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?
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