"Axel Jantsch" <axel@kth.se> wrote:
I call a C function from a Haskell program. I am using unsafePerformIO to use it outside a monad.
Even though the C function does not have any side effect, I absolutely don't want to evaluate it more than once for performance reasons. But my impression is that it is evaluated several times.
1. Can I monitor somehow how often it is evaluated?
You could wrap it (in C) with another function that keeps a counter of invocations in a static local variable. result_t originalFn (arg1_t arg1, arg2_t arg2); result_t wrappedFn (arg1_t arg1, arg2_t arg2) { static int i = 0; i++; fprintf(stderr,"originalFn called %d times\n",i); return originalFn(arg1,arg2); }
2. Can I ensure that the function is evaluated only once?
How about stating in the type of the FFI decl that the C function is pure (even if it is not)? Then be sure to bind its result in only one place. That should guarantee it is called only once. foreign import ccall originalFn :: Arg1T -> Arg2T -> IO ResultT becomes foreign import ccall originalFn :: Arg1T -> Arg2T -> ResultT Regards, Malcolm