request for reviews for my first patch -- ticket 7401

[Richard Eisenberg]

According to the Haskell 2010 report (http://www.haskell.org/onlinereport/haskell2010/haskellch11.html#x18-18200011), a datatype with no constructors cannot derive any instances.

But, instead of creating a new extension for this feature, what about just co-opting EmptyDataDecls? More concretely, I propose this:

Under H98: EmptyDataDecls allows both the declaration of empty data decls and deriving instances for them.

Under H2010: EmptyDataDecls allows deriving instances for empty data decls.

This proposal brings the annoyance that H2010 no longer implies EmptyDataDecls.

Thoughts?

Richard

On Aug 11, 2013, at 10:28 PM, Austin Seipp wrote:

> Technically, EmptyDataDecls is part of Haskell 2010, so it is standard
> these days (and H2010 is our default in GHC.)
> 
> As far as I know, the 2010 standard doesn't address this point about
> deriving for empty data decls, but I agree with your reasoning here.
> It's more consistent to have it apply to all of the base classes which
> are derivable.
> 
> On Sun, Aug 11, 2013 at 9:14 PM, Richard Eisenberg <eir at cis.upenn.edu> wrote:
>> Maybe it's best if Show, Ord, etc., echo the new behavior for Eq, if EmmptyDataDecls is specified. The reason for the check in cond_stdOK is to make sure that we're conforming to the Haskell standard. But, if the user has specified EmptyDataDecls, then we're not bound to that requirement anymore. So, to me, it seems reasonable that the "null data_cons" check should be omitted for any class if EmptyDataDecls is specified.
>> 
>> What do others think? This would go beyond the scope of the particular reported bug, but it seems to make sense to me.
>> 
>> Thanks,
>> Richard
>> 
>> On Aug 9, 2013, at 3:16 PM, Ömer Sinan Ağacan wrote:
>> 
>>> Removing `null data_cons`(compiler/typecheck/TcDeriv.lhs 1105) check
>>> for eq instance declarations fixes this, code generation part would be
>>> same(changes in TcGenDeriv.hs were wrong and TcGenDeriv needs to be
>>> unchanged)but I'm not sure how to remove that check only for eq
>>> instance(Show, Ord, etc. sould still fail with same error message and
>>> they're checked by same function, `cond_stdOK` -- line 1053).
>>> 
>>> ---
>>> Ömer Sinan Ağacan
>>> http://osa1.net
>>> 
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>> 
>> 
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> 
> 
> -- 
> Regards,
> Austin - PGP: 4096R/0x91384671
>