On Fri, 2012-02-24 at 07:49 +0100, Jos Pedro Magalhes wrote:
Hi,
2012/2/23 Maxime Henrion
> * 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? I seemed to remember a similar construct for 2-3 finger trees that would statically guarantee that the shape of the tree is valid, so I took that as an example, but I don't remember the specifics. Cheers, Maxime