[Haskell-cafe] Dynamic types through unsafeCoerce

[Alfonso Acosta]

On 12/13/06, Brandon Moore <brandonm at yahoo-inc.com> wrote:
> It looks to me like the C code could make your program segfault from a
> type missmatch, if it
> gets a handle from instantiating one descriptor, and tries to run it
> from another.

Well, the C code could even provide (by error or at will) any pointer
value which could mismatch or not. C is potentially dangerous by
definition and there is no way to cope with that whatever we export
from Haskell. This is just a particular case of that major problem, in
which we use a instance handle.

> Building code which can fail that way hopefully requires unsafeCoerce!

Any haskell code which receives a C pointer is likely to suffer from
the type-mismatch segfault problem you mentioned above.

The purpose of using unsafeCoerce in my code is to allow me to pack
some polymorphic values in a list as if they ware homogeneous. I don't
see how unsafeCoerce would help me with a type mismatch.

> I think Udo's code is the best way to go, if it solves your problem.
> Udo's code is equivalent to code similar to yours but defined around the
> existential type
>
> data Descriptor =
>     forall hd . Descriptor { -- create a new instance and return its
> handler
>                 instantiate            :: InstanceInitData -> hd,
>                 -- Run and return a new handler
>                 run                    :: hd   -> IO hd}

Hiding the hd parameter breaks the C-exported cRun function.

> Then observing that because hd is hidden you can only use an hd by
> passing it run, so you
> might as well wrap it up in that Runner type.

Which yet again, requires splitting Descriptor in two.

> If you want to be able to define several Descriptors over the same type
> hd, and ....

Different descriptors are allowed to have different handlers (void *)
in C. That's a precondition, and furthermore, the source of my
modelling problem and this thread :)