There are a number of tests which, applied to an object of a given class, produce information about that object. Consider objects of class "function". The functions is.primitive() or is.closure(), or (from rlang) is_primitive_eager() or is_primitive_lazy(), provide information about a function object. However, Using methods(class = "function") (with rlang loaded) does not return any of these functions:
[1] as.data.frame as.list coerce coerce<- fortify head latex plot print tail .
Using extends(class1 = "function", maybe = TRUE, fullInfo = TRUE) shows two superclasses, "OptionalFunction" and "PossibleMethod".
Using completeClassDefinition(Class = "function", doExtends=TRUE) provides 23 subclasses. However, it appears to me (though I am not sure of this) that all or almost all of the super- and sub-classes from these two functions are specifically of S4 classes, which I generally do not use. One of these subclasses is "genericFunction", so I tried to apply it to a base R function which I knew to be generic. Although is(object=plot, class2 = "genericFunction") returns TRUE, and plot() antedates S4 classes, there is no "is.generic" test in base R, but there is an "isGeneric" test in the methods package, which suggests to me that plot() has been rewritten as an S4 object.
At any rate, there are a lot of obvious potential properties of functions, like whether they are generic, for which there are no is.<whatever> tests that I can find, and I would like to know if there are other ways I can search for them, e.g., in packages.
A more generic way of asking this same question is whether there is any way of identifying functions that will accept objects of a specified class and not return an error or nonsense. If so I could take a list of the functions in the reccomended packages or in some specified package and test whether each returns a sensable response when handed a function. This is not exactly an answer --- such a method would return TRUE for quote(), for example -- but it would at least cut the problem down to size.
