Re: [Rd] Possible problem with S4 dispatch

From: Prof Brian Ripley <>
Date: Tue, 01 May 2007 13:09:19 +0100 (BST)

On Mon, 30 Apr 2007, Seth Falcon wrote:

> First a bit of disclaimer... I haven't isolated this problem into an
> easy to reproduce case, and I won't be surprised if the root cause is
> a fault in my code's use of name spaces or some such.

Note that you called

selectMethod("mget", signature(x="character", envir=class(LLe)))

by name rather than calling the visible function mget() (which you could have supplied as fdef). I've never really got to the bottom of the complicated searches that getGeneric() uses, but the fact that it does not just look for a visible function of that name tells you it is doing something different.

What I would check from your browser is what parent.env() shows, successively until you get to the imports and then the base namespace. If mget is not in the imports, something would seem to be up with your importing of namespaces. find() is not relevant here as namespace scoping is in play: only if the mget generic is imported will it take precedence over base:::mget. (It is not clear to me what is being browsed here, and hence what namespaces are in play.)

Perhaps we need lexical versions of find() and search() to make it easier to explore search paths from a function body.

Hope it helps: without a reproducible example I can only guess blindly.

> The error I'm seeing is one in which the desired method is not found.
> What worries me in terms of my expectations of how to debug the
> problem is that showMethods and selectMethod both find the method.
> Here is an example browser session:
> ## Input 1, a character vector
> Browse[1]> gN[1:3]
> [1] "1005_at" "1007_s_at" "1008_f_at"
> ## Input 2, an object of class AtomicAnnMap (subclass of AnnMap)
> Browse[1]> class(LLe)
> [1] "AtomicAnnMap"
> attr(,"package")
> [1] "AnnotationDbi"
> ## See what selectMethod has to say (it finds the method I'm expecting)
> Browse[1]> selectMethod("mget", signature(x="character", envir=class(LLe)))
> Method Definition:
> function (x, envir, mode = "any", ifnotfound = list(function(x) stop(paste("value for '",
> x, "' not found", sep = ""), call. = FALSE)), inherits = FALSE)
> {
> .checkNamesAreStrings(x)
> .checkNamesExist(x, names(envir))
> as.list(envir, names = x)
> }
> <environment: namespace:AnnotationDbi>
> Signatures:
> x envir
> target "character" "AtomicAnnMap"
> defined "ANY" "AnnMap"
> ## Call it (we get base::mget, not the generic and hence an error)
> Browse[1]> mget(gN[1:3], LLe)
> Error in mget(x, envir, mode, ifnotfound, inherits) :
> second argument must be an environment
> Browse[1]> find("mget")
> [1] "package:AnnotationDbi" "package:base"
> The package code I'm working with imports the package that defines the
> mget method (and this package does exportMethods(mget)), yet the
> problem seems to be that the mget generic is not found -- but I find
> it confusing that selectMethod works here.

Brian D. Ripley,        
Professor of Applied Statistics,
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

______________________________________________ mailing list
Received on Wed 02 May 2007 - 07:28:46 GMT

Archive maintained by Robert King, hosted by the discipline of statistics at the University of Newcastle, Australia.
Archive generated by hypermail 2.2.0, at Wed 02 May 2007 - 09:33:40 GMT.

Mailing list information is available at Please read the posting guide before posting to the list.