Re: [Haskell-cafe] Re: [Haskell] Re: Global Variables and IO initializers

[Robert Dockins]

As a purely practical matter, it seems like the easiest solution (to 
this particular use case) is to write a small wrapper initializer in C 
which is idempotent, then use FFI to call the wrapper, rather than 
calling the initialization directly.  This is easy enough to do with a 
static local variable:

void doInit()
{
    static int doneInit = 0;
    if( !doneInit ) {
       reallyInit();
       doneInit = 1;
    }
}
Then your haskell libs can call doInit any number of times, and
reallyInit will be called at most once.
Since your committed to FFI anyway (calling a C lib is the premise), the 
wrapper seems a small price to pay.  For Haskell-only code, something 
else would be nice.

Keith Wansbrough wrote:
Adrian Hey writes:

The problem is simple enough to restate for anyone who's interested.
"Provide a simple reliable mechanism to ensure that in a given
program run one particular top level IO operation cannot be executed
more than once."

Can you give one concrete example of an "intended application
of oneShot", so that we can either propose a concrete Haskell
implementation of it, or agree that global variables really are necessary.
Any C library which requires an explicit initialisation call before anything
in that library can be used (common enough IME). Accidental re-initialisation
(e.g. by two independent modules/libraries) will destroy any state currently
be used by the libraries existing "clients".

Great, thanks, that's just what I was hoping for - I now see the
problem you're trying to address.
--KW 8-)
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