Ah! My favourite Haskell discussion. So, consider these two programs, the first in Haskell:

    main :: IO ()
    main = do
      foo
      foo

    foo :: IO ()
    foo = putStrLn "Hello"

and the second in Python:

    def main():
      foo()
      foo()

    def foo():
      print("Hello")

For the Python one I'd say "I/O is done inside `foo` before returning". Would you? If not, why not? And if so, what purpose does it serve to not say the same for the Haskell?

My Haskell is rusty enough that I don’t know the proper syntax for it, but you can make a program that calls foo and then throws away / never uses the IO computation. Because Haskell is lazy, “Hello” will never be printed.

You can do this

    main = do
      let x = foo
      putStrLn "foo was never executed"

but you can also do this

    def main():
      x = foo
      print("foo was never executed")

What's the difference?