Hello,
I am not sure what GHC is doing, it certainly seems to be
inconsistent. In Hugs both the examples work. In case you are
interested, here is how you can get a version that works in
both Hugs and GHC (I just modified your code a little):
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
module Add where
data Zero; zero = undefined :: Zero
newtype Succ n = Succ n
class Add n m s | n m -> s where
add :: n -> m -> s
add = undefined
instance Add Zero m m
instance Add n m s => Add (Succ n) m (Succ s)
class Fib n f | n -> f where
fib :: n -> f
fib = undefined
instance Fib Zero (Succ Zero)
instance Fib (Succ Zero) (Succ Zero)
instance (Fib n fib_n,
Fib (Succ n) fib_s_n,
Add fib_n fib_s_n sum
) => Fib (Succ (Succ n)) sum
eight = fib (Succ (Succ (Succ (Succ (Succ zero)))))
two = add (Succ zero) (Succ zero)
*Add> :t eight
eight :: Succ (Succ (Succ (Succ (Succ (Succ (Succ (Succ Zero)))))))
*Add> :t two
two :: Succ (Succ Zero)
-Iavor
On 8/16/05, Dirk Reckmann
Hello Keean!
Am Dienstag, 16. August 2005 13:48 schrieb Keean Schupke:
Picked up on this late... I have working examples of add etc under ghc/ghci... I can't remeber all the issues involved in getting it working, but I can post the code for add if its any use?
Yes, that would be nice. I'd like to see 'add' working... However, after each answer to my posting, I get more confused. Simon Peyton-Jones took all of my hope to get it working, because ghc doesn't like universal quantified but uniquely idetified types (at least, this is my understanding of his email). Now you have a working 'add' typelevel program. And the most confusing part for me is, that my fibonacci number program works, even though it makes use of the not working version of add.
So, I'm really looking forward to your version!
Ciao, Dirk _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users