Proposal for stand-alone deriving declarations?

[Brian Smith]

On 11/2/06, Bulat Ziganshin <bulat.ziganshin at gmail.com> wrote:
>
> Hello Malcolm,
>
> Thursday, November 2, 2006, 12:46:43 AM, you wrote:
>
> >     instance Num (Bar z) where
> > and
> >     instance Num (Bar z)
>
> > The former declares that _no_ methods are defined (except for defaults),
> > and the latter, with your proposal, that _all_ methods are defined. The
>
> i join to this note. moreover, currently GHC supports "generics for
> the masses" that may mean very subtle semantic changes between code
> generated by these two forms :))


Thank you guys for your responses.

Effectively, every class method has a default implementation: if the
programmer does not supply a default implementation, then the the "default"
default implementation is undefined.

But, does it make sense to create a class with default methods, but which is
also derivable, such that the derived instance declarations are different
than if the default implementations are chosen? When would it make sense to
allow:

    derive instance A B

and

    instance A B

and have them both be valid (not at the same time, of course), but mean
totally different things? I think it would be very confusing, especially for
the innocent user who simply forgets the "derive" psudo-keyword.

Would it be possible to make deriving a special case of defaulting? That is,
could we define every derivable class so that every method defaulted,
instead of to undefined, to some "deriving magic," perhaps expressible in
Template Haskell? Perhaps we could say that all classes that have all their
methods defaulted are then derivable?

- Brian
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