Eq instance for Data.Data.DataType

[Eelis-]

The DataType? data type in Data.Data currently does not derive an Eq instance, even though the types of both of its fields do have Eq instances.

[Eelis-]

trivial patch deriving the Eq instance

[dreixel]

Should this instance be derived (and therefore compare both the name and the representation) or use the same approach as Constr, comparing only the representation?

See: http://www.haskell.org/ghc/dist/stable/docs/libraries/base/src/Data-Data.html#DataType

[Eelis-]

From ticket #3866 it looks like the semantics of the Eq Constr instance you refer to are considered unfortunate and are really only preserved for backward compatibility. For DataType? we have no such constraint, and I think that its Eq instance should really just do what one would expect, meaning that all the fields (rather than some subset thereof) should count.

[igloo]

It feels wrong for DataType and Constr to have Eq instances that behave differently. Should we have a  library proposal to change them both to be derived?

[igloo]

I'm going to close this, as it's not clear to me what the right thing to do is, so I think a  library proposal is the right way forward.