Partial types about vctrs HOT 4 CLOSED

I want to say "Amen!": this is something that both readr and readxl allow (and, I think, people really use) and it would be nice to make available in more circumstances.

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It now occurs to me that the way to implement this is as a new vector type. Then it would be straightforward to implement custom coercing and casting semantics using the existing framework.

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This would also be nice if we implemented a vec_assert() function - then you could assert that a data frame input had at least the columns that you needed (while not caring about any extras) .

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Some experiments checked in to local partial branch — it's totally do-able, but it requires more time than I have right now (it's a bit more complicated than I anticipated). Mostly need to think about what vec_type2(partial(), data.frame()) should return

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