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

Suppose there was such a function:

extract :: Monad m => m a -> a

Now you could write a "function" like this:

appendLine :: String -> String
appendLine str = str ++ extract getLine

Unless the extract function was guaranteed never to terminate, this would violate referential transparency, because the result of appendLine "foo" would (a) depend on something other than "foo", (b) evaluate to different values when evaluated in different contexts.

Or in simpler words, if there was an actually useful extract operation Haskell would not be purely functional.