Simon Peyton-Jones wrote:
The coverage condition is described in the paper
http://research.microsoft.com/~simonpj/papers/fd-chr
Use -fallow-undecidable-instances to make it compile.
Thanks, it seems to compile now. I've had a quick look at
the paper you mentioned and I also see the latest ghc user
guide has something to say about this too. But I'm still
not really clear about what the problem is :-(
(But I haven't really had time to study the paper properly
yet either.)
| > -- Generalsed Trie map class
| > class Ord k => GT map k | map -> k , k -> map where
|
| > -- Map type for pairs
| > newtype (GT2 map1 map2 a) = GT2 (map1 (map2 a))
|
| > -- Which is an instance of GT
| > instance (GT map1 k1, GT map2 k2) => GT (GT2 map1 map2) (k1,k2) where
Intuitively, this looks quite unambigous to me and allowing undecidable
anything worries me a bit. It makes me think my code is inherently
flakey somehow (not sure how right now though :-).
Regards
--
Adrian Hey
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