for. I’ve decided to choose the closest(although not the most useful now) to what I wanted.

Answer 1

What you want cannot work this way since you are trying to mix two monads together:

do x <- [1,2,3]
   print x

Specifically you are mixing the IO and the [] monads. In do-notation, all the statements should have the type m a for some Monad m. But in the above code, the first statement has the type [Integer] while the second statement has the type IO ().

To get the effect you want you should use the ListT monad transformer. Monad transformers allow mixing monads together in a certain order in a stack and combining their effects as needed.

import Control.Monad.Trans
import Control.Monad.Trans.List

value = do x <- ListT (return [1,2,3])
           lift (print x)

This will return a value of type ListT IO Integer. To get the IO computation out of this transformer, use runListT. Which will return a value of type IO [Integer]. This will output:

GHCI> runListT value
1
2
3
[(),(),()]

Which is equivalent to mapM print [1,2,3]. To throw away the list and get the effect of mapM_ print [1,2,3] you can use void from Control.Monad.

GHCI> void . runListT $ value
1
2
3