[Haskell-cafe] OS Abstraction module??

[Ryan Ingram]

Also, note that it's really easy to hide all the CPP uglyness in a
single file, or do it with Makefiles, or something along those lines.

By the way, you don't want to use typeclasses here; they solve the
problem of having more than one possible interface at runtime, whereas
you only care about compile-time.  The easiest way to solve that
problem is through the module system.  An example follows.

with this directory tree:
System/Abstraction.hs
System/Abstraction/POSIX.hs
System/Abstraction/Windows.hs

in System/Abstraction.hs:

module System.Abstraction (
#if POSIX
    module System.Abstraction.POSIX
#elsif WINDOWS
    module System.Abstraction.Windows
#endif
) where

#if POSIX
import System.Abstraction.POSIX
#elsif WINDOWS
import System.Abstraction.Windows
#else
#error Unknown system type
#endif

Now you can write POSIX.hs and Windows.hs independently to implement
whatever OS/foreign interface you care about, and your other code can
import System.Abstraction without needing -cpp.

Alternatively you can make POSIX.hs and Windows.hs just declare
themselves as "module System.Abstraction" and use your build system to
compile the correct one.