Proposal for stand-alone deriving declarations?

[Brian Smith]

On 11/1/06, Malcolm Wallace <Malcolm.Wallace at cs.york.ac.uk> wrote:
>
> "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
> decls?
>
>     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
> 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
> bug-free programs.


Malcolm, thank you for pointing that out. To be honest, I had not read this
sentence of the report (section 4.3.2 of
http://haskell.org/onlinereport/decls.html):

"If no binding is given for some class method then the corresponding default
class method in the class declaration is used (if present); if such a
default does not exist then the class method of this instance is bound to
undefined and no compile-time error results."

Personally, I think that the part after the semicolon is the part that leads
to subtle bugs, not Brian H's proposal. Why is this not considered a
compile-time error? There was a previous mention of it on the list [1] but
there was no explanation given. I think it would be better if the report
stated this instead:

"If no binding is given for some class method then the corresponding default
class method in the class declaration is used (if present); if such a
default does not exist then a compile-time error results."

[1] http://www.haskell.org/pipermail/haskell/2003-May/011899.html

- Brian
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