[R] Lazy Evaluation?

[Thomas Stabla]

Hello,

I've stumbled upon following problem, when trying to overload the methods
for group Math for an S4-class which contains functions as slots.


  setClass("NumFunction", representation = list(fun = "function"))

  NumFunction <- function(f) new("NumFunction", fun = f)

  square <- function(x) x^2
  NF <- NumFunction(square)

  setMethod("Math",
            "NumFunction",
            function(x){
                nfun  <- function(n) callGeneric(x at fun(n))
                tmp <- function(n) nfun(n)
                NumFunction(tmp)
            })

  sinNF <- sin(NF)
  sinNF at fun(sqrt(pi/2))

# works as expected, returns 1

# now a slightly different version of setMethod("Math", "NumFunction",
# ...), which dispenses the "unnecessary" wrapper function tmp()

  setMethod("Math",
            "NumFunction",
            function(x){
                nfun  <- function(n) callGeneric(x at fun(n))
                return(NumFunction(nfun))
            })

   sinNF <- sin(NF)
   sinNF at fun(sqrt(pi/2))

# produces an error, namely:
# Error in typeof(fdef) : evaluation nested too deeply: infinite
# recursion / options(expression=)?


Replacing the generating function NumFunction() with corresponding
new(..) calls doesn't change the outcome.

When I call the newly defined functions nfun resp. tmp from within the
function body of setMethod(), e.g.

  setMethod("Math",
            "NumFunction",
            function(x){
                nfun  <- function(n) callGeneric(x at fun(n))
                cat(nfun(1))
                NumFunction(nfun)
            })

by tmp(1) resp. nfun(1), both versions "cat" correct output when called by
sin(NF).

If I don't return NumFunction(tmp) resp. NumFunction(nfun) but tmp resp.
nfun, both versions work just fine, i.e. sin(NF)(sqrt(pi/2)) returns 1.

Would someone explain me this behavior? (R Version 1.9.0)

Best regards,
Thomas Stabla

Sourcecode can be found at:
http://www.uni-bayreuth.de/departments/math/org/mathe7/DISTR/NumFun.R