X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=GHC%2FIO.hs;h=8a2dd595b3b8b5e1bf814118163868d79ba57a5a;hb=08cc1c4f54b482dffaef1a1b43c5e0208894f643;hp=f2ccc7d5b974047d2bc20c7e981cc61cf149017a;hpb=7e8f2da24a671fa3b314e49f244cabe37af7ccd2;p=ghc-base.git

diff --git a/GHC/IO.hs b/GHC/IO.hs
index f2ccc7d..8a2dd59 100644
--- a/GHC/IO.hs
+++ b/GHC/IO.hs
@@ -1,5 +1,4 @@
-{-# OPTIONS_GHC -fno-warn-orphans #-}
-{-# OPTIONS_GHC -XNoImplicitPrelude -funbox-strict-fields #-}
+{-# OPTIONS_GHC -XNoImplicitPrelude -funbox-strict-fields -XBangPatterns #-}
 {-# OPTIONS_HADDOCK hide #-}
 -----------------------------------------------------------------------------
 -- |
@@ -102,11 +101,15 @@ This is the \"back door\" into the 'IO' monad, allowing
 this to be safe, the 'IO' computation should be
 free of side effects and independent of its environment.
 
-If the I\/O computation wrapped in 'unsafePerformIO'
-performs side effects, then the relative order in which those side
-effects take place (relative to the main I\/O trunk, or other calls to
-'unsafePerformIO') is indeterminate.  You have to be careful when 
-writing and compiling modules that use 'unsafePerformIO':
+If the I\/O computation wrapped in 'unsafePerformIO' performs side
+effects, then the relative order in which those side effects take
+place (relative to the main I\/O trunk, or other calls to
+'unsafePerformIO') is indeterminate.  Furthermore, when using
+'unsafePerformIO' to cause side-effects, you should take the following
+precautions to ensure the side effects are performed as many times as
+you expect them to be.  Note that these precautions are necessary for
+GHC, but may not be sufficient, and other compilers may require
+different precautions:
 
   * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
         that calls 'unsafePerformIO'.  If the call is inlined,
@@ -117,7 +120,7 @@ writing and compiling modules that use 'unsafePerformIO':
         two side effects that were meant to be separate.  A good example
         is using multiple global variables (like @test@ in the example below).
 
-  * Make sure that the either you switch off let-floating, or that the 
+  * Make sure that the either you switch off let-floating (@-fno-full-laziness@), or that the 
         call to 'unsafePerformIO' cannot float outside a lambda.  For example, 
         if you say:
         @
@@ -336,7 +339,4 @@ a `finally` sequel =
 -- >   evaluate x = (return $! x) >>= return
 --
 evaluate :: a -> IO a
-evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
-        -- NB. can't write
-        --      a `seq` (# s, a #)
-        -- because we can't have an unboxed tuple as a function argument
+evaluate a = IO $ \s -> let !va = a in (# s, va #) -- NB. see #2273