[Haskell-cafe] GHCi shows result of (IO a) only if (a) is in class Show

I wrote a parser and it took me a while to realize why GHCi suddenly did not show any result nor an error message anymore. My parsing function has type (IO Expression), Expression is in class Show. After changing the parser to (IO Declaration), it did not show anything anymore, because Declaration was not in class Show. When I typed (parseFile "input.txt" >>= print), I got the error message and understood what was going on. But for I while I was really confused what's happening. Just wanted to share this. Best, Daniel