ARROW-3794 [R]: Consider mapping INT8 to integer() not raw()

[romainfrancois]

R vectors of raw types are converted to arrays of type uint8

arrow::array(as.raw(1:10))$type
#> arrow::UInt8 
#> uint8

but otoh, arrays of type uint8 are converted to R integer vectors.

library(arrow, warn.conflicts = FALSE)

a <- array((-128):127)$cast(int8())
str(a$as_vector())
#>  int [1:256] -128 -127 -126 -125 -124 -123 -122 -121 -120 -119 ...

a <- array(0:255)$cast(uint8())
str(a$as_vector())
#>  int [1:256] 0 1 2 3 4 5 6 7 8 9 ...

^{Created on 2019-06-10 by the reprex package (v0.3.0.9000)}

[romainfrancois]

Not sure about the first thing, perhaps raw vectors should just be converted to int32 arrays ?

[codecov-io]

Codecov Report

Merging #4507 into master will decrease coverage by 13%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master    #4507       +/-   ##
===========================================
- Coverage   88.11%   75.11%   -13.01%     
===========================================
  Files         850       54      -796     
  Lines      105774     3110   -102664     
  Branches     1253        0     -1253     
===========================================
- Hits        93203     2336    -90867     
+ Misses      12326      774    -11552     
+ Partials      245        0      -245

Impacted Files	Coverage Δ
r/src/arrowExports.cpp	72.7% <ø> (ø)	⬆️
r/src/array_from_vector.cpp	78.29% <100%> (ø)	⬆️
r/src/array__to_vector.cpp	78.66% <100%> (+0.7%)	⬆️
python/pyarrow/ipc.pxi
cpp/src/arrow/csv/chunker-test.cc
cpp/src/parquet/column_page.h
cpp/src/parquet/bloom_filter-test.cc
cpp/src/arrow/array/builder_decimal.cc
cpp/src/plasma/client.cc
cpp/src/arrow/io/test-common.h
... and 790 more

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[nealrichardson]

The original concern that int8 map to integer seems addressed here, but i'm wondering if uint8 is most appropriate for raw. Why not the binary type that Javier mentioned in the ticket? http://arrow.apache.org/docs/format/Metadata.html#utf8-and-binary

[romainfrancois]

That does not look like the same thing to me. binary here looks more like a list of raw vectors rather than a single raw vector, which are closer to a fixed width (1) binary type: https://arrow.apache.org/docs/cpp/classarrow_1_1_fixed_size_binary_type.html

[nealrichardson]

Alright, LGTM