[Haskell-cafe] Baffled by Disk IO

SevenThunders mattcbro at earthlink.net
Sat Jul 29 19:16:40 EDT 2006

SevenThunders wrote:
> I have run into a baffling distinction between the behavior of GHCi and 
> the compiled binary from GHC.
> I suspect it's something pretty stupid on my part.
> I have the following test program
> <test.hs>
> import Matrix
> main = let
>     f1 = bRgauss 4 3
>     f2 = bRgauss 3 2
>     fu = f1 *. f2   
>     in bsave "fumat" fu
> ----------------------------------
> The type signature of bsave is
>     -- | save a matrix to a file
>     bsave :: String -> m -> IO()
>     bsave str z = do
>         k <- matindx z
>         withCString str (\x -> bmsave x k)
> and bmsave is a C routine that does a simple write to disk using fprintf. 
> If I compile using GHC and run this as test.exe it does absolutely 
> nothing, no file is actually saved.  In fact no file is saved even
> if I try to print components of fu in the IO monad.
> Whereas if I load it into GHCi and run main, everything works as 
> expected.  Well sort of.  Apparently everytime I extract something out
> of fu and use it in an IO monad it executes all the IO actions in fu, so 
> it's easy to get unwanted behavior with incautious use.
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OK it was stupid.  Apparently GHC behaves differently according to what the
name of the high level source file is.  If I renamed test.hc to main.hc
everything works the same as GHCi.  I probably should actually read the
manual some day.
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