[Haskell] Empty instance declaration

[Malcolm Wallace]

ajb at spamcop.net wrote:

> One thing that hasn't come up yet is that empty instance declarations
> are the only decent option (that I know of) that we have in the
> absence of real class aliases.

It does seem to me that compilers could reasonably distinguish between
incomplete definition:

> > class (Monad m, Ord t) => ReVars m t where { }
> > instance (Monad m, Ord t) => ReVars m t where { }

and intentionally incomplete definition:

    class (Monad m, Ord t) => ReVars m t
    instance (Monad m, Ord t) => ReVars m t

Both syntaxes (with and without the 'where') are currently legal, but
the latter is more obviously deliberate (at least to this human reader).

Regards,
    Malcolm