How to define a closure?

Question

Here I'm attempting to define a closure that contains a function which can be invoked :

myFirstClosure <- function (){

  var1 = 1;

  f1 <- function() {
    var1
  }

}

m <- myFirstClosure()
m.f1()

Error is returned : Error: could not find function "m.f1"

Why is the inner scoped function f1 not accessible as I have defined the outer scoped function myFirstClosure ?

Update :

Is it meaningless to invoke closures like :

myFirstClosure <- function (){

  var1 = 1;
  var2 = 0;

  f1 <- function() {
    var1
  }

  f2 <- function() {
    var2
  }

}

As f1 and f2 cannot be invoked independently ?

Regarding your update, take a look at `?environment`. You can place your `var1`, `var2`, `f1` and `f2` inside the same environment. — Artem Sokolov, Nov 07 '17 at 21:25
With your updates, I'd be tempted to mark as a dupe of [Return more than one object from an R function](https://stackoverflow.com/q/8936099/903061). — Gregor Thomas, Nov 07 '17 at 21:36
@Gregor: I think this question has more to do with how to abuse R to simulate classical OO paradigm. My understanding is that the OP is trying to define a "class" with "member variables" and "member functions". Returning multiple objects is a by-product of this particular solution, but I would argue that's not what the question is about. — Artem Sokolov, Nov 07 '17 at 21:42

score 6 · Answer 1 · answered Nov 07 '17 at 21:15

. is meaningless in R (except for S3 method dispatch, but that's not relevant here), and the fact that the returned function was once called f1 in the environment of the function that created it is not retained. As soon as you assign myFirstClosure() to m,

m <- myFirstClosure()

its name is m and nothing else. You can call it with m().

For additional reading, I'd suggest the Functional Programming chapters of Advanced R.

Rui Barradas · Answer 2 · 2017-11-07T21:32:12.953

You cannot use that form, m.f1() because that's not Rsyntax. You can use a similar form, with environments. Something like the following.

myFirstClosure <- function (){
    var1 <- 1
    e <- environment(myFirstClosure)
    e$f1 <- function() {
        var1
    }
    e
}

m <- myFirstClosure()
m$f1()
#[1] 1

Note however that myFirstClosure would then return an environment, not a closure. This probably defeats your original goal.
See also An Introduction to R, section 10.7 Scope, the open.account example for another way of doing something like what you seem to want.

EDIT.
To answer to the question's edit by the OP, and repeating the code above a bit, the example above can be extended to any number of functions.

myFirstClosure <- function (){
    var1 <- 1
    var2 <- 0
    e <- environment(myFirstClosure)
    e$f1 <- function() {
        var1
    }
    e$f2 <- function() {
        var2
    }
    e
}

m <- myFirstClosure()
m$f1()
#[1] 1

m$f2()
#[1] 0

please see my recent question update, I think your answer is closest to what I'm planing to use. — blue-sky, Nov 07 '17 at 21:28

score 2 · Answer 3 · answered Nov 07 '17 at 22:35

2

This achieves the behavior I'm looking for :

myFirstClosure <- function() {

  var1 <- 1;
  var2 <- 0;

  list(f1=function() { 
    var1
  },f2=function() { 
    var2
  })
}

m <- myFirstClosure()
m$f1()
m$f2()

This aided in finding solution https://www.r-bloggers.com/using-closures-as-objects-in-r/ as well as answers / comments for this question.

answered Nov 07 '17 at 22:35

blue-sky

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I highly recommend you take a look at the `R6` package. – Andrie Nov 08 '17 at 08:49

score 0 · Answer 4 · answered Nov 07 '17 at 21:20

The simplest answer here is that you have defined your closure on m itself, and need to simply invoke m

myFirstClosure <- function (){

  var1 = 1;

  f1 <- function() {
    var1
  }

}

m <- myFirstClosure()
m()

I just ran this and it outputs 1 as you desire.

I am aware that you are hoping to do something slightly different - your code reminds me of the closure-as-module pattern in javascript, where your closure-creating function outputs an object with multiple functions that can be called individually as attributes, all with a closure on variables internal to the creating function.

I don't want to say that this is impossible with R but I am not aware of how to do it. I am interested to see if other commentors can suggest a way.

Incidentally dot-to-access attribute syntax is mostly (entirely?) not available in r - usually you will use the $ syntax, but that also won't work in this instance.

How to define a closure?

4 Answers4