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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