Alfonso Acosta wrote:
Ok, instead of pushing about why I want to use unsafeCoerce (which I know it's not a good thing) I decided (as suggested by Taral) to paste a simplified example of my code.
If anyone finds a way of implementing something equivalent to this code without unsafeCoerce# and ...
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. Building code which can fail that way hopefully requires unsafeCoerce! 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} 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. If you want to be able to define several Descriptors over the same type hd, and trade handles back and forth between their run functions, then you will need some casting (but tools from Typeable and Dynamic should be enough, without bringing unsafeCoerce) Brandon