by Karl-Kuno Kunze

This post gives you an idea of what a closure is. I first motivate why it is helpful to know about the concept. Then I introduce the notion of an environment. Having this we build a closure.

Sometimes you may run into error messages like: object of type ‘closure’ is not subsettable. To understand what R wants to tell you here, the notion of a closure is of quite some help. To put it briefly: A closure is a function together with an environment.

Firstly, we look at environments and then we connect an environment with a function. That is all it needs to build a closure.

Environment

An environment is a container that holds a set of objects. As everything is an object in R, an environment is an object, too. We first create two environments and then play around with them a little:

env1 <- new.env()
env2 <- new.env()
env1[["iAm"]] <-"Environment 1"  # One way of creating objects in an environment
env2$iAm <- "Environment 2"      # Another way
get("iAm", envir = env1)         # One way of accessing objects in an environment

## [1] "Environment 1"

env2$iAm                         # Another way of accessing objects in an environment

## [1] "Environment 2"

There are several functions to access and manipulate objects in environments. Check with(), within() and attach() to name some.

Every environment resides in a parenting environment. This is the same as saying it is an object that is defined in the parenting environment.

parent.env(env1)

## <environment: R_GlobalEnv>

Closure

Now we define a function that prints the contents of object iAm. This implicitly connects the global environment to the function to make a closure.

iAm <- "R_GlobalEnv"
envFun <- function() cat("I belong to :", iAm)
envFun()

## I belong to : R_GlobalEnv

Now we connect environment env1 with the function to make another closure:

environment(envFun) <- env1
envFun()

## I belong to : Environment 1

Now we connect environment env2 with the function to make still another closure:

environment(envFun) <- env2
envFun()

## I belong to : Environment 2

Re-attaching the Global Environment works like this:

environment(envFun) <- .GlobalEnv
envFun()

## I belong to : R_GlobalEnv

If you start R from scratch and define a function, it will form a closure with the Global Environment, i.e. have the Global environment attached.

Now, if you want to provoke the error message above, this is a way to do it:

envFun[]

## Error in envFun[]: Object of type 'closure' is not subsettable

You cannot build subsets of functions, which comes quite natural – and is no surprise.