ANN: H98 FFI Addendum 1.0, Release Candidate 10

[Ross Paterson]

On Thu, Jul 03, 2003 at 06:51:31PM +1000, Manuel M T Chakravarty wrote:
> Ross Paterson <ross at soi.city.ac.uk> 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?