Rethink trailing comma behavior with subsetting

[DavisVaughan]

The terminology for what we do with trailing commas is incorrect. We don't "drop trailing commas". It is more appropriate for us to "expand unspecified trailing indices"

This requires a very small change in the code, changing to .ignore_empty = "none" here. But requires changes to the documentation and more tests.

https://github.com/r-lib/rray/blob/525539dba63435d39f828b75c2e3ed8f1e5b9ead/R/subset.R#L179

This is what we should get:

library(rray)

x <- rray(1:8, c(2,2,2))

dim(x)
#> [1] 2 2 2

# max number of commas allowed is dim-1
x[,,1]
#> <rray<int>[,2,1][2]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> [2,]    2    4

# missing dimensions are always "expanded out"

# equivalent - expanded to `x[,,]`
x[]
#> <rray<int>[,2,2][2]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> [2,]    2    4
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7
#> [2,]    6    8
x[,]
#> <rray<int>[,2,2][2]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> [2,]    2    4
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7
#> [2,]    6    8
x[,,]
#> <rray<int>[,2,2][2]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> [2,]    2    4
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7
#> [2,]    6    8

# error - max number of commas is `3 - 1 = 2`
x[,,,]
#> Error: The dimensionality of `x` is 3. Cannot subset into dimension 4.

# equivalent - expanded to `x[1,,]`
x[1]
#> <rray<int>[,2,2][1]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7
x[1,]
#> <rray<int>[,2,2][1]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7
x[1,,]
#> <rray<int>[,2,2][1]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7

# equivalent - expanded to `x[,1,]`
x[,1]
#> <rray<int>[,1,2][2]>
#> , , 1
#> 
#>      [,1]
#> [1,]    1
#> [2,]    2
#> 
#> , , 2
#> 
#>      [,1]
#> [1,]    5
#> [2,]    6
x[,1,]
#> <rray<int>[,1,2][2]>
#> , , 1
#> 
#>      [,1]
#> [1,]    1
#> [2,]    2
#> 
#> , , 2
#> 
#>      [,1]
#> [1,]    5
#> [2,]    6

# with padding - "padding applied before expansion"
# (number of dims specified = 1, pad twice)
x[pad(), 1] # == x[,,1]
#> <rray<int>[,2,1][2]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> [2,]    2    4

# (number of dims specified = 2, pad once)
x[pad(), 1, ] # == x[, 1,]
#> <rray<int>[,1,2][2]>
#> , , 1
#> 
#>      [,1]
#> [1,]    1
#> [2,]    2
#> 
#> , , 2
#> 
#>      [,1]
#> [1,]    5
#> [2,]    6

# (number of dims specified = 3, no padding)
x[pad(), 1, ,] # == x[1, ,]
#> <rray<int>[,2,2][1]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    5    7

# (number of dims specified = 2, pad once)
x[1, pad(), 1]
#> <rray<int>[,2,1][1]>
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    3

^{Created on 2019-11-26 by the reprex package (v0.3.0.9000)}