| > > class T root pos sel | pos -> root, root -> sel where
| > > f :: pos -> sel -> Bool
| > >
| > > instance T root (Any root) sel where
| > > f (ANY p) s = f p s
...
| That is not surprising. What is surprising is why GHC 6.6 accepts such
| an instance?
Well, it shouldn't. As the user manual says, the flag
-fallow-undecidable-instances lifts *both* the Paterson Conditions *and* the
Coverage condition. I stupidly forgot that the Coverage Condition is needed
both to help guarantee termination, and to help guarantee confluence (as our
own paper says!). Losing the latter is more serious, and should not be an
effect of -fallow-undecidable-instances.
This is the same issue as
http://hackage.haskell.org/trac/ghc/ticket/1241
What to do?
- Never lift the Coverage Condition
- Give it a flag all to itself -fno-coverage-condition
- Combine it with -fallow-incoherent-instances
More work is
- Implement some of the more liberal coverage conditions
described in the paper
Simon
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