Programming with Reference Class

Question

1. How to define several polymorph constructors and functions like

   function Add( x, y : Integer ) : Integer;
   begin
       Add := x + y
   end;
   
   function Add( s, t : String ) : String;
   begin
       Add := Concat( s, t )
   end;
   
   begin
       Writeln(Add(1, 2));
       Writeln(Add('Hello, ', 'World!'));
   end.
   

Can I do this only by case decission like

A<-setRefClass(Class = "A"
              ,fields = list(var1="character")
              ,methods = list(setFields=A.setFields
                             ,initialize=function(var1) {
                                if(isClass(var1,"B"))
                                   .self$var1<-as.character(var1$getFields("var1"))                                  
                                else{
                                  .self$var1<-as.character(var1)
                                }
                                .self
                              })
)

1. How to combine functional programming with objectorientated prgramming. So if I would call the functions getFields(vecB), where vecB is a vector or list of objects B. The return should be the values of each object?

    B.getFields<-function(...,values){ 
      vars<-mget(names(.refClassDef@fieldClasses), envir = attr(.self, ".xData"))
      if(missing(values)) 
        return(vars)
      if(length(vars)==1)
        return(vars[[1]])
     return(vars[names(vars) %in% values])
     }
   
        B<-setRefClass(Class = "B"
                  ,fields = list(var1 = "character")
                    )
   

2. How to debug e.g. the function initialize from class A? I tried

   A$trace("initialize")
   a<-A$new("ABC")
   initial<-a$initialize
   trace(initial,browser,where=A)
   

   but it doesnt work.

Answer 1

1. Use S4 generics and methods for polymorphism

   setGeneric("Add", function(x, y) standardGeneric("Add"))
   setMethod(Add, c("numeric", "numeric"), function(x, y) x + y)
   setMethod(Add, c("character", "character"), function(x, y) paste0(x, y))
   

   so

   > Add(1, 2)
   [1] 3
   > Add("hello ", "world")
   [1] "hello world"
   > Add("hello ", 2)
   Error in (function (classes, fdef, mtable)  : 
     unable to find an inherited method for function 'Add' for signature '"character", "numeric"'
   

   One idea to use this generic in a reference class is

   A <- setRefClass("A",
       fields=list(x="ANY"),
       methods=list(
         getX=function() {
             .self$x
         }, setX=function(x) {
             .self$x <- x
             .self
         }, addX=function(y) {
             setX(Add(getX(), y))
         }, show=function() {
             cat("class:", class(.self), "\nx:", getX(), "\n")
         }))
   

   with appropriate dispatch for functional program style:

   setMethod(Add, c("A", "ANY"), function(x, y) x$addX(y))
   setMethod(Add, c("A", "A"), function(x, y) x$addX(y$getX()))
   

   (maybe for functional programming it makes more sense to implement $addX() as A(.self, x=Add(x$getX(), y)), i.e., creating a clone of x?) to allow for

   > Add(A(x=1), A(x=2))
   class: A 
   x: 3 
   > Add(A(x="hello "), "world")
   class: A 
   x: hello world 
   > A(x=2)$addX(3)
   class: A 
   x: 5 
   

   though not A(x=1)$addX(A(x=2))

2. See this answer for one approach to returning field values:

   B = setRefClass("B", fields=c(var1="list", var2="character"),
       methods=list(getFields=function(values) {
           flds = names(getRefClass()$fields())
           if (!missing(values))
               flds = flds[flds %in% values]
           result = setNames(vector("list", length(flds)), flds)
           for (fld in flds)
               result[[fld]] = .self[[fld]]
           result
       }))
   

   One way to invoke in a functional way requires a separate implementation, e.g.,

   setGeneric("getFields", function(x, ...) standardGeneric("getFields"))
   setMethod(getFields, "B", function(x, ...) x$getFields(...))
   setMethod(getFields, "list", function(x, ...) lapply(x, getFields, ...))
   

3. Your class A example is incomplete.

   A <- setRefClass("A", methods=list(initialize=function(...) {
       message("hello A")
       callSuper(...)
   }))
   

   and then

   > A$trace("initialize", browser)
   Tracing reference method "initialize" for class "A"
   [1] "initialize"
   > A()
   Tracing .Object$initialize(...) on entry 
   Called from: eval(expr, envir, enclos)
   Browse[1]> n
   debug: {
       message("hello A")
       callSuper(...)
   }
   Browse[2]> n
   debug: message("hello A")
   Browse[2]> 
   

Answer 2

Here's a previous discussion on polymorphisms in R: What are the suggested practices for function polymorphism in R?

The way to concatenate a string is with paste.

paste("Hello, ","world",sep="")