On Sun, Jan 28, 2007 at 07:16:16PM +0000, Chris Kuklewicz wrote:
> Iavor Diatchki wrote:
> > Hello,
> > The "packCString" function (and other similar functions) in the
> > ByteString library break referential transperancy, which is one of the
> > big selling points of Haskell (and its libraries). Here is an
> > example:
> >
> > main = do x <- newCString "Hello"
> > let s = packCString x
> > h1 = B.head s
> > print s
> > -- print h1
> > poke x (toEnum 97)
> > print s
> > let h2 = B.head s
> > print h1
> > print h2
> > Output:
> > "Hello"
> > "aello"
> > 97
> > 97
> >
> > This is already confusing because the "pure" value 's' has magically
> > changed. Also notice that the evaluation order of the program
> > affects the output. If we include the commented out statement (which
> > forces h1 to be evaluated earlier) the output becomes:
> > "Hello"
> > 72
> > "aello"
> > 72
> > 97
It gets more fun:
import Foreign.C.String
import Data.ByteString
import Control.Exception
shouldBePure :: CString -> ()
shouldBePure str = packMallocCString str `seq` ()
main = do x <- newCString "Hello"
evaluate $ shouldBePure x
evaluate $ shouldBePure x
-- force a GC
evaluate $ Prelude.reverse [0..]
-->
stefan at stefans:/tmp$ ./X
*** glibc detected *** double free or corruption (fasttop): 0x08086628 ***
Aborted
Now what if GHC did more CSE... <shudder>