Nicolas,
OK, let's restrict the word side-effect to printing on the screen as the only side-effect possible.
Some recalls:
Example:
putStrLn is a side-effecting function expecting *2* arguments, the first one is the string to print, and the second one is the world state. So if you could give to arguments to putStrLn you could make a side-effect, however you don't have a value of type RealWorld.
Is this clear?
Yes! Thanks for giving such a clear and insightful explanation. I guess I forgot about the IO being an abstraction for something else. So can I clarify that it's the runtime system that triggers the side-effect, and not Haskell? I have one last question that is still confusing me: what if I have a function that reads something from a file, say, and does something depending on that input - would that be a side-effect? Consider something modifying a file outside of the Haskell world that changes a program's behaviour. Am I right in thinking that the side-effect happens at runtime - but in Haskell, the funtion is still pure. I feel, also, that as a reasonably experienced Haskell user, I am getting confused with what should be fundamental Haskell concepts. Perhaps these concepts should be made much clearer to beginners in the first instance. Thanks! Chris.