[dllthomas]

"The only conceivable value that the type IO () can have is still the value IO ()."

There is some magic around the IO type, but it's not needed to answer your question.

If you look in GHC.Types in the package ghc-prim, you can find the definition of IO:

    newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

An IO action is a function from the state of the world, to the state of the world plus a value. So the compiler may know it's giving you (), but it doesn't know what the new state-of-the-world will be.