How does the Haskell compiler “know” that IO cannot be unwrapped?

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Category:Languages

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?

 


Just and Nothing are data constructors for the type Maybe a. 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 IO differently).

unwrapIO (IO str) = str doesn't make sense in the same way unwrapMaybe (Maybe str) = str doesn't make sense. IO and Maybe are not data constructors, so you cannot pattern-match on them.

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