#9163: Ptr should have a phantom role -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 7.10.1 Component: Compiler | Version: 7.8.2 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | roles/should_compile/Roles2 Blocked By: | Blocking: Related Tickets: #9164 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I think there's a bit of a tension here between two different camps who want `Ptr`'s role signature to be phantom or representational for different purposes: 1. Some have argued that it should be phantom because that allows one to implement `castPtr` (a quite fundamental operation) efficiently via `coerce`. 2. Others have argued that it should be representational to avoid coercing between `Ptr`s whose underlying types have different representations. Personally, I find myself more aligned towards the (1) camp, for the simple reason that `Ptr`s are not intended to be a safe abstraction. There are many ways you can cause a Haskell program to segfault through reckless use of `Ptr`s (even without `castPtr`), so changing its role signature isn't going to change things in this regard. In light of this, I'm inclined to believe that the proper fix to dominic's problem is to give `Vector` a role signature of representational, as `Vector` is what is intended to be a safe abstraction. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9163#comment:34 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler