[Rd] validity testing as part of '@<-'

Duncan Murdoch murdoch at stats.uwo.ca
Fri Sep 22 01:21:46 CEST 2006

On 9/21/2006 5:29 PM, Parlamis Franklin wrote:
> 'methods' package feature request / discussion starter:
> perhaps a call to 'validObject' should occur at part of any slot  
> replacement operation (and the operation not be carried out if it  
> would invalidate the object)?  this may prevent the need for  
> prophylactic 'validObject' calls in other user-defined functions.  in  
> the example below, the slot assignment occurs even though it  
> invalidates the object.  the 'show' method then fails somewhat deep  
> in a subsequently-called coercion method.
> library(Matrix)
> test <- Matrix(1:4+0.1, nrow = 2, dimnames = list(letters[1:2],  
> LETTERS[1:2]))
> test at Dimnames <- list(1:100)
> test
> direct slot access (without a validity check) seems an easily  
> accessible loophole to S4 object "guarantees".

Often changing a single slot puts the object into a temporarily invalid 
state; others need to be changed consistently before it could pass the 
validity test.

Perhaps you could handle this case by wrapping all the calls in some 
sort of wrapper, e.g.

atomic( {obj at slot1 <- 1
          obj at slot2 <- 2 })

but requiring that would break tons of existing code, and turning on 
validObject tests everywhere would slow things down a lot.  Isn't it 
easier to just sprinkle a few more validObject() calls into your code?

Duncan Murdoch

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