Effective Haskell

Looking at the Type of main

So far we’ve skimmed past the type of one important function: main. In all Haskell executables, there should be a single main function and it should have the type main :: IO (). This is called an IO action. You’ll learn more about IO actions here. For now, it’s sufficient to know that functions with the type IO a are functions that do some sort of interaction with the real world, like printing a value to a screen or reading a file.