RE: Ptr type

I've moved this thread to https://ghc.haskell.org/trac/ghc/ticket/9163 | -----Original Message----- | From: mad.one@gmail.com [mailto:mad.one@gmail.com] On Behalf Of Austin | Seipp | Sent: 03 June 2014 14:22 | To: Richard Eisenberg | Cc: Simon Peyton Jones; ghc-devs@haskell.org | Subject: Re: Ptr type | | I think Ptr should almost certainly be representational, as it is a case | where the actual underlying value is the same, but what it points to is | not (I'll ignore castPtr here for a second). | | This makes me think of something I talked about before with Andres - in | general, phantom roles seem somewhat dangerous, and I kind of wonder if | they should be inferred by default. Often you see some code along the | lines of: | | ---------------------------------- | | module A ( Bar, newBarInt ) where | | data Bar a = Bar Int | | newBarInt :: Int -> Bar Int | | --------------------------------- | | where A is exported to the client and the module boundary enforces some | restrictions on 'Bar', like what types we can instantiate 'Bar a' | to (the example is dumb but bear with me). | | In the above example, I believe the `a` in `Bar a` would be inferred at | phantom role. | | The question I have is: what legitimate case would there be for phantom | roles like this? Such usage of phantom type parameters seems very | common, but in almost *all* cases I can't think of a reason why I would | ever want a user to be allowed to `coerce` away the type information, if | the `a` parameter was phantom. It seems like in the above example, I | would almost certainly want `a` to be representational instead. | | What other cases exist for legitimate phantom roles such as this where | we want this inference? I wonder if instead we should require an | explicit role annotation for phantoms in this case - not the other way | around. | | Ptr is a similar story - by default it would maybe seem reasonable to be | phantom, but that seems like an especially grievous error, given we | don't want people to `poke` incorrect values in or something (again, | ignoring castPtr for a second, but I think the general idea holds.) | | | On Tue, Jun 3, 2014 at 7:24 AM, Richard Eisenberg | wrote: | > It seemed clear to me that Ptr *should* be representational, thinking | > that we don't want to coerce a `Ptr Int` to a `Ptr Bool`. I don't see | > any reason why this couldn't be changed. | > | > Why is Ptr a `data` not a `newtype`? If it were a newtype, we could | > keep the role annotation and use `coerce` internally, according to the | > updated Coercible solver. | > | > However, it is crucial that FunPtr have a representational argument, | > as normaliseFfiType' depends on this fact. There is a brief comment in | > TcForeign, but clearly more comments are necessary. Will do shortly. | > If we want to change FunPtr's role, normaliseFfiType' would have to be | > updated, too, but it shouldn't be hard. | > | > More comments in the code later today. | > | > Richard | > | > On Jun 3, 2014, at 7:13 AM, Simon Peyton Jones | > wrote: | > | > Richard | > | > In GHC.Ptr we see | > | > type role Ptr representational | > data Ptr a = Ptr Addr# deriving (Eq, Ord) | > | > with no comments. Why is Ptr representational? | > | > In the same module we have castPtr, which unpacks and repacks a Ptr. | > If Ptr was phantom, we could use coerce. And that in turn would | > actually make a lot of code more efficient – there are lots of calls | to castPtr. | > | > Simon | > | > | > | > | > | > _______________________________________________ | > ghc-devs mailing list | > ghc-devs@haskell.org | > http://www.haskell.org/mailman/listinfo/ghc-devs | > | | | | -- | Regards, | | Austin Seipp, Haskell Consultant | Well-Typed LLP, http://www.well-typed.com/