[Haskell-cafe] ANN: generic-deepseq 18.104.22.168
mhenrion at gmail.com
Fri Feb 24 09:26:22 CET 2012
On Fri, 2012-02-24 at 07:49 +0100, Jos Pedro Magalhes wrote:
> 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
> 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
> 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 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.
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