R-beta: S Compatibility (again)

3. Language manipulation within R seems to be impossible.  I
   realise this may be a design limitation that nobody can do
   anything about, but it may be worth noting.

   To be more specific, the R substitute() is much more limited
   than the S version and coercion to mode "{", "call" or
   "function" are unavailable, and function objects are not
   subsetable.  as.call is useless and as.function only exists to
   issue a rather tetchy error message.

   For example, as far as I can see it is impossible to write a
   version of the S function deriv() for R, short of getting down
   to brass tacks and writing a new primitive into the base code,
   but even then you can't extend it, of course.

While language manipulation is much more limited than in S, you probably
could write a version of deriv() (if it didn't already exist) since
expressions can be manipulated quite well.

eg
R> a
expression(sin(cos(x + 1)))
R> a[[1]]
sin(cos(x + 1))
R> a[[1]][[1]]
sin
R> a[[1]][[2]]
cos(x + 1)
R> a[[1]][[2]][[2]]
x + 1
R> a[[1]][[2]][[2]][[1]]
+
R> a[[1]][[2]][[2]][[2]]
x
R> a[[1]][[2]][[2]][[3]]
[1] 1
R> a[[1]][[1]]<-as.name("cos")
R> a
expression(cos(cos(x + 1)))


Thomas Lumley
------------------------
Biostatistics		
Uni of Washington	
Box 357232		
Seattle WA 98195-7232	
------------------------

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