arrow

ARROW-3439: [R] R language bindings for Feather format

``` r
library(arrow, warn.conflicts = FALSE)
library(tibble)

tib <- tibble(x = 1:10, y = rnorm(10), z = letters[1:10])
tib
#> # A tibble: 10 x 3
#>        x       y z
#>    <int>   <dbl> <chr>
#>  1     1  0.315  a
#>  2     2 -1.16   b
#>  3     3  0.0980 c
#>  4     4  0.836  d
#>  5     5 -1.12   e
#>  6     6  0.118  f
#>  7     7  2.74   g
#>  8     8 -0.323  h
#>  9     9  0.291  i
#> 10    10  1.40   j
tf <- tempfile()
write_feather(tib, tf)

# reads back into an arrow::Table
tab1 <- read_feather(tf)
tab1
#> arrow::Table

# convert that to tibble
as_tibble(tab1)
#> # A tibble: 10 x 3
#>        x       y z
#>    <int>   <dbl> <chr>
#>  1     1  0.315  a
#>  2     2 -1.16   b
#>  3     3  0.0980 c
#>  4     4  0.836  d
#>  5     5 -1.12   e
#>  6     6  0.118  f
#>  7     7  2.74   g
#>  8     8 -0.323  h
#>  9     9  0.291  i
#> 10    10  1.40   j
identical(tib, as_tibble(tab1))
#> [1] TRUE

# read it through reticulate
library(reticulate)
pa <- import("pyarrow")

# read straight to a data frame via pyarrow.feather.read_feather
tab2 <- pa$feather$read_feather(tf)
# just so that we cna compare
attr(tab2, "pandas.index") <-  NULL
tab2
#>     x          y z
#> 1   1  0.3148670 a
#> 2   2 -1.1583740 b
#> 3   3  0.0980070 c
#> 4   4  0.8360379 d
#> 5   5 -1.1171395 e
#> 6   6  0.1177422 f
#> 7   7  2.7407804 g
#> 8   8 -0.3229787 h
#> 9   9  0.2910557 i
#> 10 10  1.4015148 j

identical(tib, as_tibble(tab2))
#> [1] TRUE
```

<sup>Created on 2018-11-12 by the [reprex package](https://reprex.tidyverse.org) (v0.2.1.9000)</sup>

Author: Romain Francois <romain@purrple.cat>

Closes #2947 from romainfrancois/ARROW-3439/feather and squashes the following commits:

8d708e21d <Romain Francois> added `mmap` argument to all methods of feather_table_reader
fd1b7d5b5 <Romain Francois> additional test using mmap_open() and file_open() directly
da9df5d69 <Romain Francois> + feather prefix -> feather_table_writer and feather_table_reader
d5f886d19 <Romain Francois> Add mmap argument to table_reader so that character method uses mmap_open instead of file_open if mmap = TRUE
14b169dd6 <Romain Francois> until we figure out how to probably do double dispatch and 🤐 check
43cab0629 <Romain Francois> Fixes for write_feather
2a62b7222 <Romain Francois> using construct and STOP_IF_NOT_OK (rebase)
8825b3e5b <Romain Francois> lint 🤷
♀️
a144cfd73 <Romain Francois> exposing feather::TableReader
61fec7187 <Romain Francois> 👶 support for feather::TableWriter
432588cf6 <Romain Francois> Message and MessageReader
7751a1c18 <Romain Francois> s/construct/shared_ptr/, add unique_ptr R function. support for unique_ptr.
8a695b881 <Romain Francois> lint
df5fdba49 <Romain Francois> R6 class support for arrow::ipc::Message backed by a unique_ptr
56ff849f3 <Romain Francois> using struct input_parameter<const std::shared_ptr<T>& instead of the heavier Exporter class