Quoth Yves Pares, ...
If StablePtrs cannot have their pointed value modified (either C or Haskell-side), that mostly limits their interest, doesn't it?
I'm not sure I follow what's happening in the two cases you mention, but in case it helps, I use StablePtr as a way to smuggle Haskell values through a non-Haskell layer. On the other side, they are just things - not really pointers, so while void* is all right as long as it's the right size, it is a little misleading. "Tag" might be a better word. castStablePtrToPtr doesn't change that - you still get a tag, not a memory address. I can think of no use for it, unless you want to call a function that happens to use a Ptr () without expecting it to point somewhere in the memory address sense. Since they're Haskell values, it would be perverse to modify them. I just want them back, in a Haskell function dispatched by the other layer. If I want to pass data per se to the other side, I have to marshal it to be readable CInts and such, and then pass it as a Ptr. And/or if the data is to be modified by the other side, same deal, I have to un-marshal it back into Haskell. I actually use them with a C++ layer, where of course the dispatch happens through C++ object member functions. That allows me to map Haskell functions to the C++ API in a C++ derived class that just maintains a table of the actual Haskell `member functions', and a StablePtr for the actual Haskell data. Donn