* José Pedro Magalhães
Hi,
I would prefer it if packages would use DefaultSignatures conditionally, instead of creating new packages just to define a generic variant of a function. In general, though, I think it's good practice to define the defaults as separate functions (like `genericSize`) and export them, so that those not using DefaultSignatures can still write an instance easily.
Do you propose exposing genericSize conditionally? I'm strongly against this. I think that packages' interfaces may not depend on the compiler or compilation options. Only the implementation of the intreface may.
As for the concerns about people using DefaultSignatures and not realizing their code is not portable, I guess we could reject programs that use a default signature (when writing an empty instance) unless they enable -XDefaultSignatures. Then people would know they are relying on an extension.
Can't we simply ignore default signatures (and corresponding default definitions) when DefaultSignatures is off? E.g. instance Show Foo is accepted in any case, but has different semantics depending on whether DefaultSignatures is on. This way it's more Haskell2010 compatible. I realise that changing semantics is not very good, but people usually test this kind of code anyway, so I don't think it will be a big problem in practice. In that case it'd make sense to add a separate flag (-XUseGenericDefinition?) whose impact on the program semantics is more obvious from its name. Roman