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

This doesn't exist because Monad is a pattern for composition, not a pattern for decomposition. You can always put more pieces together with the interface it defines. It doesn't say a thing about taking anything apart.

Asking why you can't take something out is like asking why Java's Iterator interface doesn't contain a method for adding elements to what it's iterating over. It's just not what the Iterator interface is for.

And your arguments about specific types having a kind of extract function follows in the exact same way. Some particular implementation of Iterator might have an add function. But since it's not what Iterators are for, the presence that method on some particular instance is irrelevant.

And the presence of fromJust is just as irrelevant. It's not part of the behavior Monad is intended to describe. Others have given lots of examples of types where there is no value for extract to work on. But those types still support the intended semantics of Monad. This is important. It means that Monad is a more general interface than you are giving it credit for.