support for "AsIs"

[romainfrancois]

This breaks packages biotmle, FindMyFriends, incadata, radiant.multivariate, sitar on dplyr 1.0.0 rev deps.

library(vctrs)
 
x <- I("")
vec_c(x)
#> Error: Can't find vctrs or base methods for concatenation.
#> vctrs methods must be implemented for class `AsIs`.
#> See <https://vctrs.r-lib.org/articles/s3-vector.html>.

chops <- vec_chop(x, list(1L))
vec_unchop(chops, list(1L))
#> Error: Can't find vctrs or base methods for concatenation.
#> vctrs methods must be implemented for class `AsIs`.
#> See <https://vctrs.r-lib.org/articles/s3-vector.html>.

^{Created on 2020-03-11 by the reprex package (v0.3.0)}

[DavisVaughan]

If we don't want to propagate AsIs, one idea is to add ptype2 and cast methods that strip it and then re-call vec_ptype2() and vec_cast() with the underlying data. We'd need to add vec_ptype2.<type>.AsIs() methods to everything though, but maybe there is a better idea for that.

library(vctrs)

x <- I("asis")
vec_c(x)
#> Error: Can't find vctrs or base methods for concatenation.
#> vctrs methods must be implemented for class `AsIs`.
#> See <https://vctrs.r-lib.org/articles/s3-vector.html>.

strip_as_is <- function(x) {
  class(x) <- setdiff(class(x), "AsIs")
  x
}

vec_ptype2.AsIs <- function(x, y, ..., x_arg = "x", y_arg = "y") {
  vec_ptype2(strip_as_is(x), y)
}

vec_ptype2.character.AsIs <- function(x, y, ..., x_arg = "x", y_arg = "y") {
  vec_ptype2(x, strip_as_is(y))
}

vec_ptype2(x, x)
#> character(0)
vec_ptype2(x, "x")
#> character(0)
vec_ptype2("x", x)
#> character(0)


vec_cast.AsIs <- function(x, to, ...) {
  vec_cast(x, strip_as_is(to))
}

vec_cast.character.AsIs <- function(x, to, ...) {
  vec_cast(strip_as_is(x), to)
}

vec_cast(x, x)
#> [1] "asis"
class(vec_cast(x, x))
#> [1] "character"

vec_cast(x, "x")
#> [1] "asis"
vec_cast("x", x)
#> [1] "x"


vec_c(x,x)
#> [1] "asis" "asis"
class(vec_c(x,x))
#> [1] "character"

vec_c(x, "x")
#> [1] "asis" "x"
vec_c("x", x)
#> [1] "x"    "asis"

^{Created on 2020-03-11 by the reprex package (v0.3.0)}

[hadley]

Hmmm, this is going to require a little thought because AsIs behaves like a subclass of any existing class, which because we no longer rely on inheritance in vctrs is going to require an arbitrary and unknown number of methods.

Should we add something in vec_default_ptype2() or vec_ptype()?