[GHC] #2756: state hack causes unneeded value to be evaluated

[GHC]

#2756: state hack causes unneeded value to be evaluated
---------------------------------+------------------------------------------
    Reporter:  int-e             |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Component:  Compiler        
     Version:  6.10.1            |    Severity:  normal          
    Keywords:                    |    Testcase:                  
Architecture:  Unknown/Multiple  |          Os:  Unknown/Multiple
---------------------------------+------------------------------------------
 The following code prints "{{{Main: Why?}}}" when compiled with {{{ghc
 -O1}}}. With {{{-fno-state-hack}}} it works fine.

 {{{
 data X = X ()

 {-# NOINLINE newX #-}
 newX :: () -> IO X
 newX n = do
     let {-# NOINLINE value #-}
         value = n
     return (X value)

 main = do
     x <- newX (error "Why?")
     case x of
         X _ -> return ()
 }}}

 Both pragmas are needed to exhibit the bug.

 (In my actual code, {{{value}}} is an {{{unsafePerformIO}}} -- this was
 the motivation for adding the pragmas.)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2756>
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