Try registering a chooseOpsMethod() for a stepper

[DavisVaughan]

CC @hadley @t-kalinowski @ltierney - This is an example of testing out chooseOpsMethod() to prove that it works as expected. And it looks good to me!

I think we will actually end up registering chooseOpsMethod.vctrs_vctr(), which will handle it correctly for all vctrs subclasses, like this almanac stepper. So I won't end up merging this.

The only problem that can come up is if both classes register a chooseOpsMethod() method with conflicting mx and my methods, like what I do below by registering a method for <Date>. That can result in silent weird results, which is a little scary, but I still think this feature is a big improvement.

Before (R 4.2):

library(almanac)

# A Friday
x <- as.Date("2023-04-21")

# Boo
x + workdays(1)
#> Warning: Incompatible methods ("+.Date", "+.vctrs_vctr") for "+"
#> [1] "2023-04-22"

workdays(1) + x
#> Warning: Incompatible methods ("+.vctrs_vctr", "+.Date") for "+"
#> <stepper[1]>
#> [1] 19469

x - workdays(1)
#> Warning: Incompatible methods ("-.Date", "-.vctrs_vctr") for "-"
#> [1] "2023-04-20"

workdays(1) - x
#> Warning: Incompatible methods ("-.vctrs_vctr", "-.Date") for "-"
#> <stepper[1]>
#> [1] -19467

After (r-devel):

library(almanac)

# A Friday
x <- as.Date("2023-04-21")

# Yay! ("steps" over the weekend)
x + workdays(1)
#> [1] "2023-04-24"

workdays(1) + x
#> [1] "2023-04-24"

x - workdays(1)
#> [1] "2023-04-20"

# Correct!
workdays(1) - x
#> Error in `vec_arith()` at vctrs/R/type-vctr.R:659:4:
#> ! <stepper> - <date> is not permitted


# Let's try also registering a method for Date
chooseOpsMethod.Date <- function(x, y, mx, my, cl, reverse) {
  TRUE
}

# Boo
# - Silently uses the `.Date` method because it found it "first"
# - `workdays(1)` is stored as `1` plus attributes internally, so it is the same as `+ 1`
x + workdays(1)
#> [1] "2023-04-22"

# - Silently uses the `.almanac_stepper` method because it found it "first"
workdays(1) + x
#> [1] "2023-04-24"

^{Created on 2023-04-19 with reprex v2.0.2}