I have a small C interface with a C type DB. DB handles are only administed on the C side; at the Haskell end they are a black box. The Haskell side never sees DB values, just values of type DB * (pointers to DB). I'm wondering how to represent this.
The usual is something like: data DB -- note that we have no constructors foreign import ... :: blah -> IO (Ptr DB)
One solution would be just to represent the equivalent of DB * as type DBP = Addr (or Ptr () or something) but then of course it might be confused with any other Addr.
I think Addr is deprecated (in favour of Ptr and FunPtr). It doesn't even appear in section 3.2. [The other type that you might expect but which was dropped was 'Word' - the reasoning being that it is an obstacle to writing portable code. 'Int' should be excluded for the same reason but that was even more controversial than omitting Word :-)] I'd say that 'Ptr ()' is discouraged partly because it's an accident waiting to happen and also because it prevents you from creating meaningful instances of Storable.
A rather more ingenious solution would be something like
newtype DBPInternal = DBPInternal Int type DBP = Ptr (Ptr DBPInternal)
I think this might actually work in that a DBP would be marshallable and couldn't be confused with any other type not involving DBPInternal. Elegant it isn't, since it involves a pretend pointer inside the Ptr just to get the types working.
I think this is too ingenious for me. Not knowing about the common trick of writing a datatype with no data constructors (note: not sanctioned by H98), I can see why you might have gone for: newtype DB = DB_Dont_Use_Me Int type DBP = Ptr DB or newtype DB = DB_Dont_Use_Me Int foreign import ... :: blah -> IO (Ptr DB) the idea being that no-one would ever use the data constructor or the Int. I'm a bit puzzled by the double-layer of pointers though. -- Alastair Reid reid@cs.utah.edu http://www.cs.utah.edu/~reid/