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"....

Yeah? Has the S language finally got object-oriented programming right this time? — Spacedman
– Spacedman, Commented Dec 2, 2011 at 8:21
If by right, you mean has implemented in a way that you're familiar with from languages like java, then yes. — hadley
– hadley, Commented Dec 2, 2011 at 13:00
I meant in the way that S3 is clearly wrong (but its a very moby hack) and S4 is just unclearly wrong (see above). But yeah, I think in the limit as t->Inf then R->python... Lets get rid of curly braces from R and use indentation.... — Spacedman
– Spacedman, Commented Dec 2, 2011 at 13:51
With reference classes you'd get 'self' but loose the 'isA' relationship Spacedman asked for (at least in my experiments -- a more complete example?). Also you'd have reference semantics on your pow exponent, which could be quite confusing though consistent with the python implementation. — Martin Morgan
– Martin Morgan, Commented Dec 2, 2011 at 14:40

Martin Morgan · Accepted Answer · 2011-12-02 15:19:49Z

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

I might just give you that. Although if you try and do an anonymous object: new("Pow")(3) it actually gets the \@pow slot from another new object created by the eval rather than the one created by the initial "new" call. Not that it makes any difference, and its a very perverse thing to do anyway. eval is nearly always evil.

IRTFM · Accepted Answer · 2011-11-30 15:47:07Z

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

Jialin Ma · Accepted Answer · 2017-08-26 09:45:41Z

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.

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