arrow

ARROW-7028: [R] Date roundtrip results in different R storage mode

Although it's to some extent tolerated that Date vectors are backed by `integer` vectors, it's much more common that they are backed by numeric:

``` r
.Internal(inspect(Sys.Date()))
#> @7fb95b8178c0 14 REALSXP g0c1 [OBJ,ATT] (len=1, tl=0) 18425
#> ATTRIB:
#>   @7fb9586b1808 02 LISTSXP g0c0 [REF(1)]
#>     TAG: @7fb957004fd0 01 SYMSXP g0c0 [MARK,REF(25905),LCK,gp=0x6000] "class" (has value)
#>     @7fb95b817888 16 STRSXP g0c1 [REF(1)] (len=1, tl=0)
#>       @7fb9571c1530 09 CHARSXP g0c1 [MARK,REF(191),gp=0x61] [ASCII] [cached] "Date"
.Internal(inspect(as.Date("1982-05-15")))
#> @7fb95c10eb58 14 REALSXP g0c1 [OBJ,ATT] (len=1, tl=0) 4517
#> ATTRIB:
#>   @7fb9587cbc48 02 LISTSXP g0c0 [REF(1)]
#>     TAG: @7fb957004fd0 01 SYMSXP g0c0 [MARK,REF(25948),LCK,gp=0x6000] "class" (has value)
#>     @7fb95c10eb20 16 STRSXP g0c1 [REF(1)] (len=1, tl=0)
#>       @7fb9571c1530 09 CHARSXP g0c1 [MARK,REF(192),gp=0x61] [ASCII] [cached] "Date"
```

<sup>Created on 2020-06-12 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>

So I've changed `Converter_Date32` to convert to a numeric vector:

``` r
library(dplyr, warn.conflicts = FALSE)

tmp = tempdir()
dat = tibble(tag = as.Date("2018-01-01"))
dat2 = tibble(tag2 = as.Date("2019-01-01"))

arrow::write_parquet(dat, file.path(tmp, "dat.parquet"))
dat = arrow::read_parquet(file.path(tmp, "dat.parquet"))

typeof(dat$tag)
#> [1] "double"
typeof(dat2$tag2)
#> [1] "double"

bind_cols(dat, dat2) %>%
  mutate(comparison = if_else(TRUE, tag, tag2))
#> # A tibble: 1 x 3
#>   tag        tag2       comparison
#>   <date>     <date>     <date>
#> 1 2018-01-01 2019-01-01 2018-01-01
```

<sup>Created on 2020-06-12 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>

Note: `dplyr::if_else` is due a rewrite, probably in `funs::` that will be more `vctrs`-correct and perhaps would deal with the issue:

``` r
library(dplyr, warn.conflicts = FALSE)

today_dbl <- Sys.Date()
today_int <- structure(as.integer(today_dbl), class = "Date")

dplyr::if_else(TRUE, today_dbl, today_int)
#> Error: `false` must be a `Date` object, not a `Date` object.
```

<sup>Created on 2020-06-12 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>

This might be a usecase for `vctrs::vec_cast_common()`. ping @lionel- @DavisVaughan:

``` r
library(dplyr, warn.conflicts = FALSE)

today_dbl <- Sys.Date()
today_int <- structure(as.integer(today_dbl), class = "Date")
.Internal(inspect(
  vctrs::vec_cast_common(today_dbl, today_int)
))
#> @7ffe825f1688 19 VECSXP g0c2 [] (len=2, tl=0)
#>   @7ffe823aeaa0 14 REALSXP g0c1 [OBJ,REF(12),ATT] (len=1, tl=0) 18425
#>   ATTRIB:
#>     @7ffe823b5408 02 LISTSXP g0c0 [REF(1)]
#>       TAG: @7ffe80804fd0 01 SYMSXP g1c0 [MARK,REF(33031),LCK,gp=0x6000] "class" (has value)
#>       @7ffe823aea68 16 STRSXP g0c1 [REF(65535)] (len=1, tl=0)
#>  @7ffe809c1530 09 CHARSXP g1c1 [MARK,REF(395),gp=0x61] [ASCII] [cached] "Date"
#>   @7ffe8265eb30 14 REALSXP g0c1 [OBJ,REF(1),ATT] (len=1, tl=0) 18425
#>   ATTRIB:
#>     @7ffe82664938 02 LISTSXP g0c0 [REF(1)]
#>       TAG: @7ffe80804fd0 01 SYMSXP g1c0 [MARK,REF(33031),LCK,gp=0x6000] "class" (has value)
#>       @7ffe82d15d60 16 STRSXP g1c1 [MARK,REF(65535)] (len=1, tl=0)
#>  @7ffe809c1530 09 CHARSXP g1c1 [MARK,REF(395),gp=0x61] [ASCII] [cached] "Date"
```

<sup>Created on 2020-06-12 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0)</sup>

Closes #7415 from romainfrancois/ARROW-7028

Lead-authored-by: Romain Francois <romain@rstudio.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>