[Haskell-cafe] Sub-optimal [code]

[Andrew Coppin]

>> I tried -O2 -fno-cse. No difference.
>>
>> I also tried -O2 -fno-full-laziness. BIG DIFFERENCE.
>
> See also the very old GHC ticket at
> http://hackage.haskell.org/trac/ghc/ticket/917

I don't know if that's the problem or not, but it might plausibly be.

Here's the smallest version of the program that I could come up with 
[which still misbehaves]:

module Main (main) where

import System.IO
import System.Random

main = do
   file_batch "01-Uniform" random_byte_uniform

random_byte_uniform :: IO Int
random_byte_uniform = randomRIO (0x00, 0xFF)

random_file :: String -> Int -> IO Int -> IO ()
random_file f n rnd = do
   putStrLn $ "Save: " ++ f ++ " [" ++ show n ++ " bytes]"
   h <- openFile f WriteMode
   hSetBinaryMode h True
   mapM_ (\ _ -> rnd >>= hPutChar h . toEnum) [1..n]
   hClose h

file_batch :: String -> IO Int -> IO ()
file_batch f rnd =
   mapM_
     (\ k ->
       mapM_
         (\ n ->
           random_file
             (f ++ "-" ++ show k ++ "x-" ++ [n])
             (10 * 1024 * 1024 * k)
             rnd
         )
         "ABCD"
     )
     [1..4]

If main calls random_file directly, the program seems to work OK, so the 
problem seems to be file_batch. Maybe. I don't really know. I had a go 
at playing with -ddump-simpl, but that just generated a 8 KB file which 
is utterly incomprehensible. (Well, the -O0 variant is just about 
comprehensible. The -O2 variant isn't. But it appears that *everything* 
gets inlined into main...)

If anybody can figure out what's happening here, I'd be interested to know.