Re: FW: [Haskell-cafe] hsc2hs and Storable (especially) unsafe

Somebody claiming to be Simon Peyton-Jones wrote:
>| It so happens that on 64 bit OS X, haskell's Int is 64 bits, while C's
>| int is
>| 32 bits.  hsc2hs's #poke does no typechecking at all, so if you have
>|
>| (#poke Struct, int_field) structp int -- where int :: Int

One should not do this.

That said, the tool should not allow this.

>| It's probably not even correct to
>| assume Double = CDouble and Float = CFloat, though it seems likely to be
>| always true.

base has a configure check for these, but I have been unable to determine 
what it does.  It fails on my QNXNTO cross-compiler and so I've been 
manually hacking this assumption in there, but ... yeah ... probably not 
safe.

>| I carefully vetted all my #pokes but I'm annoyed that hsc2hs and Foreign
>| let me get away with this.  If Storable is for marshalling types to and
>| from C, then non-C types should not be Storable!

Well, except that you might just be round-tripping some Haskell data through 
(void*) in C, and so the interpretation on the C side does not matter in 
that case.

>| While we're at it, fromIntegral is not a safe way to convert a haskell
>| type to a C one.  But it sure is a tempting one

It is the one I always use, but I'm very aware that the behaviour is likely 
to be bad.  That said, the behaviour of `fromIntegral` moving to a smaller 
type is always bad.  This should probably exist:

fromIntegralSafe :: (Integral a, Ord a, Bounded b, Integral b) => a -> Maybe b
fromIntegralSafe x
	| toInteger x > toInteger (maxBound `asTypeOf` v) = Nothing
	| toInteger x < toInteger (minBound `asTypeOf` v) = Nothing
	| otherwise = Just v
	where
	v = fromIntegral x

>| Or maybe I shouldn't be using hsc2hs in the first place.

Hard to say.  I've used it a couple times, mostly when there are C enums 
involved.  Usually in my FFI work I just write the imports by hand (though 
this also allows for dangerously writing `Int` when you mean `CInt`, etc).

-- 
Stephen Paul Weber, @singpolyma
See <http://singpolyma.net> for how I prefer to be contacted
edition right joseph

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