On Sat, May 19, 2007 at 08:40:46PM +0100, Matthew Sackman wrote:
The following doesn't seem to work. Is this a limitation of the current implementation or will it never work? Are there any work arounds without introducing extra type params into the data type E?
class G a b | a -> b where data E a :: * wrap :: b -> E a unwrap :: E a -> b
instance G a b where data E a = EC b -- this line - the b is not in scope. wrap = EC unwrap (EC b) = b
I get "Not in scope: type variable `b'". I was rather hoping it would be in scope.
Having thought about this for some time, I think I can see a reason that this might not be possible. Classes are open, thus the a -> b does not ensure that there will only ever be one E a for each unique a in G a b, given multiple modules.
From the paper [0], pg 10:
"We could also in principle allow an associated type to mention only a subset of its parent class parameters; but then we would need to make extra tests to ensure the instance declarations did not overlap taking into account only the selected class parameters to ensure the type translation described by \Omega is confluent." Well, I suppose that given the open type classes, the instance declarations can overlap, despite the fundep. Does this mean that until we get closed classes, this sort of thing is impossible (without ugly workarounds)? Any thoughts at all? Anyone? Is it the case that associated types really allow nothing that can't be done with fundeps? [0] "Associated Types with Class" http://www.cse.unsw.edu.au/~chak/papers/CKPM05.html Matthew -- Matthew Sackman http://www.wellquite.org/