[Rd] Missing args with a default value in S4 methods

[John Chambers]

This has essentially nothing to do with methods, but rather with the 
treatment of missing arguments.

Consider:
 > foo <- function(x,...)bar(x,...)
 > bar <- function(x, y=12, z, ...) {cat(missing(y), "\n"); cat(y, "\n")}

This is the same argument-matching as your example, since the generic 
and method have different formal arguments.  And indeed,

 > foo("a",,z=99)
TRUE
Error in cat(y, "\n") : argument is missing, with no default

The error message is correct, but the argument in question is not "y" 
but "..1".  This is constructed and passed down as a special R object 
representing "missing-argument-with-no-default".   (Splus would have 
worked as you expected, because missingness there is a property of the 
function call, not of the object corresponding to the formal argument.)


Herve Pages wrote:
> Hi,
>
>
> Strange things happen with missing args in S4 methods:
>
>   > setGeneric("mygen", signature="x", function(x, ...) standardGeneric("mygen"))
>   [1] "mygen"
>
>   > setMethod("mygen", "character", function(x, y=12, z, ...) {cat(missing(y), "\n"); cat(y, "\n")})
>   [1] "mygen"
>
>   > mygen("aa", z=99)
>   TRUE
>   12
>
>   > mygen("aa", , 99)
>   TRUE
>   Error in cat(y, "\n") : argument is missing, with no default
>                                       ^^^^^^^       ^^^^^^^^^^
>                                        TRUE          NOT TRUE!
>
>
> For "normal" functions, things work as expected:
>
>   > myfun <- function(x, y=12, z, ...) {cat(missing(y), "\n"); cat(y, "\n")}
>
>   > myfun("aa", z=99)
>   TRUE
>   12
>
>   > myfun("aa", , 99)
>   TRUE
>   12
>
> And with S3 generics too:
>
>   > dd <- data.frame(aa=letters[1:9], ii=9:1)
>   > head(dd, z="ignored")
>     aa ii
>   1  a  9
>   2  b  8
>   3  c  7
>   4  d  6
>   5  e  5
>   6  f  4
>
>   > head(dd, , "ignored")
>     aa ii
>   1  a  9
>   2  b  8
>   3  c  7
>   4  d  6
>   5  e  5
>   6  f  4
>
> Cheers,
> H.
>
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