[Rd] generics for primitive functions

[John Chambers]

Probably a bug, but not at all the one you imply.  You have found a way 
to subvert a guarantee in R that methods for primitives can never be 
redefined for basic data types.

As you no doubt found, but didn't show us, so long as you say sqrt(4) 
the result is correct.  (Your "Works" should really read "Gotcha!").

Since a large amount of effort and not a little programming pain has 
been spent retaining the purity of primitive calls to satisfy the 
efficiency lobby, it's not clear what should happen when one insists on 
calling the generic function directly.  Either an error or just a call 
to the primitive would be the two likely candidates.  Not what happens 
in your first example though.

If you really want a "numeric" class that does something different for 
sqrt(), define an S4 class that contains "numeric"
 
 > setClass("WeirdNumber", contains="numeric")
[1] "WeirdNumber"
 > setMethod("Math", "WeirdNumber", function(x) "Weird")
[1] "Math"
 > xx = new("WeirdNumber", 1:10)
 > sqrt(xx)
[1] "Weird"


Parlamis Franklin wrote:
> i think these two code snippets exhibit a bug.  they are identical  
> but for the inclusion of an initial line in snippet [2]
>
> [1]
> setMethod("Math", signature(x = "numeric"), function(x) "Works")
> getGeneric("sqrt")(4)
>
> [2]
> getGeneric("sqrt")(4)
> setMethod("Math", signature(x = "numeric"), function(x) "Works")
> getGeneric("sqrt")(4)
>
> these are my results, when each is run from a fresh R session
>
> [1]
>  > setMethod("Math", signature(x = "numeric"), function(x) "Works")
> [1] "Math"
>  > getGeneric("sqrt")(4)
> [1] "Works"
>
> [2]
>  > getGeneric("sqrt")(4)
> `__Deferred_Default_Marker__`
>  > setMethod("Math", signature(x = "numeric"), function(x) "Works")
> [1] "Math"
>  > getGeneric("sqrt")(4)
> `__Deferred_Default_Marker__`
>
> the section in "How Methods Work" that deals with this is unfinished,  
> but i think it suggests that getGeneric("sqrt")(4) should work out of  
> the box.
>
> franklin parlamis
>
>  > version
>                 _
> platform       powerpc-apple-darwin8.7.0
> arch           powerpc
> os             darwin8.7.0
> system         powerpc, darwin8.7.0
> status         beta
> major          2
> minor          4.0
> year           2006
> month          09
> day            22
> svn rev        39471
> language       R
> version.string R version 2.4.0 beta (2006-09-22 r39471)
>
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