More helpful error when trying to write out data frames with list columns

[jimjam-slam]

library(tidyverse)
#> Warning: package 'ggplot2' was built under R version 3.5.1
#> Warning: package 'dplyr' was built under R version 3.5.1
diamonds %>% nest(-cut) %>% write_csv('test.csv')
#> Error in stream_delim_(df, path, ...): Don't know how to handle vector of type list.

Created on 2018-12-06 by the reprex package (v0.2.0).

It would be more useful (especially in non-interactive use) if the write_ functions reported which column it was unable to handle. My feeling is this error either happens because:

• Someone forgot to drop an intermediate list column (eg. nested source data or a list of models) before exporting;
• They accidentally used regular map() instead of a type-coercive variety like map_dbl to extract a parameter; or
• They don't understand that flat files can't store more complex list columns.

I understand that there's some discussion in #303 about potentially handling simple list columns, but given the variety of objects that end up in list columns accidentally, I imagine this problem will still exist even if that issue gets resolved. Actually implementing a PR for this might be a little out of my depth, but I'm willing to give it a go over Xmas.

[lambdamoses]

I'm working on this

[lock]

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