Proposal for stand-alone deriving declarations?
Malcolm.Wallace at cs.york.ac.uk
Wed Nov 1 16:46:43 EST 2006
"Brian Smith" <brianlsmith at gmail.com> writes:
> > http://haskell.org/haskellwiki/GHC/StandAloneDeriving
> Basically, the syntax
> for instances and derived instances would be identical, but derived
> instances would just omit the "where" clause, while non-derived instances
> would need the "where" clause.
This is a really _bad_ idea for readability. In a year's time, when
this discussion is forgotten, who would be able to tell at a glance the
deep and real semantic difference between the following two instance
instance Num (Bar z) where
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
real killer is that both of these decls are already valid in Haskell'98,
but they mean the _same_ thing (the former, if you were wondering).
This is a sure recipe for introducing new and subtle bugs into existing
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