As you will know, for any identifier ident in C, you can write &ident to obtain the memory location (pointer) to ident, regardless of whether that location is on the heap or on the stack. Whilst working with C types, it is often helpful to have access to this functionality.
Is there an equivalent operation in Haskell in the FFI?
If you have a C identifier you're importing into Haskell, you can convert it to a pointer to that address via the "&" syntax in FFI declarations, like so:
foreign import ccall "&" bar :: Ptr CInt
Typically this is quite a rare thing to do, as almost everything you import is already a pointer.
Haskell values themselves may also be turned into pointers, using stable pointers, to ensure they're unaffected by the garbage collector.