dynamically add function to r6 class instance

Question

I'm trying to forget refclasses (R5) and move to R6 but there is a problem with dynamic code. I would add a new function and it works in R5:

clsTrn <- setRefClass("clsTrn",
  fields = list(x = "numeric"),
  methods = list(
    add_function = function(rcode) {
      eval(parse(text=rcode), envir=.self)
    }
  )
)  

cls <- clsTrn$new(x=4)
cls$x
# [1] 4
cls$add_function("predict = function(y) {return(.self$x*y)}")

cls$predict(3) 
#[1] 12

Similar code doesn't work for R6.

library(R6)

clsTrnR6 <- R6Class("clsTrnR6",
  lock=FALSE,
  public = list(
    x = NA,
    initialize = function(x) {
      self$x <- x
    },
    add_function = function(rcode) {
      eval(parse(text=rcode), envir=self)
    }
  )
)  


clsR6 <- clsTrnR6$new(x=4)
clsR6$x
#[1] 4

clsR6$add_function("predict = function(y) {return(self$x*y)}")
# Błąd weval(expr, envir, enclos) : nie udało się znaleźć funkcji '='
clsR6$predict(3)

Adding predict in class definition changes nothing, the same error. Is there any solution? Thanks in advance.

> sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=pl_PL.UTF-8       LC_NUMERIC=C               LC_TIME=pl_PL.UTF-8        LC_COLLATE=pl_PL.UTF-8     LC_MONETARY=pl_PL.UTF-8   
 [6] LC_MESSAGES=pl_PL.UTF-8    LC_PAPER=pl_PL.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=pl_PL.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] R6_2.0

loaded via a namespace (and not attached):
[1] codetools_0.2-8 rpart_4.1-5     tools_3.1.1    
> 

Added: After great @G.Grothendieck answer, I have string based function definition, but maybe there is more elegant solution.

library(R6)

clsTrnR6 <- R6Class("clsTrnR6",
  lock=FALSE,
  public = list(
    x = NA,
    initialize = function(x) {
      self$x <- x
    },
    add_function = function(name, meth) {
      self[[name]] <- meth
      environment(self[[name]]) <- environment(self$add_function)
    },
    add_function2 = function(name, meth) {
      eval(parse(text=paste0("predict <- ",meth)))
      self[[name]] <- predict
      environment(self[[name]]) <- environment(self$add_function)
    }
  )
)  

clsR6 <- clsTrnR6$new(x=4)
clsR6$x

#[1] 4

clsR6$add_function2("predict", "function(y) y*self$x")
clsR6$predict(11)

#[1] 44

Answer 1

Try this. Like the reference class example it adds a function to the object (not the class). Here name is a character string containing the name of the function/method and meth is the function/method itself:

clsTrnR6 <- R6Class("clsTrnR6",
  lock=FALSE,
  public = list(
    x = NA,
    initialize = function(x) {
      self$x <- x
    },
    add_function = function(name, meth) {
      self[[name]] <- meth
      environment(self[[name]]) <- environment(self$add_function)
    }
  )
)  
clsR6 <- clsTrnR6$new(x=4)
clsR6$x
#[1] 4
clsR6$add_function("predict", function(y) y*self$x)
clsR6$predict(11)
## 44

Added Note that this is also easy to do using proto. It does not require a special add_function. We will use an upper case P to denote the proto object that plays the role of a class (called a "Trait" in the proto vignette) and use lower case p to denote the proto object that plays the role of an instance:

library(proto)

P <- proto(new = function(., x) proto(x = x))
p <- P$new(x = 4)

p$predict <- function(., y) .$x * y
p$predict(11)
## 44

Although its common to use . to refer to the object in proto you can use the name self (or any name you like) in place of . if you prefer.