Out of curiosity, I wondered what would happen if I made up a State# RealWorld token and passed it into an IO action. My intuition was that State# RealWorld is elided in the final program, so none of the IO actions should actually be evaluating it. But when running the below program, I get an error.…

State# is not a lifted type, so it can’t lazily hide a bomb. Compare: {-# LANGUAGE MagicHash, UnboxedTuples #-} import GHC.Prim main :: IO () main = pure () where rw :: State# RealWorld rw = error "real world inspected!" The rw binding is strict, as if you had written !rw.

RealWorld evaluated?

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brandonchinn178 June 17, 2025, 4:19am 5

Ah no, realWorld# is what I was asking for. Thanks!

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RealWorld evaluated?

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