Obviously, the following function is impossible, because it is impossible to unwrap an IO value permanently (ignoring unsafePerformIO or similar):
unwrapIO :: IO String -> String unwrapIO (IO str) = str
However, similar functions such as the following are possible:
unwrapJust :: Maybe String -> String unwrapJust (Just str) = str unwrapJust Nothing = "ignore this plz"
I fully understand the reasoning behind why #2 is possible but #1 is not, but I do not understand how. Can I also make my own types that are not unwrappable?
Nothing are data constructors for the type
IO has no data constructors to speak of (in GHC it actually has constructors but they're really implementation details of GHC, and other implementations might define
unwrapIO (IO str) = str doesn't make sense in the same way
unwrapMaybe (Maybe str) = str doesn't make sense.
Maybe are not data constructors, so you cannot pattern-match on them.