[Rd] Is structure(NA, class = c("def", "condition")) a valid 'condition' object?

[Henrik Bengtsson]

Thank you, Duncan.

It sounds like you're reading it the same as I, i.e. what
typeof(<condition>) should be is not specified.

Using list() in my tiny example was a thinko - NULL would indeed have
been better.

/Henrik

On Wed, Oct 8, 2025 at 5:32 AM Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
>
> 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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>