Dusan Kolar wrote:

Hello all,

the following program changes behavior if translated using different options for ghc compiler. Is this correct or not? I used The Glorious Glasgow Haskell Compilation System, version 6.6. And I would expect in both cases behavior 1.

But I may be wrong...

You do not get to expect that unsafe* function are unaffected by optimizations.

Thanks for any resolution.

Dusan

*Program:*

$ cat Test.hs import System.IO.Unsafe import System.IO

main = do putStr "Start...\n" putStr $ show $ falling' falling10000 putStr "\n" putStr $ show $ sorted' rising10000 putStr "Stop...\n"

sorted' [] = True sorted' [_] = True sorted' l = sx l 1 where sx [] _ = True sx [_] _ = True sx (x:l@(y:ys)) n = if n==1000 then unsafePerformIO (putChar '.' >> hFlush stdout) `seq` x<=y && sx l 1 else x<=y && (sx l $! n+1)

falling' [] = True falling' [_] = True falling' l = sx l 1 where sx [] _ = True sx [_] _ = True sx (x:l@(y:ys)) n = if n==1000 then unsafePerformIO (putChar '.' >> hFlush stdout) `seq` x>=y && sx l 1 else x>=y && (sx l $! n+1)

rising10000 = [0..100000]

falling10000 = [100000,99999..0]

-- EOF

the expression

unsafePerformIO (putChar '.' >> hFlush stdout)

has type "()" and does not depend on anything in scope. The `seq` command demands that this be evaluated to see it is bottom or the value "()", but then it never needs to be re-evaluated, as Haskell may assume it is will be immutably the value "()".

*Behavior 1:*

$ ghc Test.hs -o test $ ./test Start... ....................................................................................................True

....................................................................................................TrueStop...

There it is re-evaluated.

*Behavior 2:*

$ rm test Test.o Test.hi $ ghc -O2 Test.hs -o test $ ./test Start... .True .TrueStop...

There it is evaluated once and cached.