[R] Ask if an object will respond to a function or method

Martin Morgan martin.morgan at roswellpark.org
Thu Mar 31 22:30:57 CEST 2016

On 03/31/2016 04:00 PM, Paul Johnson wrote:
> In the rockchalk package, I want to provide functions for regression
> objects that are "well behaved." If an object responds to the methods
> that lm or glm objects can handle, like coef(), nobs(), and summary(),
> I want to be able to handle the same thing.
> It is more difficult than expected to ask a given fitted model object
> "do you respond to these functions: coef(), nobs(), summary()." How
> would you do it?
> I tried this with the methods() function but learned that all methods
> that a class can perform are not listed.  I'll demonstrate with a
> regression "zz" that is created by the example in the plm package.
> The coef() function succeeds on the zz object, but coef is not listed
> in the list of methods that the function can carry out.
>> library(plm)
>> example(plm)
>> class(zz)
> [1] "plm"        "panelmodel"
>> methods(class = "plm")
>   [1] ercomp          fixef           has.intercept   model.matrix
>   [5] pFtest          plmtest         plot            pmodel.response
>   [9] pooltest        predict         residuals       summary
> [13] vcovBK          vcovDC          vcovG           vcovHC
> [17] vcovNW          vcovSCC
> see '?methods' for accessing help and source code
>> methods(class = "panelmodel")
>   [1] deviance      df.residual   fitted        has.intercept index
>   [6] nobs          pbgtest       pbsytest      pcdtest       pdim
> [11] pdwtest       phtest        print         pwartest      pwfdtest
> [16] pwtest        residuals     terms         update        vcov
> see '?methods' for accessing help and source code
>> coef(zz)
>     log(pcap)      log(pc)     log(emp)        unemp
> -0.026149654  0.292006925  0.768159473 -0.005297741
> I don't understand why coef(zz) succeeds but coef is not listed as a method.

coef(zz) finds stats:::coef.default, which happens to do the right thing 
for zz but also 'works' (returns without an error) for things that don't 
have coefficients, e.g., coef(data.frame()).

stats:::coef.default is

 > stats:::coef.default
function (object, ...)

Maybe fail on use, rather than trying to guess up-front that the object 
is fully appropriate?

Martin Morgan

> Right now, I'm contemplating this:
> zz1 < - try(coef(zz))
> if (inherits(zz1, "try-error")) stop("Your model has no coef method")
> This seems like a bad workaround because I have to actually run the
> function in order to find out if the function exists. That might be
> time consuming for some summary() methods.
> pj

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