Commit a5b43a2a authored by simonmar's avatar simonmar
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[project @ 2004-04-20 15:49:58 by simonmar]

- comments on INLINability of unsafePerformIO

- change unsafeInterleaveIO from NOINLINE to INLINE(!)  we believe
  this is safe.  Interestingly, there's now a good reason to use
parent 5c925279
......@@ -181,13 +181,19 @@ help of 'unsafePerformIO'. So be careful!
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
......@@ -195,6 +201,10 @@ unsafeInterleaveIO (IO m)
(# 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
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