In chp 7 on understanding IO, there's also another great section that explains how IO evaluation is often confused. I share an example from the book that reads, writes, and prints files.

    Memory intensive func because it attempts to read _all_ files at once because the execution of reading and writing actually only happens when the print fn is called (putStrLn files). makeAndReadFile is actually doing both reading and writing - a normal task in all other languages.

    slow =  
      let  files = mapM makeAndReadFile [1..500] ::  IO  [ String ]     
      in  files >>= (putStrLn . show)


    Efficent version. Here we force reads and writes to actually occur per file instead of waiting til the print. Seems like a pretty easy step to misunderstand.

    safe ::  IO  ()  safe =    
    foldl ( \ io id ->   io >> makeAndShow id  ) (return ()) [1..500]

* it has never been an issue for me

* I don't think the performance implications are nearly as hard to understand as often implied

* in the extreme you can just enable the STRICT language pragma in your project and forget about it :)

Really, lazy IO is a much larger source of problems, and even there, after a week or two writing IO people just learn to write code that doesn't break due to it.