[Rd] Make as.factor an S3 generic?

[Martin Morgan]

as.factor / as.ordered is not written as a generic. This differs from 
as.numeric, as.matrix, and other as.*. The following seems to address 
this and does not break make check-all.

FWIW, the patch is against r55563, because with r55564 I see

/home/mtmorgan/src/R-devel/src/main/dounzip.c:75:15: error: storage size 
of ‘dt’ isn’t known
/home/mtmorgan/src/R-devel/src/main/dounzip.c:88:5: warning: implicit 
declaration of function ‘mktime’
make[3]: *** [dounzip.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory `/home/mtmorgan/bin/R-devel/src/main'
make[2]: *** [R] Error 2
make[2]: Leaving directory `/home/mtmorgan/bin/R-devel/src/main'
make[1]: *** [R] Error 1
make[1]: Leaving directory `/home/mtmorgan/bin/R-devel/src'
make: *** [R] Error 1


Index: src/library/base/R/factor.R
===================================================================
--- src/library/base/R/factor.R (revision 55563)
+++ src/library/base/R/factor.R (working copy)
@@ -45,7 +45,9 @@
  }

  is.factor <- function(x) inherits(x, "factor")
-as.factor <- function(x) if (is.factor(x)) x else factor(x)
+as.factor.default <- function(x, ...)
+    if (is.factor(x)) x else factor(x, ...)
+as.factor <- function(x, ...) UseMethod("as.factor")

  ## Help old S users:
  category <- function(...) .Defunct()
@@ -245,7 +247,10 @@
  ordered <- function(x, ...) factor(x, ..., ordered=TRUE)

  is.ordered <- function(x) inherits(x, "ordered")
-as.ordered <- function(x) if(is.ordered(x)) x else ordered(x)
+as.ordered.default <- function(x, ...)
+    if(is.ordered(x)) x else ordered(x, ...)
+as.ordered <- function(x, ...)
+    UseMethod("as.ordered")

  Ops.ordered <- function (e1, e2)
  {
Index: src/library/base/man/factor.Rd
===================================================================
--- src/library/base/man/factor.Rd      (revision 55563)
+++ src/library/base/man/factor.Rd      (working copy)
@@ -10,7 +10,9 @@
  \alias{is.factor}
  \alias{is.ordered}
  \alias{as.factor}
+\alias{as.factor.default}
  \alias{as.ordered}
+\alias{as.ordered.default}
  \alias{is.na<-.factor}
  \alias{Math.factor}
  \alias{Ops.factor}
@@ -40,8 +42,8 @@
  is.factor(x)
  is.ordered(x)

-as.factor(x)
-as.ordered(x)
+as.factor(x, \dots)
+as.ordered(x, \dots)

  addNA(x, ifany=FALSE)
  }

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