[Rd] as(1:4, "numeric") versus as.numeric(1:4, "numeric")

[Seth Falcon]

On 3/31/10 4:52 PM, John Chambers wrote:
> The example is confusing and debatable, but not an obvious bug.  And
> your presentation of it is the cause of much of the confusion
> (unintentionally I'm sure).

To restate the issue (I think):

In a new R session if you happen to call:

   selectMethod("coerce", c("integer", "numeric"))

*Before* having made a call like as(1:4, "numeric") then there is a 
side-effect of creating definition "A" of the integer => numeric coerce 
method.  From this point forward all calls to as(x, "numeric") when x is 
"integer" will return as.numeric(x).

If instead you do not call selectMethod, then when calling as(x, 
"numeric") for x "integer" you get definition "B", the documented 
behavior, which simply returns x.

Presumably there are other similar cases where this will be an issue.

So while I agree this could be considered obscure, this qualifies as a 
bug in my book.  It seems desirable that selectMethod not change the 
state of the system in a user-visible fashion.  And calling 
selectMethod, or any other function, should not alter dispatch unless 
documented to do so.

I'm also suspicious of the behavior of the strict argument:

 > class(as(1:4, "numeric"))
[1] "integer"
 > class(as(1:4, "numeric", strict = TRUE))
[1] "integer"
 > class(as(1:4, "numeric", strict = FALSE))
[1] "integer"

Is that intended?

+ seth