One may read this message as proving True === False without resorting to IO. In other words, referential transparency, or the substitution of equals for equals, may fail even in expressions of type Bool. This message is intended as an indirect stab at lazy IO. Unfortunately, Lazy IO and even the raw unsafeInterleaveIO, on which it is based, are sometimes recommended, without warnings that usually accompany unsafePerformIO. http://www.haskell.org/pipermail/haskell-cafe/2013-March/107027.html UnsafePerformIO is known to be unsafe, breaking equational reasoning; unsafeInterleaveIO gets a free pass because any computation with it has to be embedded in the IO context in order to be evaluated -- and we can expect anything from IO. But unsafeInterleaveIO has essentially the same code as unsafeInterleaveST: compare unsafeInterleaveST from GHC/ST.lhs with unsafeDupableInterleaveIO from GHC/IO.hs keeping in mind that IO and ST have the same representation, as described in GHC/IO.hs. And unsafeInterleaveST is really unsafe -- not just mildly or somewhat or vaguely unsafe. In breaks equational reasoning, in pure contexts. Here is the evidence. I hope most people believe that the equality on Booleans is symmetric. I mean the function (==) :: Bool -> Bool -> Bool True == True = True False == False = True _ == _ = False or its Prelude implementation. The equality x == y to y == x holds even if either x or y (or both) are undefined. And yet there exists a context that distinguishes x == y from y ==x. That is, there exists bad_ctx :: ((Bool,Bool) -> Bool) -> Bool such that *R> bad_ctx $ \(x,y) -> x == y True *R> bad_ctx $ \(x,y) -> y == x False The function unsafeInterleaveST ought to bear the same stigma as does unsafePerformIO. After all, both masquerade side-effecting computations as pure. Hopefully even lazy IO will get recommended less, and with more caveats. (Lazy IO may be superficially convenient -- so are the absence of typing discipline and the presence of unrestricted mutation, as many people in Python/Ruby/Scheme etc worlds would like to argue.) The complete code: module R where import Control.Monad.ST.Lazy (runST) import Control.Monad.ST.Lazy.Unsafe (unsafeInterleaveST) import Data.STRef.Lazy bad_ctx :: ((Bool,Bool) -> Bool) -> Bool bad_ctx body = body $ runST (do r <- newSTRef False x <- unsafeInterleaveST (writeSTRef r True >> return True) y <- readSTRef r return (x,y)) t1 = bad_ctx $ \(x,y) -> x == y t2 = bad_ctx $ \(x,y) -> y == x