How to extract value from monadic action

Question

Is there a built-in function with signature :: (Monad m) => m a -> a ?

Hoogle tells that there is no such function.

Can you explain why?<

Answer 1

There is probably a better answer than this, but one way to see why you cannot have a type (Monad m) => m a -> a is to consider a null monad:

data Null a = Null

instance Monad Null where
    return a = Null
    ma >>= f = Null

Now (Monad m) => m a -> a means Null a -> a, ie getting something out of nothing. You can't do that.