[Haskell-cafe] Dynamic types through unsafeCoerce

Alfonso Acosta alfonso.acosta at gmail.com
Sat Dec 9 16:09:26 EST 2006

On 12/9/06, Taral <taralx at gmail.com> wrote:
> On 12/9/06, Alfonso Acosta <alfonso.acosta at gmail.com> wrote:
> > I've been using Data.Dynamic but the Typeable requirement doesn't go
> > well with FFI declarations (which don't accept type contexts).
> Can you be a little more specific?

Functions like this one are not directy exportable

myfunc :: (Tyeable a, Typeable b) => MyType a b ...

> > mt2mgt :: MyType a b -> MyGenType
> > mt2mgt = MyGenType.myToDyn
> >
> > mgt2mt :: MyGentype -> MyType a b
> > mgt2mt (MyGenType dyn) = myfromDyn dyn
> >

> mgt2mt . mt2mgt :: MyType a b -> MyType c d
> Yes, it's dangerous. The reason Dynamic requires Typeable is to be
> able to check that you're casting Dynamic back to the original type.

Uhm thinking about it right now I realized that mgt2mt is hidden to
the user, so I guess it wouldn't be dangerous if transformations like
the following one are not.

MyType Int Char ---> MyGenType ----> MyType a b

So to summarize, Is transforming a monomorphic type to it's
polymorphic equivalent through unsafeCoerce a dangerous operation?

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