Re: [Haskell-cafe] ANN: generic-deepseq 1.0.0.0

On Sat, 2012-02-25 at 11:38 +0100, José Pedro Magalhães wrote:

2012/2/25 Andres Löh > Would you have an example of a type for which it would be useful to have > a DeepSeq instance, and that would require a V1 instance? I cannot think > of one now; I originaly thought it would be necessary to permit deriving > DeepSeq instances for types tagged with "void" types, but as José > explained, in that case, the V1 instance isn't needed because those void > types don't show up in the representation.

While void datatypes are rare, it just doesn't make sense to exclude them. It's an arbitrary restriction. Here's a constructed example:

data X a = C1 Int | C2 a data Z -- empty

type Example = X Z

We're using Z as a parameter to X in order to exclude the use of the C2 case. Without a V1 case, you cannot use deepSeq on values of type Example.

Yes, I agree. There should be a V1 instance, and it should return `undefined`. This gives the expected behavior of `seq` on an empty datatype, I think. If there is no V1 instance, you'll get a type-checking error (no instance for V1), preventing generic deepseq on any datatype that happens to use an empty datatype in its definition.

Thanks for all the input guys. I have just released generic-deepseq 2.0.1.0 to hackage, with fixed U1 and V1 instances per this discussion. Cheers, Maxime