Lexical Scoping: eval(expr,envir=)

On Thu, 18 Nov 2004 12:05:34 +0100, Eric Lecoutre
<lecoutre at stat.ucl.ac.be> wrote :

Hi R-listers,

I am trying to better undertand what we would call "functional paradigm" 
use of S/R to better map my programming activities in other languages.

This little function is aimed to create an object (at the end end, it would 
have it's own class):

--
  myObject =function(){
    list(
      a=1,
      foo=function(b)
      {
      cat("b:",b)
      cat("\na:", a)
      }
    )
  }
--
To my minds, "a" would be a property of the object and "foo" one of it's 
method.

To work with lexical scoping, you could do it as

 myObject  <- function(){
    a <- 1 
     list(
       a=function() a,
       "a<-" = function(x, value) a <<- value,
       foo=function(b)
       {
       cat("b:",b,"\n")
       cat("a:", a,"\n")
       }
     )
   }

Then you can access the a property pretty easily, but changing it is
really ugly:

m <- myObject()
m$a()

[1] 1

m$foo()

Error in cat("b:", b, "\n") : Argument "b" is missing, with no default

m$foo(1)

b: 1 
a: 1

(m$"a<-")(m, 4)
m$foo(1)

b: 1 
a: 4 

It would be slightly nicer (but still ugly) if this worked:

(m$a)(m) <- 4

Error: invalid function in complex assignment

but it doesn't.

Duncan Murdoch