#9164: Hunt down uses of `castPtr` ------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- This bug is a fork from #9163: nomeata: Slightly off topic, but maybe `GHC.IO.Handle.Text` can be written so that `castPtr` is not needed. It seems it always works with `Ptr Word8`, but exposes an API that uses `Ptr a` as a generic pointer. The type `a` is never needed inside `GHC.IO.Handle.Text`... I tried it, and `hPutBuf` & Co are called with `Ptr Word8` (sometimes called LitString) in all cases but `BufWrite.bPutCStringLen`, where we have a `Ptr CChar`, where `CChar` is a newtype. So by using coerce there (and `unsafeCoerce` during bootstrapping), we get the best of boths worlds: The more restrictive role and the free behaviour. SPJ: Well regardless of the decision about the role of `Ptr`, what you describe sounds like an improvement anyway, getting rid of (always suspicious) `castPtr` calls. If you grep in the base library you'll find quite a few others. Can you eliminate them in the same way? I think this would be a nice improvement, if you cared to push it through. simonmar: So I've been assuming that `castPtr` was free all this time, and it turns out it isn't! Please make it free. We can't change the type of `hPutBuf` and friends without potentially breaking code. There's no better choice than `Ptr a` here, because we don't know the type of the underlying data. Making it `Ptr Word8` would just force the caller to use `castPtr` sometimes without any benefit. e.g. `Ptr CChar` is also a sensible choice, but it could in reality be anything. It does look like the two `castPtrs` in `bufWrite` are unnecessary, though. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9164 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler