2012/2/25 Andres Löh <andres.l...@googlemail.com> > > 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. Cheers, Pedro
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