join for different monads in Haskell

Question

I know I can use join to remove the first level of list

join [["dog"]] " output ["dog"]

But why the following is not working

join $ (Just ["dog"]) 
got error:
Couldn't match type ‘[]’ with ‘Maybe’

We know Maybe and [] are both Monad in Haskell,

join(Monad m)=> m(m a) -> m a

Maybe is Monad

[] is Monad

What is wrong with that?

It seems to me join only for the same type of Monad, please correct me if I'm wrong.

Is there any function similar to join for any Monads like the example I's given above?

I know I can do it as following

fromJust (Just ["dog"]) 
"output: ["dog"]

But I need to know Just in advance.

• chepner: join doesn't simply remove an outer monad; it merges them, in a monad-specific way. Even join [["dog"]] doesn't simply remove the outer list; it concatenates the inner lists: join [["dog"], ["cat"]] == ["dog", "cat"].
• Aron Lee: Can you clarify what is "monad-specific way", any reference or doc would be appreciated.
• Adam Smith: @AronLee join is defined as part of the definition of a Monad. Each Monad defines join differently.

Answer 1

The m in join :: Monad m => m (m a) -> m a is the same Monad everywhere it appears. If you had Just (Just "dog"), then you have two Maybe monads and your join will work perfectly. Similarly to your list example.

Pulling things out of a Monad cannot be generalized, which is part of the power of Monads. Consider that if you could do that, it would be trivial to strip out the type safety of the IO Monad!

To go from Just ["dog"] -> ["dog"], you just need a Maybe a -> a You could use Data.Maybe.fromJust, but maybe is safer (what if you have Nothing instead?)