concept putStrLn in category haskell

This is an excerpt from Manning's book Get Programming with Haskell. Login to get full access to this book.

If main isn’t a function, it should follow that neither is putStrLn. You can quickly clear this up by looking at putStrLn’s type:

putStrLn :: String -> IO ()

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As you can see, the return type of putStrLn is IO (). Like main, putStrLn is an IO action because it violates our rule that functions must return values.