[Haskell-cafe] Re: Can my type be allowed as return type in FFI?
briqueabraque at yahoo.com
Wed Dec 10 11:17:26 EST 2008
> > > foreign import "nameOfFunction" nameOfFunction
> > > :: IO MyType
> > > Is it possible to write a new MyType and make it allowed as
> > > a return type from foreign functions? Is changing the
> > > compiler the only way to do that?
> > Of course you're not really returning a MyType but a pointer
> > to one. (...)
This would solve half my problem. Can I always trust that? I've
been told before that everytime a C function returns a struct it
is actually returning a pointer, but I wasn't able to find that
written in stone so that I can trust it to be valid anywhere
Haskell will run and for any return type (like a struct containing
just a char, for instance).
How does that pointer work? My code didn't provide it to the C
function, so, I believe I don't have to deallocate it. Where does
the memory it points to live? Did Haskell FFI engine gave that
pointer to the C function? How much time is it going to live?
Enough so I can 'peek' it?
> > Using raw Ptrs is not very nice, so we would usually either
> > wrap that in a ForeignPtr or use a Storable instance to get a
> > MyType directly.
OK. I've instances of Storable for all the appropriate types.
> > If do recommend reading the FFI spec. It's quite readable and
> > explains a lot of the issues.
Sure. It helped a lot. Actually, I've been able to do everything I
needed so far with just the spec and Foreign.*.
> I would also add the chapter 17 of Real World Haskell. you can
> read it here:
Nice. It shows unsafePerformIO. Some functions returning structs
are side effect free. I'm writing bindings I want to put on
hackage, so what do you think would fit better the taste of
Haskell programmers: pure functions using unsafePerformIO or
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