Besides `conditionMessage` and `conditionCall`, base R also has methods
defined for `as.character` and `print`, but they appear to make no
assumptions about the object other than having `conditionMessage` and
`conditionCall` defined.
The help page is silent about what type of thing `conditionCall()`
should return, but the objects produced by the standard condition
functions will return the `call` argument, which defaults to `NULL`, but
could be a "call expression".
So I'm not sure your definition of the `conditionCall()` methods is
going to work: `list()` doesn't return an expression. Returning
`NULL` would be better.
Of course, in S3 "valid" isn't defined formally; it just means something
that won't mess up. So it's quite possible `list()` is okay.
Duncan Murdoch
On 2025-10-07 7:42 p.m., Henrik Bengtsson wrote:
I think structure(NA, class = c("def", "condition")) is a valid
'condition' object. Am I wrong?
BACKGROUND:
The abstract 'condition' class: why type or mode can a 'condition' object have?
In help("condition"), we can read that:
"Conditions are objects inheriting from the abstract class condition. ..."
and then it specifies the API, i.e. the methods it should support, e.g.
"The functions conditionMessage and conditionCall are generic
functions that return the message and call of a condition."
Then we have several functions for creating 'condition' objects, e.g.
simpleCondition
function (message, call = NULL)
{
class <- c("simpleCondition", "condition")
structure(list(message = as.character(message), call = call),
class = class)
}
AFAIK, all of them create 'condition' object of type 'list'.
CAN CONDITIONS BE ENVIRONMENTS OR ATOMIC OBJECTS?
However, is the list type a requirement? I cannot find it specified
anywhere. The way I interpret help("condition") and how it is
carefully written using terms like "abstract class" and not mentioning
the type anywhere, I take it as:
cnd1 <- structure(new.env(), class = c("abc", "condition"))
and
cnd2 <- structure(NA, class = c("def", "condition"))
are both valid 'condition' objects, as long as we define the S3
methods for `conditionMessage()` and `conditionCall()`, e.g.
conditionMessage.abc <- function(c) "boom"
conditionCall.abc <- function(c) list()
conditionMessage.def <- function(c) "boom"
conditionCall.def <- function(c) list()
FWIW, I create 'condition' objects of type NA in my 'R.oo' package
going back ~25 years.
Thanks,
Henrik
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