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