[Haskell-cafe] ANN: generic-deepseq 1.0.0.0

[Maxime Henrion]

On Fri, 2012-02-24 at 09:32 +0100, Jos Pedro Magalhes wrote:
> 
> 
> 2012/2/24 Maxime Henrion <mhenrion at gmail.com>
>         On Fri, 2012-02-24 at 07:49 +0100, Jos Pedro Magalhes wrote:
>         > Hi,
>         >
>         > 2012/2/23 Maxime Henrion <mhenrion at gmail.com>
>         >
>         >         > * Why do you have the instance:
>         >         >
>         >         > instance GDeepSeq V1 where grnf _ = ()
>         >         >
>         >         > The only way to construct values of a void type is
>         using ⊥.
>         >         And I
>         >         > would expect that rnf ⊥ = ⊥, not (). I think the
>         best thing
>         >         is to just
>         >         > remove the V1 instance.
>         >
>         >
>         >         This would have the consequence that any type tagged
>         with a
>         >         phantom type
>         >         (for whatever reason) couldn't be used with deepseq,
>         it would
>         >         return
>         >         bottom. What if I want to deepseq a 2-3 finger tree
>         tagged
>         >         with a
>         >         type-level natural that ensures the proper shape of
>         the tree
>         >         statically?
>         >         It seemed to me that I should be able to do that;
>         this is why
>         >         I added
>         >         this V1 instance.
>         >
>         > I'm not sure I understand your comment... V1 should only be
>         used for
>         > datatypes without constructors, such as `data Empty`.
>         
>         
>         Yes, such as the usual type-level naturals (not using
>         DataKinds):
>         
>         data Z
>         data S n
>         
>         Those can be used to tag a type which also contains actual
>         values that
>         you would want to deepseq? For example, a length-type vector?
> 
> But in those cases they are used as tags, not as values, and hence do
> not show up in the generic representation. So if all you want is to be
> able to deepseq a value of a type like
> 
>         data Proxy t = Proxy
> 
> even if your value is of type `Proxy Ze`, you shouldn't need a `V1`
> instance.

Oh, ok; in that case, I probably don't need that V1 instance indeed. I
should probably write QuickCheck tests using the ChasingBottoms package
in order to ensure correct behaviour of this code.

Thanks,
Maxime

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