[Haskell-cafe] RE: [Haskell] line-based interactive program

Simon Marlow simonmar at microsoft.com
Wed Jul 13 10:46:20 EDT 2005

On 08 July 2005 18:22, Olaf Chitil wrote:

> In fact, unsafeInterleaveIO shows up limitations of the IO monad.
> Without this strange primitive (what is actually unsafe about it?)

Just picking up on this point: unsafeInterleaveIO is so named because it
lets you write programs that don't obey the usual laws.  For example:

main = do
  r <- newIORef 0
  a <- unsafeInterleaveIO $ readIORef r
  b <- unsafeInterleaveIO $ do writeIORef r 42; return 0
  print (a + b)

if you change (a + b) to (b + a), you get a different answer.


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