Gerrit,
Thanks for your help. Yes, your suggestion below will allow me to
give only an instance declaration for B1 (and get the A declaration
for free, if you will).
However, I meant to suggest that there were other T's which are not
instances of B1 but of B2. I will be back in the same situation for
those T's as I was previously in with T. I can't add
instance B2 a => A a
in addition to
instance B1 a => A a
because of the duplicate instance situation. So, my problem remains.
--
Robin Bate Boerop
On 22-Apr-06, at 2:01 PM, Gerrit van den Geest wrote:
Robin,
The following does NOT work, because of a duplicate instance
declaration for A:
class A a
class B1 b1
class B2 b2
instance B1 x => A x
instance B2 x => A x -- duplicate instance, won't compile
data T = T
instance B1 T
Yes, this doesn't work and I think there is no GHC extension that
supports this.
The following DOES work, but it requires that I explicitly give
instance declarations of A for all instances of B1 and B2:
class A a
class A a => B1 a
class A a => B2 a
data T = T
instance A T -- I don't want to have to specify this!
instance B1 T
If you replace the instance you don't want to specify with:
instance B1 a => A a
and use the following flags to start ghc:
-fglasgow-exts -fallow-undecidable-instances
You can add other datatypes, and you only have to give an instance
for class B1.
Good luck!
Gerrit
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