It's great if the instances preserve the abstraction.
As I said, I'm not a fan of Data, but it makes certain things very convenient.
On Sun, Apr 19, 2009 at 11:41 PM, Duncan Coutts
On Sun, 2009-04-19 at 13:58 -0700, Ashley Yakeley wrote:
I will not expose any private structure of types via instances of the Data class. I can't see what benefits SYB brings that it outweighs such loss of abstraction. And if someone else wants to shoot themselves in the foot by using standalone deriving, that's their problem.
It's ok, abstraction is preserved by standalone deriving. You can only derive for types where you can import the constructors.
However, this might not be an issue, as I believe all the constructors of all or almost all types in time are public. If it is possible for a user of time to hand-write instances of Data using only what time exposes, then it is safe to include those instances in the package.
Right, that seems like a sensible solution, that is to include Data instances that preserve the abstraction. As you say, that can be tested by initially writing the instances outside the package so that you can be sure it's only using the public interface.
Duncan
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