[Haskell-cafe] OS Abstraction module??

[Neil Mitchell]

Hi Bill,

> > >     I am really talking about a module or perhaps a Haskell class that
> > > provides notion for multiple threads of execution, semaphores, .. that
> > > "hides" POSIX vs Win32 APIs .. i.e. the underlying OS APIs would be
> totally
> > > hidden.
> >
> > I think you are thinking in a "C" way. In Haskell, portable is the
> > default. If you want to stop your code being portable, you have to go
>
>
>      ^^ how? If I define something like "class OS where ...." and define a
> POSIX instance of "class OS" and a Win32 API instance.. function calls will
> be to the instances and hence the OS APIs are visible. Yes?

OK, I think I'm slowly figuring out what you are meaning. You want to
write some code which runs on POSIX, and some which runs on Win32,
each of which bind to some foreign library which is different on both
operating systems.

The key thing to note about this is that whether a program is running
on Windows or Posix is fixed at compile time. Things like classes can
do some of what you seem to be after, but it tends to be easier to
reach for the C pre processor.

Let's take a simple login scenario. In Linux you enter the user name
using the call getLine, and in Windows you call something else like
getInputString. The way I would structure this would be:

getUserName :: IO String
#ifdef WINDOWS
getUserName = getInputString
#else
getUserName = getLine
#endif

(Note - it wouldn't be WINDOWS - I can't remember what the blessed CPP
for distinguishing Windows and Posix is)

Then your logic code can call getUserName, and the platform
differences are papered over.

You could use classes, but for something which is OS dependent, I'd
tend to use the CPP.

Thanks

Neil