To add to lgas' point: The distinction between functions with side effects and descriptions of what IO will be done at run-time has many practical consequences

To add to lgas' point: The distinction between functions with side effects and descriptions of what IO will be done at run-time has many practical consequences. For example, if you execute the Haskell code greet name = do putStrLn ("Hello, " ++ name) repeat3 action = do putStrLn "Doing it thrice." action action action putStrLn "Done it thrice." main = repeat3 (greet "themagicalcake") you get the following output: Doing it thrice. Hello, themagicalcake Hello, themagicalcake Hello, themagicalcake Done it thrice. But if you execute the similar-looking Python script def greet(name): print ('Hello %s' % name) def repeat3(action): print ('Doing it thrice.') action() action() action() print ('Done it thrice.') if __name__ == "__main__": repeat3(greet('themagicalcake')) you get Hello themagicalcake Doing it thrice. Traceback (most recent call last): File "test.py", line 10, in <module> repeat3(greet('themagicalcake')) File "test.py", line 5, in repeat3 action() TypeError: 'NoneType' object is not callable instead. The Haskell value `greet "themagicalcake"` has type `IO ()`: it is a value that describes IO to be done at runtime. In contrast, the Python value returned by the call `greet('themagicalcake')` has type `NoneType`, and this value does not describe anything. Calling `greet` itself had the side-effect of printing "Hello themagicalcake".