[Haskell-cafe] unsafeSTToIO and stToIO

[Xiao-Yong Jin]

Hi,

Between the following two functions

stToIO        :: ST RealWorld a -> IO a
stToIO (ST m) = IO m

unsafeSTToIO :: ST s a -> IO a
unsafeSTToIO (ST m) = IO (unsafeCoerce# m)

All I can see is that the safe one uses RealWorld instead of
an arbitrary thread s used in the unsafe one.  I really
don't understand the difference between these two.  Why is
the one without RealWorld unsafe?

I tried google, but couldn't find anything helpful.
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