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