Force single evaluation?

Axel Jantsch axel at
Wed Dec 20 10:10:00 EST 2006

I tried both alternatives:

foreign import ccall originalFn :: Arg1T -> Arg2T -> IO ResultT

foreign import ccall originalFn :: Arg1T -> Arg2T -> ResultT

but in both cases the C function originalFn is called several times for
the same arguments. And it is bound only in one place.

I don't understand why.
Maybe the problem is that the data marshaling has to be done within the

My simplified codes is as follows:

f :: InparT -> IO CInt
f inPar = do
          inP <- malloc
          poke inP inPar

          r <- cfun inP 

          free inP
          return r

foreign import ccall "cfun" :: Ptr InparT -> IO CInt

So even if I make cfun pure (which it is), the enclosing function f is

Shall I pretend f is pure and wrap it into unsafePerformIO? 


Malcolm Wallace <Malcolm.Wallace at> wrote:

> "Axel Jantsch" <axel at> 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
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