On Thu, Jul 03, 2003 at 06:51:31PM +1000, Manuel M T Chakravarty wrote: > Ross Paterson <[EMAIL PROTECTED]> wrote, > > > The new wording: > > > > \code{unsafePerformIO} may compromise typing; to avoid this, the programmer > > should ensure that the result of \code{unsafePerformIO} has a monomorphic > > type. > > > > rules out the following: > > > > my_hash :: Storable a => a -> Int > > my_hash a = fromIntegral $ unsafePerformIO $ > > allocaBytes (sizeof a) $ \p -> do > > let size = fromIntegral (sizeOf a) > > c_memset p 0 size > > poke p a > > hash_bytes p size > > > > foreign import ccall unsafe "memset" > > c_memset :: Ptr a -> CInt -> CSize -> IO () > > foreign import ccall unsafe > > hash_bytes :: Ptr a -> CSize -> IO CInt > > Why is this ruled out? hash_bytes returns a `CInt', which > is a monomorphic type.
The argument of unsafePerformIO has type forall a. Storable a => a -> CInt > > Manuel writes: > > > However, it is possible to construct examples that are deterministic, > > > but still dubious from a typing perspective. Let's assume a C routine > > > > > > void *foo(); > > > > > > that *always returns the same pointer* to a buffer area. To > > > bind this in Haskell as > > > > > > foreign import ccall foo :: Ptr a > > > > > > is problematic[1]. > > > > (a) It's constant across a run of the program, but its value still depends > > on the environment, and > > Yes, and that's nothing that we want to rule out. A > standard idiom for obtaining constant values from C is > > -= In C land =- > > int my_const () > { > ... > return ...; > } > > -= In Haskell land =- > > const :: Int > const = unsafePerformIO my_const > > foreign import ccall my_const :: IO Int > > All that's required here is that my_const() is constant > within a program run. Shouldn't it be constant in a global sense, e.g. getpid wouldn't be allowed? _______________________________________________ FFI mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/ffi