Laws for GHC.Generics

[Gershom B]

Yes, you're right about `to . from`. I had it backwards.

Vis a vis:

> If either function is partial, you will be in a world of pain if
> you try to use something like generic serialization.

I'm not sure I understand. Deserialization is already partial if we're
considering the source as "all possible strings of bits" and the
target as a specific datatype? I don't see how having the conversion
from a generic be partial as well makes things any different?

(i.e. i think we're in a retract situation already)

-g



On Mon, Mar 20, 2017 at 5:16 PM, David Feuer <david.feuer at gmail.com> wrote:
> Ryan Scott has argued that abstract types should not have Generic instances
> at all. I'll let him make those arguments himself, but I think I should
> mention that it's really hard to write instances that set up all the
> appropriate type-level meta-data. Some of it (like the extra-full package
> name) may only be doable with Template Haskell. Furthermore, some of the
> metadata types change from GHC version to version.
>
> As for laws, I think you *probably* meant
>
> to . from = id
>
> That is, that you can convert a value to its generic representation and back
> and get the "same" value back.
>
> I think that's likely sufficient for most purposes, but probably not all. At
> the very least, it's something people using your instance would need to be
> aware of. If either function is partial, you will be in a world of pain if
> you try to use something like generic serialization.
>
> On Mar 20, 2017 4:53 PM, "Gershom B" <gershomb at gmail.com> wrote:
>
> I had a bit of discussion today on how to handle generic instances for
> abstract types.
>
> It resolved to the following question, in a sense -- what are the laws
> for the Generic typeclass?
>
> Should require that `from` and `to` be an isomorphism of data types
> (i.e. that `from . to` and `to . from` both be the identity) or should
> we require only the weaker one-sided retract condition (i.e. that
> `from . to` be the identity.). If the latter, is it better that `from`
> be partial (which i  prefer) or that it quotient together "equivalent"
> representations (i.e. representing a map as a `fromList` as
> syb-generics do).
>
> The documentation doesn't seem to provide clear guidance here, and it
> might be worth discussing if some should be added.
>
> Cheers,
> Gershom
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