OK I have tested with today's GHC 7.6.2, which is very slightly later than the
release candidate.
When I add (EuclideanRing (UPol k)) to the signature for cubicExt, the whole of
demotest compiles. So that works.
Your misconception is here:
| c) Pol3_ has
| instance (LinSolvRing (Pol a), CommutativeRing a) =>
| LinSolvRing (UPol (Pol a))
| But this does not overlap with Field k => LinSolvRing (UPol k).
|
| Because Pol a is never of a Field nor of EuclideanRing.
When matching instances, GHC does not take account of the context of the
instance. Say you have
data T a = ...
data S1 = ...
data S2 = ...
[a] instance Num a => Num (T a) where ...
[b] instance Show a => Num (T a) where ...
instance Num S1
and suppose you need an instance of (Num (T S1)). Then although [a] and [b]
overlap, you might say we should use [a], since S1 is an instance of Num, but
not an instance of Show. But GHC does not do this. It matches instances only
based on the bit after the "=>".
I'm sorry, but that's the way it is. (There are good reasons for this.)
If you don't care which of the overlapping instances is picked, then you can
use -XIncoherentInstances. But you have to set that flag at the *instance
declaration* not at the *use* of the instance. In this case it would be the
instance for LinSolvRing (UPol k)
Simon
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