[R] Need Advice: Considering Converting a Package from S3 to S4

[Terry Therneau]

 For 90 percent of what I do I strongly prefer the loose (S3) rather than the 
rigid (S4) classes.  So I'm closer to Rolf.  My summary of S4 vs S3
 
A large increment in 
  1. nuisance to write
  2. difficulty to debug
  3. ability to write very obscure code 
  4. design
  
Gain
  5. ability to direct automatic conversions
  6. validate the contents of a class object
  
  For simple objects 5 and 6 can be critical.  Consider a date for instance, 
which will often be turned into a character, added or subtracted as a numeric, 
plotted, etc.  Conversely, aspects of 1-4 are less worrisome for a simple 
object, particularly #4: I have a reasonable chance of "getting it right" the 
first time.
  
  For a complex object such as the result of a coxph fit 
  	fit <- coxph(Surv(time, status) ~ age + sex + treatment)

#5 makes no sense at all: as.numeric(fit)???  Number 4 and 6 are really hard;
after 15+ years of tuning I am still modifying the list of components in a coxph 
object.  I know more about the computational aspects of Cox models than almost 
anyone and still it's not enough.  Changes are harder with rigid classes.

  With reference to #3 above, for your amusement, look at
     www.netfunny.com/rhf/jokes/98/May/straustrup.html
the key line (to me) being "..every C++ programmer feels bound by some mystic 
promise to use every damm element of the languange on every project..."

Terry T.