Define S4 class inheriting from function-CodePudding

I'm trying to write an S4 class that specifically returns a numeric vector of the same length as the input. I think I'm close; the problem I'm having now is that I can only create new classes from functions that live in my GlobalEnv.

library(S4Vectors)

setClass("TransFunc", contains = c("function"), prototype = function(x) x)

TransFunc <- function(x) {
  if (missing(x)) return(new("TransFunc"))
  new2("TransFunc", x)
}

.TransFunc.validity <- function(object) {
  msg <- NULL
  if (length(formals(object)) > 1) {
    msg <- c(msg, "TransFunc must only have one argument.")
  }
  res1 <- object(1:5)
  res2 <- object(1:6)
  if (length(res1) != 5 || length(res2) != 6) {
    msg <- c(msg, "TransFunc output length must equal input length.")
  }
  if (!class(res1) %in% c("numeric", "integer")) {
    msg <- c(msg, "TransFunc output must be numeric for numeric inputs.")
  }
  if (is.null(msg)) return(TRUE)
  msg
}

setValidity2(Class = "TransFunc", method = .TransFunc.validity)

mysqrt <- TransFunc(function(x) sqrt(x))
mysqrt <- TransFunc(sqrt) ## Errors... why??
## Error in initialize(value, ...) : 
##   'initialize' method returned an object of class “function” instead 
##   of the required class “TransFunc”

The benefit to having a class inherit from function directly is the ability to use them as regular functions:

mysqrt(1:5)
## [1] 1.000000 1.414214 1.732051 2.000000 2.236068 
body(mysqrt) <- expression(sqrt(x)^2)
mysqrt(1:10)
##  [1]  1  2  3  4  5  6  7  8  9 10

Why does it error when passing functions outside the global env?

CodePudding user response：

It does not work for sqrt because sqrt is primitive.

I am not aware of any functions that take only one argument and aren't primitive. Therefore I cut your validity down to demonstrate how your code works with other functions from the preloaded packages:

 #using your class definition and counstructor
 .TransFunc.validity <- function(object) {
   msg <- NULL
   res1 <- object(1:5)
   if (!class(res1) %in% c("numeric", "integer")) {
     msg <- c(msg, "TransFunc output must be numeric for numeric     inputs.")
   }
   if (is.null(msg)) return(TRUE)
   msg
  }  

  setValidity2(Class = "TransFunc", method = .TransFunc.validity)

Here are the results for the default version of mean

mymean <- TransFunc(mean.default)
mymean(1:5)
[1] 3

Here is a workaround by modifying initialize for your class to catch primitives and turn them into closures:

#I modified the class definition to use slots instead of prototype
setClass("TransFunc", contains = c("function"))

TransFunc <- function(x) {
if (missing(x)) return(new("TransFunc"))
new2("TransFunc", x)
}
 
# Keeping your validity I changed initilalize to:

 setMethod("initialize", "TransFunc",
      function(.Object, .Data = function(x) x , ...) {
          if(typeof(.Data) %in% c("builtin", "special"))
                    .Object <- callNextMethod(.Object, function(x) return(.Data(x)),...)
              
          else 
             .Object <- callNextMethod(.Object, .Data, ...)
                                              
          
          .Object                                    
                                              
      })

I got the following results

mysqrt <- TransFunc(sqrt)
mysqrt(1:5)
[1] 1.000000 1.414214 1.732051 2.000000    2.236068

EDIT:
in the comments @ekoam proposes a more general version of initilaize for your class:

setMethod("initialize", "TransFunc", function(.Object, ...) 
 {maybe_transfunc <- callNextMethod();
      if (is.primitive(maybe_transfunc)) 
          [email protected] <- maybe_transfunc 
      else .Object <- maybe_transfunc; 
 .Object})