Understanding lazy evaluation in R

Question

I'm trying to understand just how lazy evaluation in R works. Does it only apply to the evaluation of function arguments? Because that I understand, e.g.

f <- function(x = x, y = x*2) {
  c(x, y)
}

f(2)
[1] 2 4

But in other languages, e.g. Haskell, lazy evaluation means that a function call only gets evaluated if it's ever actually used. So I would expect something like this to run in an instant:

g <- function(x) {
  y <- sample(1:100000000)
  return(x)
}

g(4)

But it clearly evaluates the sample call even though its result doesn't get used.

Could somebody explain exactly how this works, or point me in the direction of where it is explained in detail?

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Hans Ekbrand Hans Ekbrand · Accepted Answer · 2017-03-09T15:16:01

As you already have found out, R does not use lazy evaluation in the general sense. But R does provides that functionality, if you need it, by the function delayedAssign()as shown below:

> system.time(y <- sample(1E8))
   user  system elapsed 
  7.636   0.128   7.766 
> system.time(length(y))
   user  system elapsed 
      0       0       0 
system.time(delayedAssign("x", sample(1E8)))
   user  system elapsed 
  0.000   0.000   0.001 
> system.time(length(x))
   user  system elapsed 
  7.680   0.096   7.777

As you can see, y is evaluated immediately, so to determine the length of y takes no time at all. x on the other hand, is not evaluated when it is created, only a promise to evaluate x is returned by delayedAssign(), and only when we actually need a value of x, in this case to determine its length, x is evaluated.

It does not matter if the expression is placed in a function or executed in the global enviroment, so the encapsulation of the expression within a function which you did in your example, does not really add anything, which is why I excluded it. But if you want to be sure, try:

a.f <- function(z) { delayedAssign("x", sample(1E8)); return(z+1) }
system.time(a.f(0))
   user  system elapsed 
      0       0       0

Understanding lazy evaluation in R

1 Answers