[Hs-Generics] Re: Splitting SYB from the base package in GHC 6.10

[José Pedro Magalhães]

Hello,

On Fri, Sep 5, 2008 at 14:34, Jules Bean <jules at jellybean.co.uk> wrote:

> José Pedro Magalhães wrote:
>
>> Complex has a derived instance. In Data.Complex:
>>
>>    data (RealFloat a) => Complex a
>>      = !a :+ !a      # if __GLASGOW_HASKELL__
>>            deriving (Eq, Show, Read, Data)
>>    # else
>>            deriving (Eq, Show, Read)
>>    # endif
>>
>> Was there a problem with this?
>>
>>
> Yes, isn't it exactly the same problem as Ratio?
>
> The derived instance 'leaks' the implementation detail of a complex number
> as a real and imaginary component. As part of a large structure I might want
> to apply some operation to all the 'Doubles' therein, but certainly not to
> the components of any Complex Doubles. You can imagine particular situations
> where I want to apply only to the real component, or something more subtle.
>
> You can also imagine that an alternative implementation of Complexes in
> polar coordinates (with a suitable solution to the principle value problem)
> is supposed to be abstractly the same, but would gain a totally different
> Data instance.


I think the question here is if the datatype if abstract or not. Complex
exports its (:+) constructor, so the implementation details are not hidden.
Therefore it is correct to traverse its arguments. Ratio, however, does not
export its (:%) constructor: Ratio is abstract, while Complex is not.

I guess then Complex should have a Data instance as it is above.

But regarding Ratio, do we want to change its gfoldl to match its current
gunfoldl (as suggested before), or go the other way around, and change
its gunfold, toConstr and dataTypeOf to match its gfoldl?

If we change its gfoldl, the instance is fully defined, but does not respect
the data abstraction.

If we go the other way around, the instance becomes partial (gunfold and
toConstr return errors), but respects the data abstraction.

In any case, the behaviour would differ from the current. Any opinions on
this?


Thanks,
Pedro
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