[Haskell-cafe] GHC optimization changes evaluation strategy?

13 Mar 2007

      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...

  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

*Behavior 1:*

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

*Behavior 2:*

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

-- 

 Dusan Kolar            tel: +420 54 114 1238
 UIFS FIT VUT Brno      fax: +420 54 114 1270
 Bozetechova 2       e-mail: kolar@fit.vutbr.cz
 Brno 612 66
 Czech Republic

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