Accessing slots of an S4 function superclass

Question

Can I create an S4 superclass of "function" and access the slots of that object from the function call? At the moment I have:

> setClass("pow",representation=representation(pow="numeric"),contains="function")
[1] "pow"
> z=new("pow",function(x){x^2},pow=3)
> z(2)
[1] 4

Now what I really want is for the function to be x to the power of the @pow slot of itself, so if I then do:

> z@pow=3 

I get cubes, and if I do:

> z@pow=2

I get squares.

But I don't see how to get a reference to 'self' like I would do in Python. I'm guessing its somewhere in the environment somewhere...

Here's how it works in python:

class Pow:
    def __init__(self,power):
        self.power=power
        self.__call__ = lambda x: pow(x,self.power)

p = Pow(2) # p is now a 'squarer'
print p(2) # prints 4

p.power=3 # p is now a 'cuber'
print p(2) # prints 8

Couldn't really be easier, and I didn't even have to do "import antigravity"....

Answer 1

Resorting to a little language manipulation

setClass("Pow", representation("function", pow="numeric"),
         prototype=prototype(
           function(x) {
               self <- eval(match.call()[[1]])
               x^self@pow
           }, pow=2))

and then

> f = g = new("Pow")
> g@pow = 3
> f(2)
[1] 4
> g(2)
[1] 8

although as Spacedman says things can go wrong

> f <- function() { Sys.sleep(2); new("Pow") }
> system.time(f()(2))
   user  system elapsed 
  0.002   0.000   4.005 

A little more within the lines but deviating from the problem specification and probably no less easy on the eyes is

setClass("ParameterizedFunFactory",
         representation(fun="function", param="numeric"),
         prototype=prototype(
           fun=function(x, param) function(x) x^param,
           param=2))

setGeneric("fun", function(x) standardGeneric("fun"))
setMethod(fun, "ParameterizedFunFactory",
          function(x) x@fun(x, x@param))

with

> f = g = new("ParameterizedFunFactory")
> g@param = 3
> fun(f)(2)
[1] 4
> fun(g)(2)
[1] 8

Answer 2

I think it depends what you really want. Is this implementation any closer to your goal?

setClass("pow",representation=representation(pow="numeric"),contains="function")
z=new("pow",function(x, pow=3){x^pow})
>  z(2)
[1] 8
 z(2,4)
#[1] 16

Answer 3

It seems that the parent function can be accessed via sys.function.

setClass("pow", slots = c(pow = "numeric"), contains = "function")
z <- new("pow", function (x) x^(sys.function()@pow), pow = 2)
z(6)
# [1] 36
z@pow <- 3
z(6)
# [1] 216

I don't know whether sys.function exists when this question was firstly discussed, though.