Make unsafePerformIO lazy

[ghc-base.git] / GHC / IOBase.lhs

diff --git a/GHC/IOBase.lhs b/GHC/IOBase.lhs
index 27f7dab..499899a 100644 (file)
--- a/GHC/IOBase.lhs
+++ b/GHC/IOBase.lhs
@@ -215,7 +215,7 @@ help of 'unsafePerformIO'.  So be careful!
 -}
 {-# NOINLINE unsafePerformIO #-}
 unsafePerformIO        :: IO a -> a
-unsafePerformIO (IO m) = case m realWorld# of (# _, r #)   -> r
+unsafePerformIO (IO m) = lazy (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
@@ -223,6 +223,22 @@ unsafePerformIO (IO m) = case m realWorld# of (# _, r #)   -> r
 -- not at all.  If we let the compiler see the application of the IO
 -- to realWorld#, it might float out part of the IO.
 
+-- Why is there a call to 'lazy' in unsafePerformIO?
+-- If we don't have it, the demand analyser discovers the following strictness
+-- for unsafePerformIO:  C(U(AV))
+-- But then consider
+--     unsafePerformIO (\s -> let r = f x in 
+--                            case writeIORef v r s of (# s1, _ #) ->
+--                            (# s1, r #)
+-- The strictness analyser will find that the binding for r is strict,
+-- (becuase of uPIO's strictness sig), and so it'll evaluate it before 
+-- doing the writeIORef.  This actually makes tests/lib/should_run/memo002
+-- get a deadlock!  
+--
+-- Solution: don't expose the strictness of unsafePerformIO,
+--          by hiding it with 'lazy'
+
+
 {-|
 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
 When passed a value of type @IO a@, the 'IO' will only be performed