ioToST :: IO a -> ST RealWorld a
ioToST (IO m) = (ST m)
+-- This relies on IO and ST having the same representation modulo the
+-- constraint on the type of the state
+--
+unsafeIOToST :: IO a -> ST s a
+unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
+
-- ---------------------------------------------------------------------------
-- Unsafe IO operations
>
> main = do
> writeIORef test [42]
-> bang \<- readIORef test
+> bang <- readIORef test
> print (bang :: [Char])
This program will core dump. This problem with polymorphic references
unsafePerformIO :: IO a -> a
unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
+-- Why do we NOINLINE unsafePerformIO? See the comment with
+-- GHC.ST.runST. Essentially the issue is that the IO computation
+-- inside unsafePerformIO must be atomic: it must either all run, or
+-- not at all. If we let the compiler see the application of the IO
+-- to realWorld#, it might float out part of the IO.
+
{-|
'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
When passed a value of type @IO a@, the 'IO' will only be performed
when the value of the @a@ is demanded. This is used to implement lazy
file reading, see 'System.IO.hGetContents'.
-}
-{-# NOINLINE unsafeInterleaveIO #-}
+{-# INLINE unsafeInterleaveIO #-}
unsafeInterleaveIO :: IO a -> IO a
unsafeInterleaveIO (IO m)
= IO ( \ s -> let
in
(# s, r #))
+-- We believe that INLINE on unsafeInterleaveIO is safe, because the
+-- state from this IO thread is passed explicitly to the interleaved
+-- IO, so it cannot be floated out and shared.
+
-- ---------------------------------------------------------------------------
-- Handle type
-- enough information to identify the handle for debugging. A handle is
-- equal according to '==' only to itself; no attempt
-- is made to compare the internal state of different handles for equality.
+--
+-- GHC note: a 'Handle' will be automatically closed when the garbage
+-- collector detects that it has become unreferenced by the program.
+-- However, relying on this behaviour is not generally recommended:
+-- the garbage collector is unpredictable. If possible, use explicit
+-- an explicit 'hClose' to close 'Handle's when they are no longer
+-- required. GHC does not currently attempt to free up file
+-- descriptors when they have run out, it is your responsibility to
+-- ensure that this doesn't happen.
data Handle
= FileHandle -- A normal handle to a file
-- but not less frequently, than specified above.
-- The output buffer is emptied as soon as it has been written out.
--
--- Similarly, input occurs according to the buffer mode for handle {\em hdl}.
+-- Similarly, input occurs according to the buffer mode for the handle:
--
-- * /line-buffering/: when the buffer for the handle is not empty,
-- the next item is obtained from the buffer; otherwise, when the
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