Simon Peyton-Jones wrote:
| t1 = runST (trav f [1..10] (1,52) >>= \ (s::STRef s (Set Int)) -> seen s)
try
t1 = runST ( (trav f [1..10] (1,52) >>= \ s -> seen s) :: forall s. ST s [Int] )
No, the problem is that t1 should use another implementation than t2. This version cannot discriminate between different implementations anymore. (And logically it did not compile.) In 6.4.2 I can choose the implementation just by type annotation but that's now impossible...
What's interesting about this example is that you aren't trying to bind the type variable 's'. You'd be quite happy to write \ (s :: STRef _ (Set Int)) -> seen s
Note the wildcard "_". Arguably, one could loosen the rules in this case. This is an avenue that I have seen suggested before, but which I have, for one, not yet explored. Maybe others have better ideas.
For me it would exactly solve the problem. So maybe one could 'quick hack' it into ghc!? Regards, MR -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---