unique.fact() needed

[jmbarbone]

unique.default() rebuilds factors -- which leaves off the fact class

library(mark)
str(unique(fact(1:4)))
#>  Factor w/ 4 levels "1","2","3","4": 1 2 3 4

unique.default
#> function (x, incomparables = FALSE, fromLast = FALSE, nmax = NA, 
#>     ...) 
#> {
#>     if (!is.object(x)) 
#>         return(.Internal(unique(x, incomparables, fromLast, nmax)))
#>     if (is.factor(x)) {
#>         z <- .Internal(unique(x, incomparables, fromLast, min(length(x), 
#>             nlevels(x) + 1L)))
#>         return(factor(z, levels = seq_len(nlevels(x)), labels = levels(x), 
#>             ordered = is.ordered(x)))
#>     }
#>     z <- .Internal(unique(x, incomparables, fromLast, nmax))
#>     if (inherits(x, "POSIXct")) 
#>         structure(z, class = class(x), tzone = attr(x, "tzone"))
#>     else if (inherits(x, "Date")) 
#>         structure(z, class = class(x))
#>     else z
#> }
#> <bytecode: 0x0000000013dd7e18>
#> <environment: namespace:base>

^{Created on 2022-01-20 by the reprex package (v2.0.1)}

[jmbarbone]

With enhancements from #69 , this could be:

unique.fact <- function(x, ...) {
  att <- attributes(x)
  struct(
    att$uniques,
    class = att$class,
    levels = att$levels,
    unique = att$unique,
    na = att$na
  )
}