Add warning about parsing errors (like readr does)

[matthiasgomolka]

I noticed that vroom does not throw a warning when the parsing according to a column specification fails. This seems very dangerous, since the use would have to check manually if the parsing went fine. For large data (where I would like to use vroom instead of readr) this is likely to be infeasible. Here is an example:

library(readr)
library(vroom)

# readr + correct specification
(read_csv(
    "A,B\n1,2020-01-01",
    col_types = cols(A = col_integer(), B = col_date())
))
#> # A tibble: 1 x 2
#>       A B         
#>   <int> <date>    
#> 1     1 2020-01-01


# readr + incorrect specification
(read_csv(
    "A,B\n1,2020-01-01",
    col_types = cols(B = col_integer(), A = col_date())
))
#> Warning: 2 parsing failures.
#> row col               expected actual         file
#>   1   A date like              1      literal data
#>   1   B no trailing characters -01-01 literal data
#> # A tibble: 1 x 2
#>   A              B
#>   <date>     <int>
#> 1 NA            NA
# readr warns about parsing errors


# vroom + correct specification
(vroom(
    "A,B\n1,2020-01-01",
    col_types = cols(A = col_integer(), B = col_date())
))
#> # A tibble: 1 x 2
#>       A B         
#>   <int> <date>    
#> 1     1 2020-01-01


# vroom + incorrect specification
(vroom(
    "A,B\n1,2020-01-01",
    col_types = cols(B = col_integer(), A = col_date())
))
#> # A tibble: 1 x 2
#>   A              B
#>   <date>     <int>
#> 1 NA            NA
# vroom does not warn about parsing errors

^{Created on 2020-07-03 by the reprex package (v0.3.0)}

Would it be possible to more or less copy & paste the respective code from readr to vroom to gain the safety? Alternatively, vroom could fall back to character, like data.table does when a column can't be parsed as specified.

[jimhester]

Yes, this is a known limitation in the current implementation. It is not trivial to do however as the parsing is done lazily, but it is planned to add something like readr's warnings for future work.

[jimhester]

Fixed by #284